Nut Proteins as Plant-Based Ingredients: Emerging Ingredients for the Food Industry

Abstract

This review explores the growing interest in and potential applications of proteins extracted from nuts in the food industry. With an increasing shift toward plant-based diets and sustainable food sources, the nutritional value and functional properties of nut proteins have gained significant attention. The composition, digestibility, and bioavailability of these proteins are discussed, emphasizing their role as high-quality substitutes for animal-based proteins. The text further delves into the technological applications of nut proteins, focusing on their ability to stabilize emulsions, enhance texture, and contribute to the development of innovative food products. This review highlights the diverse range of nuts and their unique protein profiles, underscoring the importance of combining different plant protein sources to achieve a well-balanced amino acid composition. As the food industry seeks novel and sustainable protein alternatives, the utilization of nut proteins emerges as a promising avenue with considerable nutritional benefits.

1. Introduction

In recent years, nutrition has advanced with new discoveries about the role of proteins in the diet. Proteins are crucial for the body as they contribute to tissue construction and repair [1,2,3]. The demand for a healthy diet has led to a growing interest in isolated nut proteins, such as chestnuts, hazelnuts, and walnuts, to meet nutritional needs. In this article, we will explore these proteins in detail, including their origin, benefits, applications, and scientific evidence.

Proteins are essential for the growth, maintenance, and repair of the human body, as they are composed of amino acids necessary for vital biological functions [4]. The need for proteins varies based on age, sex, physical activity, and personal goals. Isolated nut proteins, which have undergone purification to remove fats and carbohydrates, offer a high concentration of protein [5].

These proteins offer significant benefits by providing a source of high-quality protein, often with a complete amino acid profile [6,7,8]. This makes them valuable for muscle growth, post-exercise recovery, and overall health [9,10,11]. Moreover, the absence of unwanted fats and carbohydrates (saturated or trans fats and sugars associated with intolerances such as lactose intolerance) makes these proteins suitable for individuals with food intolerances or dietary restrictions. Their versatility allows them to be included in various foods and beverages [12,13].

Isolated vegetable proteins also possess remarkable technological and functional properties, such as gelation, emulsification, and thickening, enhancing the texture and stability of food products [14,15]. For example, they can be used in meat alternative production, providing a similar texture and flavor [16,17]. They are common in bakery products, desserts, and beverages to improve texture and moisture retention.

These properties have a significant impact on the food industry, driving the consumption of plant-based products [16]. Isolated nut proteins meet the demand for sustainable and healthy foods.

In this article, we will deepen the understanding of isolated nut proteins, exploring their origin, benefits, functional properties, and applications, based on clinical studies and recent research, providing a comprehensive insight into this topic. After reading, you will have a solid understanding of isolated nut proteins and how to effectively integrate them into your diet to meet your nutritional needs.

2. Protein Composition of Nuts

In recent years, there has been a notable increase in the use of nuts for food purposes. Whole seeds, oils, flours, and by-products from cakes contain a wealth of nutrients and bioactive compounds and are therefore being widely promoted by nutrition professionals and food industry companies. This growing interest in healthy ingredients transcends borders and leads to the exploration of new trends. Both scientists and the food industry are researching the partial replacement of animal proteins with plant proteins. This movement not only enhances the health benefits of meat products but also addresses ethical and environmental concerns. These changes reflect the growing concern of scientists, activists, and governments regarding the reduction in meat production, encouraging more sustainable food choices [18]. Plant proteins have emerged as economical and versatile alternatives in human nutrition, playing a significant role in food composition. This growing consumption pattern has the potential to reduce the demand for animal proteins, potentially impacting production and eventually leading to a decrease in the prices of animal-based products [19].

Nuts are a nutrient-rich food that can be consumed fresh, roasted, or preserved, bringing a variety of health benefits to humans. Due to their nutritional content being rich in unsaturated fatty acids, such as oleic acid and linoleic acid, and low in saturated fatty acids, they are recommended for healthy diets. In addition, nuts contain bioactive lipids, such as phytosterols, tocopherols, and tocotrienols, contributing to cardiovascular health [20].

In addition to their lipid properties, nuts are also an excellent source of proteins, fibers, vitamins such as folic acid, niacin, and vitamin B6, and essential minerals such as calcium, magnesium, and potassium. They also contain a variety of phenolic compounds, including phenolic acids, flavonoids, lignin, hydrolysable tannins, and proanthocyanidins (condensed tannins), which have antioxidant and anti-inflammatory properties, contributing to overall health [21].

In the current scenario of the food industry, there is a growing demand for innovative options from nuts and legumes. These alternatives have stood out not only for their diversity and sensory appeal but also for their durability, catering to a variety of consumers, including vegetarians and omnivores. This movement has driven research in the field of nut proteins, suggesting promising future directions. In this context, plant proteins gain prominence as viable substitutes for animal proteins. Intensive research on these proteins opens doors for innovation in the food industry, enabling the development of products that meet the demands of consumers concerned with ethical, environmental, and health issues [22].

A crucial aspect of this advancement is the use of protein isolates and concentrates. These play an essential role, especially when proteins need to be isolated and applied in specific formulations (

Table 1. Extraction yield of nut proteins.

Nuts	Scientific Nomenclature	Protein Yield (%)	References
Brazil nut	Berthollethia excelsea	15−20	[24]
Peanut	Arachis hypogaea	8.88−12.7	[25]
Pecan nut	Carya illinoinensis	8	[26]
Macadamia	Macadamia integrifolia	9.23	[27]
Macaúba nut	Acronomia aculeata	28.61	[28]
Cashew nut	Anacardium occidentale L.	42	[29]
Hazelnut	Corylus avellana	35−41	[30]
Pistachio	Pistacia vera	34.6−46.1	[31]
Baru almond	Dipteryx alata	26−30	[32]

). A detailed understanding of these components is vital for creating high-quality food products, ensuring not only their commercial viability but also consumer satisfaction [23].

In addition to this aspect, the amino acid composition varies among different nuts, meaning their amino acid profiles are different. Plant proteins are deficient in essential exogenous amino acids and have a lower content or are primarily lacking in lysine, isoleucine, tryptophan, methionine, and valine. To replace large amounts of meat proteins in a product with plant-based proteins, it would be advisable to use two or three types of complementary amino acid compositions from different plants’ raw materials. This would provide the body with all the essential exogenous amino acids using the principle of complementation. It is necessary to provide the correct content of essential exogenous amino acids because they are responsible for proper “protein turnover,” i.e., the constant exchange of body proteins [33]

Recently, plant proteins have become essential in the food industry, being widely incorporated as functional ingredients. In particular, proteins derived from oilseeds have gained prominence, being meticulously studied for application in foods. Table 2 exemplifies some of these proteins and the distribution of essential amino acids, highlighting their nutritional potential [21].

Proteins in food play a crucial role in construction within biological systems, facilitating the restoration of cutaneous and bone tissues, as well as repairing damaged cells. Essential amino acids have a fundamental chemical composition, contributing to the production of human proteins. These proteins perform various physiological functions, including the transport of nutrients and hormones, cell replication, and the maintenance of organic functions [34].

Table 2. Approximate composition of essential amino acids in nuts.

Food	Leucine (%)	Valine (%)	Lysine (%)	Phenylalanine (%)	Cysteine (%)	Threonine (%)	Tyrosine (%)	References
Brazil nut	82.4	49.2	37.4	71.8	95.9	26.4	71.8	[35]
Peanut	70.3	39.5	38.8	87.8	16.4	22.1	87.8	[36]
Hazelnut	74.0	46.6	29.3	73.6	24.2	29.5	73.6	[37]
Baru nut	77.8	51.8	48.4	77.2	22.0	44.9	77.2	[38]
Cashew nut	45.9	56.5	45.9	72.6	28.1	32.2	72.6	[39]
Pistachio	75.6	56.9	46.4	73.2	24.1	29.7	73.2	[32]

Table 2. Approximate composition of essential amino acids in nuts.

Food	Leucine (%)	Valine (%)	Lysine (%)	Phenylalanine (%)	Cysteine (%)	Threonine (%)	Tyrosine (%)	References
Brazil nut	82.4	49.2	37.4	71.8	95.9	26.4	71.8	[35]
Peanut	70.3	39.5	38.8	87.8	16.4	22.1	87.8	[36]
Hazelnut	74.0	46.6	29.3	73.6	24.2	29.5	73.6	[37]
Baru nut	77.8	51.8	48.4	77.2	22.0	44.9	77.2	[38]
Cashew nut	45.9	56.5	45.9	72.6	28.1	32.2	72.6	[39]
Pistachio	75.6	56.9	46.4	73.2	24.1	29.7	73.2	[32]

3. Technological Properties of Nut Proteins

3.1. Solubility and Water Absortion Capacity

Solubility plays a fundamental role in the characteristics of proteins found in nuts, closely related to the interaction of these proteins with water molecules, peptide bonds, hydrogen bonds, and the side chains of amino acid residues. This solubility is of great importance for food products containing proteins as it affects functional properties such as emulsification, foam formation, rheological behavior, and surface activity [40,41]. Several factors, including temperature, extraction method, pH, ion concentration, and the intrinsic characteristics of proteins in nuts, play a critical role in solubility and, therefore, in the properties of these proteins [40].

Proteins found in nuts have lower solubility around their isoelectric point (pI), which typically ranges from pH 3.0 to 5.0. However, solubility increases at pH values away from the pI. This suggests that these proteins have an acidic nature [40,42,43,44]. The lower solubility at the isoelectric point is a result of the net charge of the peptides, reducing electrostatic repulsion between proteins due to hydrophobic interactions, as well as increasing surface hydrophobicity [45]. On the other hand, solubility increases at neutral to alkaline pH, as plant proteins have a globular structure that enhances solubility under these conditions [40].

pH plays a crucial role in protein solubility because the electric charges of proteins can result in repulsive electrostatic forces that keep the proteins dispersed in solution. However, near the isoelectric point, proteins tend to aggregate due to strong interactions, resulting in reduced solubility. The addition of salts in small quantities can increase protein solubility, but higher concentrations can lead to ionic interactions that decrease solubility [43].

The solubility of nut proteins can be improved through gentle heat treatments, but it can also result in aggregation and reduced solubility in specific cases. Unconventional methods, such as the application of high pressure, can also affect solubility, with specific parameters playing a significant role. Furthermore, protein phosphorylation can significantly increase solubility, introducing negative charges and enhancing electrostatic repulsion between protein molecules. The solubility of nut proteins can vary widely among different types of nuts, with some, such as almonds, demonstrating good solubility over a wide pH range, while others have a more restricted solubility [46].

Water Absorption Capacity (WAC) is another important property of nut proteins, influenced by factors such as ionic strength, pH, temperature, and pressure. WAC can be improved by optimizing these factors. Increasing salt concentration can affect WAC, initially increasing it but decreasing it as salt concentration rises, due to the interaction between salt ions and proteins [43]. In acidic pH, lower values of this property are obtained in nut proteins. It is improved when at alkaline pH [40,43]. This occurs in these matrices due to aggregate formation that occurs at the isoelectric point, resulting from strong molecular interactions and affecting low WAC [43]. When adjusting the pH as a strategy to improve the properties of proteins found in nuts, an increase in the amount of bound water is observed due to the increased electrical charge and polarity of the proteins, resulting from the ionization of amino acid groups [47].

Regarding the concentration of salt ions, Yuliana et al. [43] noted that WAC increased from 1.23 to 2.04 cm³·g⁻¹ as the NaCl concentration was increased from 0 to 0.5 mol·dm⁻³. Subsequently, there was a moderate decrease in WAC with further increases in NaCl concentration. The authors explain that as the salt concentration increases, most of the available water binds to salt ions, intensifying intermolecular interactions between proteins. This results in protein dehydration and, consequently, a reduction in WAC.

Table 3 presents different nut proteins along with their WAC and solubility under different conditions of pH, salt ion concentrations, and temperature, clearly illustrating the direct impact of these factors on the characteristics of these proteins. It is evident that all these factors significantly influence WAC and solubility properties. For instance, an increase in salt concentration from 0 to 0.4 M can improve WAC by up to 12.6% [40]. However, WAC tends to decrease as salt concentration increases due to the phenomenon known as “salting in”. Similarly, solubility responds directly to ionic strength, increasing with the addition of salt but decreasing as salt concentration becomes higher [40].

While thermal treatment can lead to protein denaturation, it is noteworthy that when optimized with other processes, it can result in a clear improvement in nut protein solubility. Additionally, the adoption of advanced technologies, such as high pressure, also has a positive impact on this property. Treatment with these new technologies reduces particle size, thereby contributing to increased protein solubility [30,41,48]. However, it is important to note that excessively high levels of pressure (150 MPa), in some cases, can lead to reduced solubility due to overprocessing, increasing interactions between proteins and water, resulting in protein denaturation after treatment [41].

Table 3. Water absorption capacity and solubility of different nut proteins.

Material	Condition	Water Absorption Capacity	Solubility	References
Cashew nut peel protein isolate	pH 3 and 8/pH 3 and 11	1.2 and 2.97 cm3 water/g protein	48.44% and 84.75%	[43]
	NaCl 0.5 M and 2 M	2.04 and 1.73 cm 3 water/g protein	75.37% and 66.72%
Hazelnut protein isolate	pH 3 and 4/pH 8	1.24 g and 2.88 g water/g protein	67.19%	[40]
	NaCl 0.2 M and 1 M	2.67 g and 2.49 g water/g protein	-
Pistachio protein isolate	Control	2.25 mL g−1	-	[49]
	Boiling	2.80 mL g−1	-
	Autoclave	2.70 mL g−1	-
Cashew nut protein isolate	Control	1.00 mL g−1	-
	Boiling	3.05 mL g−1	-
	Autoclave	4.20 mL g−1	-
Chestnut protein isolate	Control	1.05 mL g−1	-
	Boiling	2.60 mL g−1	-
	Autoclave	3.10 mL g−1	-
Almond protein isolate	pH 3–4/pH 7–9	-	80–90%/70–80%	[46]
Hazelnut floor	High pressure (0–100 MPa/150 MPa)	-	Improvement in solubility (~87%) in emulsions and foam/Decrease in solubility	[41]
Macadamia protein isolate	Control/Quitosan	-	86.49%/97.32%	[44]

Table 3. Water absorption capacity and solubility of different nut proteins.

Material	Condition	Water Absorption Capacity	Solubility	References
Cashew nut peel protein isolate	pH 3 and 8/pH 3 and 11	1.2 and 2.97 cm3 water/g protein	48.44% and 84.75%	[43]
	NaCl 0.5 M and 2 M	2.04 and 1.73 cm 3 water/g protein	75.37% and 66.72%
Hazelnut protein isolate	pH 3 and 4/pH 8	1.24 g and 2.88 g water/g protein	67.19%	[40]
	NaCl 0.2 M and 1 M	2.67 g and 2.49 g water/g protein	-
Pistachio protein isolate	Control	2.25 mL g−1	-	[49]
	Boiling	2.80 mL g−1	-
	Autoclave	2.70 mL g−1	-
Cashew nut protein isolate	Control	1.00 mL g−1	-
	Boiling	3.05 mL g−1	-
	Autoclave	4.20 mL g−1	-
Chestnut protein isolate	Control	1.05 mL g−1	-
	Boiling	2.60 mL g−1	-
	Autoclave	3.10 mL g−1	-
Almond protein isolate	pH 3–4/pH 7–9	-	80–90%/70–80%	[46]
Hazelnut floor	High pressure (0–100 MPa/150 MPa)	-	Improvement in solubility (~87%) in emulsions and foam/Decrease in solubility	[41]
Macadamia protein isolate	Control/Quitosan	-	86.49%/97.32%	[44]

3.2. Emulsification and Oil Absorption Capacity

Vegetable proteins have important emulsifying properties, but their effectiveness as surfactants is slower due to their large molecular size. To act as emulsifiers, proteins need partial denaturation at the interface, exposing hydrophobic amino acids. This restructuring enables proteins to position hydrophobic amino acids in the oily phase and hydrophilic ones in the aqueous phase. However, the emulsifying capacity of proteins, in terms of oil retention per gram of protein before phase inversion, is usually lower compared to low-molecular-weight emulsifiers. Once an emulsion is formed, proteins stabilize the emulsified oil droplets, creating a protective layer that prevents coalescence and emulsion separation [50].

Proteins from nuts, like hazelnut, macadamia, peanut, pistachio, cashew, and chestnut, are promising natural emulsifiers due to their amphiphilic properties, suitable for plant-based and vegan products. However, excessive surface hydrophobicity can sometimes reduce solubility, impacting emulsifying capacity. The balance between surface hydrophobicity and electrostatic interactions plays a crucial role in determining their food application properties [51].

The ability to stabilize oil–water interfaces and create resulting emulsions is a key characteristic of vegetable proteins due to their amphiphilic nature. The emulsifying properties of nut-derived vegetable proteins have gained increased attention in research due to their ecological sustainability and positive impact on health [52]. However, emulsions stabilized by nut proteins are typically unstable due to low water solubility and a tendency to aggregate [42,53]. To improve their effectiveness as emulsifiers, synergistic treatments or combinations with other compounds are used alongside proteins, as well as changes in pH and the addition of salt ions. Various technologies can assist in this process, such as ultrasound and microwaves, which enhance emulsification by increasing the content of peptides and phenolic compounds while improving overall process efficiency [54].

Another technique to enhance the emulsifying properties of nut proteins is Pickering emulsions, which use solid particles to stabilize oil–water interfaces. When nut proteins serve as the main emulsifiers, polysaccharides, like chitosan hydrochloride, cellulose nanocrystals, inulin, and xanthan gum, enhance emulsion stability. These polysaccharides create strong electrostatic repulsion and electrostatic deposition, resulting in a highly stable emulsion [44,55,56]. Nut-derived proteins, like those from macadamia and pecan, have shown excellent performance in emulsion stability due to their synergistic adsorption of solid particles at the oil–water interface, resulting in smaller droplets, which is crucial for Pickering emulsions [57].

Chitosan hydrochloride composites significantly improve emulsion stability compared to isolated protein alone. This improvement is linked to the concentration of the chitosan composite. Stable emulsions with a walnut flavor and a creamy texture can be created, which are stable in acidic conditions [51]. Research on improving the functional properties of almond flour protein through enzymatic hydrolysis found that enzymatic treatment increased protein solubility and reduced emulsifying capacity. The emulsion was further improved by altering pH, ionic strength, temperature, and pressure [40,43,49,58].

Nut proteins have gained attention for their potential as a soy substitute in various food products like baked goods and meat alternatives, thanks to their impressive functional properties [59]. Recent research has focused on factors influencing oil-holding capacity in nut proteins, such as variations in pH, ionic strength, temperature, and pressure [40,43,49,58].

Table 4. Oil absorption capacity and emulsification capacity of different nut proteins.

Material	Condition	Oil Absorption Capacity	Emulsification Capacity	Reference
Cashew nut peel protein isolate	pH 7/pH 3 e 10	4.28 cm3 oil/g protein	40% e 70%	[43]
	NaCl (0.5 M e 2 M)	-	72.0% e 56.0%
Hazelnut protein isolate	pH 4 e pH 8/pH 3 e pH 8	0.65 g e 1.91 g (oil/g)	26.15 e 85.21 m 2 g−1	[40]
	NaCl (0–1 M)/NaCl (0.2 M e 0.8 M)	-	54.28 e 41.27 m 2 g−1
Pistachio protein isolate	Control	3.10 mL g−1	25.2%	[49]
	Boiling	3.65 mL g−1	21.6%
	Autoclave	4.40 mL g−1	4.1%
Cashew nut protein isolate	Control	4.30 mL g−1	28.8%
	Boiling	5.10 mL g−1	12.1%
	Autoclave	5.40 mL g−1	10.5%
Chestnut protein isolate	Control	3.15 mL g−1	27.8%
	Boiling	3.20 mL g−1	3.1%
	Autoclave	3.30 mL g−1	4.1%
Almond protein isolate	-	4.85 mL oil/g de protein	-	[58]

presents various nut proteins under different conditions and how these conditions affect oil retention capacity and emulsifying ability. It is observed that the oil retention capacity of hazelnut protein isolate is improved under alkaline pH, but ionic concentration does not affect this property [40]. Thermal treatments result in better oil-holding capacity, making these flours more suitable for food applications. However, it reduces the emulsifying capacity, which is unsuitable for products like sauces, ice creams, and soups [44,55,57,60].

In summary, nut proteins have significant potential in various food applications, especially when combined with appropriate additives, improved processing techniques, and modified pH conditions. These proteins offer unique functional properties and can serve as effective emulsifiers and oil absorbers, enhancing the stability and quality of a wide range of food products.

3.3. Stabilizer

In general, proteins can act as emulsion and foam stabilizers. In emulsions, they reduce the interfacial tension between water and oil by surrounding fat droplets, delaying coalescence through electrostatic and steric repulsion and by increasing the viscosity of the continuous phase [61]. Emulsion stability relates to the contact between oil globules. Protein–oil–water emulsions’ texture, formation, and stability are influenced by chemical and physical factors, including processing conditions, solvent conditions (pH, temperature, and salts), protein characteristics (surface reactivity, solubility, concentration, size, and conformation), distribution, continuous phase viscosity, fat globule size, and aqueous/lipid phase volume ratio [61].

Proteins are amphipathic, exposing hydrophobic segments to the lipid interface and polar ionic groups to the aqueous phase. They can migrate, absorb, unfold, reorganize, and interact at the interface. Protein rearrangement depends on flexibility, conformational stability, and environmental conditions such as pH, temperature, and ion concentration [62]. Better-structured protein films provide resistance to shear, deformation, and compression, leading to improved emulsion stability.

In foams, proteins can also act as stabilizers by binding to water molecules and enclosing air bubbles. The aqueous phase is stabilized by repulsive forces in surfactant films. Air bubbles tend to appear due to capillary forces but are neutralized by electrostatic or steric repulsion from protein–water interaction, delaying their appearance. However, when they are close, van der Waals attractive forces can lead to bubble coalescence, possibly through desorption and coalescence or rupture. Foam stability, like emulsions, depends on membrane film properties, such as mechanical resistance, viscoelasticity, and diffusivity [63].

Nuts are traditionally known for their high fat content, but they also contain various macronutrients, including proteins, carbohydrates, and fats. Nuts have a relatively high protein content, making them potential alternative protein sources [6]. Edible nut seeds contain approximately 7–25% protein, with the quantity and type varying depending on the seed type, which often have proteins rich in amino acids such as glutamine, asparagine, and arginine. Despite the notable protein content in nuts, studies on their functional properties are limited. Generally, using whole seeds, even if of good quality, is not economically viable for producing protein isolates and concentrates, unlike using defatted seed flour [64].

Brazil has a wide variety of edible nuts used in the food industry as seasonings, condiments, for oil extraction, beverage production, and as raw materials. Most of them are rich in macronutrients, bioactive compounds [65], monounsaturated fatty acids, polyphenols, and other phytochemicals [66]. The most sold worldwide include almonds, pistachios, pine nuts, macadamia nuts, hazelnuts, European walnuts, Brazil nuts, pecans, and cashews [67].

Studies on emulsion and foam stability using nut proteins as surfactant compounds have been conducted. The emulsion and foam stability formed by isolated baru almond (Dipteryx alata Vog) protein (BPI) was analyzed [68]. BPI showed 53.9% emulsion stability, indicating that soluble proteins were capable of unfolding and reducing the interfacial tension at the oil–water interface. In its form, BPI maintained 96% of the initial volume formed after 60 min of beating at pH 7.0.

The same properties were analyzed by Mao and Hua [69] for isolated protein (WPI), concentrated protein (WPC), and defatted walnut flour (DFWF) from English walnuts (Juglans regia L.). Emulsion stability was higher for WPI (83.33%), followed by WPC (76.92%) and DFWF (66.78%) at pH 11. The difference in emulsion stability was attributed to differences in protein concentration and surface hydrophobicity. The highest stability occurs when there is more protein–protein interaction at the oil–water interface. The same order was observed for foam stability, with WPI at 30.56 ± 2.35% at pH 11. This result suggests that WPI has a more flexible structure in aqueous solutions and interacts more strongly at the air–water interface compared to WPC. In basic solutions, the increased protein charge weakens hydrophobic interaction, making the molecule more soluble and flexible, allowing more spreading at the air–water interface. Like in emulsions, foam stability is also influenced by protein concentration, increasing viscosity and facilitating the formation of a cohesive, multilayer protein film at the interface [70].

Ogunwolu et al. [29] also analyzed these properties for cashew nut isolated protein (CNPI) and concentrated protein (CNPC) (Anacardium occidentale L.). CNPC showed 13.7% and 153% emulsifying and stability capacity, respectively. In contrast, CNPI showed lower emulsion-forming capacity (12.5%) but higher stability (447%). This result is consistent with Kinsella, Damodaran, and German [63], who state that emulsion formation capacity is inversely proportional to protein concentration. At low concentrations, proteins spread on the surface before being adsorbed, controlled by diffusion. However, at high concentrations, the energy activation barrier does not allow proteins to diffuse. According to Tsai, Cassens, and Briskey [71], less concentrated proteins enable better chain unfolding during shear in the emulsification process. Also, these chains adhere through hydrophobic interaction to oil droplets, providing a larger free area on the surface, which may facilitate emulsion formation. Regarding foam-forming properties, CNPC showed 40% foam formation capacity and 40% stability, while CNPI showed 45% foam-forming capacity and 55% stability.

Eltayeb et al. [72] found greater foam stability (40 min) for peanut bambara protein isolate (IPAB) at pH 9.0, with around 84% of the foam volume remaining stable. Foam made from defatted peanut bambara flour (FDAB) had the highest stability (5 min) at pH 4.5. In line with this result, the highest emulsion stability was also at pH 4.5 for FDAB, which retained 50% of the aqueous phase volume after 48 h. The lowest emulsion stability was at pH 3.0, with 70% of this volume separated. The highest and lowest emulsion stability for IPAB was obtained at pH 4.5 and 6.0, respectively, with 55% and 95% of the aqueous phase volume separated after 48 h, respectively.

Sharma et al. [73] studied the functional properties of proteins derived from almonds, Brazil nuts, cashews, hazelnuts, macadamia nuts, pine nuts, pistachios, Spanish peanuts, Virginia peanuts, and soybeans for subsequent comparison. The foam stability (<1 h) formed by all protein isolates prepared from these raw materials ranged from “poor” to “reasonable”. The protein content of the isolates varied from 69.23% (pine nuts) to 92.72% (almonds), except for soy (94.80%). Hazelnut protein isolate showed the lowest formation and stability of foam compared to the others. The same authors also reported that the ability to form and stabilize foams of the protein isolates studied is not related to protein concentration since almonds, cashews, and soy had similar protein levels (92.72%, 92.29%, and 94.80%, respectively), yet almond and cashew protein isolates had significantly lower foam-forming and stabilizing capacities compared to soy protein isolate.

Vioque and colleagues [74] obtained a protein isolate (97.82% protein content) extracted from rapeseed (Brassica campestris L.) and analyzed various techno-functional properties, including emulsion and foam stability. The highest emulsion stability (60−70%) was achieved with a 3.1% degree of hydrolysis, while the non-hydrolyzed rapeseed protein isolate showed very low stability (<10%). The authors report that hydrolysis exposes hydrophobic groups in amino acid residues, which can interact with oil, while the hydrophilic fraction interacts with water. Similarly, the highest foam stability (69%) was also achieved when the rapeseed protein isolate underwent 3.1% hydrolysis. According to the authors, highly hydrolyzed protein isolates can create foam, but they lack the strength to maintain it for an extended period due to the reduced peptide size.

Teixeira et al. [65] found greater emulsion stability (50–60%) from defatted sapucaia nut flour (Lecythis pisonis Cambess.) when the oil was extracted with petroleum ether using the Soxhlet method, resulting in lower relative crystallinity (32.80%). According to some authors, protein content, its interaction with polysaccharides, remaining oil content in the flour, and certain lipids like monoacylglycerols and diacylglycerols, which can act as surfactants [75], may affect the emulsifying properties of sapucaia nut defatted flour. Similarly, greater foam stability (80–90%) was also achieved using this oil extraction method.

Maciel et al. [76] found low values for emulsifying and foaming properties in a pecan nut cake (PNC) [Carya illinoinensis (Wangenh.) K. Koch] with 21.87% protein content. According to the authors, PNC exhibits low protein–protein interaction, leading to the formation of very fine, low-viscosity foam with dispersed and unstable molecules. In the emulsion, the authors believe that the high fiber content (13.01%) may have influenced the low values for these properties.

Table 5. Emulsion and foam stability of nut proteins.

Material	Protein Content (%)	Emulsion Stability (%)	Foam Stability (%)	References
Baru nut protein isolate	88.4	53.9 ± 4.2	96	[68]
Walnut protein isolate	90.50	30.30 ± 0.25	30.56 ± 2.35	[69]
Walnut protein concentrate	75.56	27.45 ± 0.34	28.18 ± 1.04
Walnut defatted flour	52.51	25.26 ± 0.27	10.23 ± 2.15
Cashew nut defatted flour	-	128 ± 0.03	8.00 ± 2.00	[29]
Cashew nut protein isolate	-	447 ± 2.00	55.0 ± 2.00
Cashew nut protein concentrate	-	153 ± 0.03	40.0 ± 2.00
Bambara defatted flour	17.70 ± 0.44	50	50	[72]
Bambara protein isolate	85.97 ± 1.41	45	84
Sapucaia nut defatted flour	49.28 ± 0.54	50–60	80–90	[65]
Macadamia defatted flour	33.12–36.45	53.52–54.26	56.27–75.07	[77]
Tigernut (Cyperus esculentus) flour	7.15–9.70	-	50.60–58.99	[78]

provides a compilation of the values found for emulsion and foam stabilities using nut proteins.

3.4. Viscosity

Viscosity measures a fluid’s resistance to flow when force is applied. In ideal solutions, shear stress is directly proportional to the applied shear rate; these are known as Newtonian fluids. However, protein solutions often exhibit pseudoplastic or shear-thinning behavior, where viscosity decreases as shear rate increases. This behavior arises from proteins aligning with the flow [70,79]. Complex protein interactions, including shape, size, protein–solvent interactions, hydrodynamic volume, and molecular flexibility in the hydrated state, determine viscosity. When dissolved in water, proteins absorb water and expand, increasing their volume compared to dehydrated proteins [70,79].

Gocer and Koptagel [80] analyzed the rheological properties of kefir made with almond (KA), peanut (KAM), hazelnut (KAV), walnut (KN), and cashew (KC) proteins. KAM exhibited the highest apparent viscosity on the first day of storage (103.03 mPa·s), while KN had the lowest (4.50 mPa·s). KA had a higher consistency coefficient (1036.06 mPa·s·n), an essential parameter for measuring the average viscosity of non-Newtonian fluids. KN also had the lowest value in this regard (741.52 mPa·s·n). KAM and KN showed the highest and lowest thixotropy, with values of 814.06 Pa·s⁻¹ and 35.47 Pa·s⁻¹, respectively. The pseudoplastic viscosity behavior of kefir made with nuts is due to the weakening of protein interactions caused by weak electrostatic and hydrophobic bonds [81]. The viscosity of nut-based kefir may decrease with increased storage time due to the degradation of protein structure by bacterial proteolysis [82].

Gul and colleagues [40] examined the rheological properties of isolated hazelnut protein at different pH values and ionic strengths. The flow behavior of 4% hazelnut protein suspensions was shear-thinning due to protein–protein interactions during high shear forces. For pH changes, the highest shear stress at higher shear rates was found at pH 3.0, 4.0, and 7.0. Ionic strength did not affect shear-thinning behavior, with the highest shear stress in suspensions containing 0.6 M NaCl. The consistency coefficient was influenced by pH variations, with the highest at pH 3.0 (0.142 Pa·s·n) and the lowest at pH 6.0 (0.003 Pa·s·n). At pH 6.0, the flow behavior was like Newtonian fluids, but pH changes made it pseudoplastic. The flow behavior index changed with increasing ionic strength, with the lowest flow behavior index at the 0.6 M concentration, attributed to higher protein surface hydrophobicity.

Similarly, Saricaoglu et al. [41] observed shear-thinning behavior in isolated hazelnut protein. High-pressure homogenization (HAP) degraded protein molecules and the network structure through covalent and hydrogen bond hydrolysis, hydrophobic, and electrostatic interactions, increasing flow properties. Apparent viscosity at a 50 s⁻¹ shear rate was affected by HAP, with the highest (1.301 Pa⁻¹) in the control sample and the lowest (0.049 Pa⁻¹) at 150 MPa. Tatar, Tunç, and Kahyaoglu [83] analyzed the rheological properties of Tumbul hazelnut protein extracted from defatted nut flour in Turkey. Shear stress was higher at low shear rates, gradually decreasing with increasing rates, tending toward Newtonian behavior. At 5 °C, shear stress ranged from 0.14 to 0.79 Pa, and at 20 °C, it ranged from 0.65 to 1.57 Pa for Tumbul hazelnut protein. Apparent viscosity was directly proportional to protein concentration but decreased with temperature increase. This was unlike the 80 °C temperature, which led to a rapid increase in viscosity due to protein denaturation. Shi et al. [84] studied the effects of partial trypsin hydrolysis, transglutaminase (TGase) crosslinking, and their combination on the rheological properties of Juglans regia L. walnut protein isolates. Viscosity decreased gradually with an increased shear rate in all samples. TGase treatment alone increased viscosity, but there was no difference when combined with trypsin hydrolysis. However, viscosity decreased when TGase treatment followed trypsin hydrolysis due to TGase crosslink degradation. The sequence and conditions of processing can thus alter the final product’s viscosity.

Lei et al. [85] investigated the effect of adding NaCl on the rheological properties of walnut protein (PIN) bound to κ-carrageenan gum (KC). The highest apparent viscosity was observed when 15 mM Na⁺ was added, enhancing hydrophobic and electrostatic interactions between PIN and KC, forming higher molecular weight aggregates responsible for increased apparent viscosity. Sharma et al. [73] also studied the rheological properties of proteins derived from almonds, Brazil nuts, cashews, hazelnuts, macadamias, pine nuts, pistachios, Spanish peanuts, Virginia peanuts, and soybeans for later comparison in their study. The apparent viscosity of protein solutions was directly proportional to protein concentration, with significant differences observed only in protein solutions of concentrations of 7% and 10%. Cashew protein exhibited the highest apparent viscosity among the samples.

3.5. Gel Formation

Emulsion gels encompass solid, viscous, and colloidal materials that exhibit behavior related to emulsions or gels. Defined in various ways, methods can be described with gels filled with emulsion, where oil droplets are added to a crosslinked biopolymer system, or where the droplets form their own gel system, called particulate emulsion gels [86]. In the food industry, the production of emulsion gels and the development of various biopolymers are advantageous for enhancing sensory properties, texture, and reducing fat content in different foods [87]. Emulsion droplets are also used to fortify foods as they can be enriched with microcapsules containing bioactive compounds [88].

Classified as three-dimensional matrices in food and pharmaceutical products, protein gels earn this title due to their textural characteristics and nutritional value [89]. Thermal treatments are the main methods used in protein gels through covalent bonds, eliminating the need for chemical crosslinkers. They are typically divided into two stages: partially denaturing protein molecules, leading to the breakdown of the native structure exposing buried residues, followed by denatured proteins linked together through hydrophobicity, hydrogen bonds, or sulfide structures to form a three-dimensional network [90,91].

The selection of the method is directly linked to the matrix and the preparation of emulsion gels. Polysaccharides or proteins act as emulsifying agents. For emulsion gel preparation, a polysaccharide is used, followed by mechanical or ultrasonic homogenization with magnesium and calcium to induce the gelation process; as well as neutral gels, agar and insulin are applied as polysaccharides [92,93]. The search for more sustainable foods is a current trend. Food emulsions with plant-based proteins are an interesting alternative for gel production. Legumes such as beans, chickpeas, soy, lentils, and peas are used for gel formation. Two phases are required to formulate the gel: the initial phase involves blending oil and the emulsion gel-forming mixture, and the final phase involves adding the emulsion through different gelation methods [94,95,96].

Considered one of the most sensitive methods to ensure texture in various food products, the gelation technique is also used in the encapsulation of bioactive compounds and the formulation of new products within the food industry. Properties such as water, oil, and air absorption make the food gel a three-dimensional polymer network [97,98]. In rheology, gelation is a method that involves storing the product through a viscoelastic system where the storage capacity is greater than the loss capacity [99]. Thus, some functional properties also undergo changes depending on the processing of protein ingredients present in the formulation, as well as their gel formation and water retention [98].

3.6. Nut Proteins as Nutritional and Functional Alternatives

Considering that the current consumer profile is undergoing transformations, the food industry is seeking viable alternatives, especially with plant proteins, to provide products for all audiences [100]. Groups such as vegans and vegetarians, who consume little or no animal-origin proteins in their diet, seek to include alternatives such as beans, chickpeas, lentils, peas, grains like quinoa, wheat, and barley, as well as nuts and seeds like almonds, pumpkin seeds, and chia. With high nutritional and functional value, plant proteins are amino acid polymers absorbed at interfaces [101,102,103]. Seen as a good alternative due to their biological and nutritional value, plant proteins are options for consumers with lactose and casein intolerance. They are widely used to ensure quality and texture in baking products due to their ability to form protein networks in doughs [104].

The “plant-based” market offers healthier options by having reduced allergenicity and being more sustainable compared to animal-origin proteins [100,103]. The nutritional value of plant proteins varies significantly based on the availability of essential amino acids, protein digestibility, and physicochemical properties. Their functionality is directly linked to their purpose, the methods used in processing, and the chosen extraction method. In addition, they provide health benefits to consumers, such as lower consumption of the saturated fat and cholesterol present in animal-origin proteins [100,105].

Rich in essential fatty acids, nuts present a relevant nutritional attribute for human health, with high levels of vitamins, minerals, antioxidants, and amino acids. The proper consumption of nuts in a balanced diet is associated with numerous health benefits, such as non-communicable chronic diseases (diabetes, cancer, respiratory diseases, and cardiovascular diseases) and infections [106]. According to Cui et al. [107], when evaluating the ultrasonic effects of proteins from Cyperus Esculents seeds (tiger nut) with the aim of exploring ultrasonic potency with Scanning Electron Microscopy. The results obtained showed some polar and hydrophobic groups in the proteins when exposed to after treatment, thus increasing the binding area between the protein molecules and water, improving solubility and hydrophobicity on the protein surface, enhancing its ability to form gels [107].

3.7. Texture

Texture, comprising rheological and structural properties, as well as the geometric and surface features of foods acknowledged by mechanical, visual, and auditory receptors, is the most crucial characteristic for the palate. Texture emerges when food undergoes mechanical changes such as cuts, bites, or compression. It is under these conditions that attributes like crispiness, resistance, fibrousness, roughness, cohesiveness, and granularity can be evaluated. Texture properties are divided into three classes: geometric, mechanical, and consistency [108].

Defined as the primary sensation for food, texture can be masked by taste and flavor. Naturally, individuals exhibit unique textural food selectivity and preferences for tastes and flavors, influencing not only consumption but also aversion to certain foods [109,110]. Textural testing instruments are considered tools that detect and quantify physical and sensory perception parameters, including chewiness, food structure derivation, and the detection of senses such as touch and pressure [111].

As a sensory property linked to nutritional content, color, and flavor, texture directly affects the acceptability of foods. Related to food content and its macro- and microscopic formation, texture is fundamental for food safety and quality. Thermal treatments and processing methods impact texture properties. Combinations with high-pressure and different temperature treatments are used to achieve positive effects on texture, color, and flavor in a specific food. Moreover, foods exposed to pressure, when compared to those processed with heat, show improvements in textural and color properties [111].

With the excessive growth of the population, the substitution of animal protein by plant-based alternatives has become increasingly necessary and gained momentum in the current consumer landscape. Negative environmental impacts, such as the release of high levels of carbon dioxide from animal protein production, and the association of excessive red meat consumption with the development of numerous human health issues, are significant factors driving this substitution [112].

To assess the cutting test directly linked to the texture of a food, Lima et al. [106] compared nutrition bars made from Brazil nuts and Baru almonds. They found that the Baru bar was considered harder than the Brazil nut bar. However, when evaluating the force, the results were inversely proportional, with the Baru bars showing lower values compared to the Brazil nut bar. This higher firmness value is linked to the protein content, a determining factor for a stronger gel network that makes the product firmer. Baru almonds have a higher protein content, with Baru almonds at 24.27 g and Brazil nuts at 17.92 g. Higher firmness occurs as higher protein content leads to more interactions between molecules and the formation of aggregates [113].

4. Toxicity

Aflatoxin is a secondary metabolite produced by fungi, with species such as Aspergillus flavus and A. parasiticus described as the most aggressive toxin producers. However, it is known that other representatives of the genus, such as A. nomius, also produce aflatoxin (toxigenic fungi). Aflatoxins, produced by Aspergillus flavus and Aspergillus parasiticus, are carcinogenic toxins found in nuts, peanuts, pistachios, and dried figs.

These harmful substances are notable for their resilience, allowing them to develop in various substrates even under adverse storage conditions. Aflatoxin B1, one of the aflatoxins, is recognized as the most toxic and carcinogenic. Due to this danger, the concentration of aflatoxins in nuts is strictly regulated in various countries around the world, from their separation and storage to their distribution [35].

These secondary metabolites are produced by fungi either in the field or during storage (depending on environmental conditions or the substrate), posing health risks to humans and animals. As a predominantly tropical climate country, Brazil has all the conditions that lead to food contamination by aflatoxins, which has been frequently observed at high levels, especially in peanuts and other nuts and seeds [114].

5. Application of Nut Proteins in Foods

The incorporation of ingredients in food products plays a crucial role in the industry, spanning various sectors such as dairy, vegetable oils, cakes, and protein bars. Despite the past prevalence of animal-derived proteins, plant-based proteins now play a significant role in the global diet. They not only provide essential nutrients but also meet the demands of the food industry and specific consumers, such as vegans and vegetarians.

5.1. Extruded

Ačkar and colleagues [115] emphasize the fundamental role of extrusion in the industrial processing of solid raw materials, combining various operations into a single device called an extruder. In the food industry, this method offers significant advantages both operationally and technologically, leading to its growing adoption. Extrusion technology is intensely researched in terms of texturization properties, nutritional characteristics, and the sustainability of extruded products. However, most research focuses on the parameters of intermediate products, assuming their consumption in pure form. While some studies address nutritional properties and sustainability, they often overlook texture and processing aspects.

Cheng et al. [116] found that the extrusion technique is widely employed in the manufacturing of various products, including breakfast cereals, snacks, and assorted savory treats, using cornmeal, rice, wheat flour, or potatoes, resulting in different shapes and textures. Although a common practice, the application of extrusion in legume flour is an emerging research area, except in the case of soy. However, many corn-based snacks available in the current market contain high levels of sodium and saturated fat, which can be concerning for health. Nevertheless, starch is often used in extruded products due to its expansion ability. However, when used as the main ingredient, it results in products with reduced nutritional value.

Extrusion has emerged as a promising strategy in the quest to develop walnut protein-based products that successfully mimic the texture of animal meat. This manufacturing process allows for the restructuring of walnut proteins, resulting in final products that not only offer significant nutritional benefits but also resemble conventional meat in terms of texture, chewiness, and juiciness [117].

5.2. Plant Beverages

Nutritional beverages derived from plant extracts of oilseeds, cereals, and legumes are a viable alternative due to low production costs and diversity, including options like soy, rice, coconut, and almond “milk,” among others. In recent years, individuals with specific dietary restrictions have become more prevalent, contributing to innovation in the food industry. Commonly responsible for most food allergies are milk, fish, shellfish, soy, eggs, wheat, peanuts, and nuts. Among the various reactions to consuming these foods, intolerance requires excluding the product from the diet, necessitating the use of a “substitute” from a nutritionally sound source, providing the necessary amounts for everyone with possible supplementation [118].

As a result, the food industry encompasses various functions, leading to the development of substitutes to promote the acceptability of alternative food sources to animal protein, such as beverages, cakes, and oils, among others. Plant-based drinks, made from plant proteins and the like, are widely discussed. One popular plant-based beverage is almond milk, which has a high protein content and the advantage, in relation to cow’s milk, of staying fresh for a longer period. Another relevant beverage is coconut milk, obtained by filtering ground coconut pulp, considered an excellent calorie source due to its high content of fatty acids that strengthen the immune system to control viruses and bacteria [119].

As a value-added product, plant-based beverages and their derivatives are part of a limited market space, with high costs and often low availability. The industry is refining techniques to attract more consumers and meet the demand of those with allergies or intolerance to animal milk and even vegans. The market is promising because it offers a healthy consumption alternative with functional properties [119].

Table 6. Examples of nut protein in plant beverages.

Product	Ingredient	Reference
Baru almond-based symbiotic beverage	Water, baru almonds, probiotics, and sugar	[120]
Brazil nut and Baru almond beverage	Water, Brazil nuts, baru almonds, crystal sugar, CMC, citric acid, potassium sorbate, and sodium benzoate	[121]
Fermented drink made from cashew nuts	Water, cashew nuts, sugar, and probiotics	[122]
Sapucaia almond-based beverage	Water and sapucaia almond cake	[123]
Prebiotic drink based on cashew nuts and pineapple juice	Water, cashew nuts, concentrated pineapple juice, sugar, and prebiotic	[124]
Prebiotic drink based on cashew nuts and mango juice	Water, cashew nuts, mango juice, sugar, and prebiotics	[125]
Cashew nut and carob beverage	Water, cashew nuts, sugar, k-caragenase, cocoa powder, and carob powder	[126]

exemplifies some plant-based protein beverages.

5.3. Emulsions

While there has been considerable progress in researching the technological properties and applications of plant proteins, there remains a gap in the scientific literature regarding the exploration of proteins from oilseeds, such as nuts, for use in food formulations [127]. In this context, we highlight some of the applications, primarily focusing on the ability of these proteins to act as efficient stabilizers in emulsions [56].

In Pickering emulsions created with peanut protein isolate (PPI) nanoparticles through thermal aggregation and Na+-induced induction, an increase in nanoparticle concentration resulted in slightly smaller emulsion droplets and enhanced stability against coalescence and creaming [128]. Additionally, interactions between cellulose nanocrystals and peanut protein play a crucial role in regulating the interfacial properties of Pickering emulsions, resulting in long-term stability and attractive rheological properties for various applications in food formulations [129].

The addition of 4 to 8% peanut protein in a fish oil (FO) emulsion and incorporation into surimi gel showed significant improvements in hardness, adhesiveness, and chewiness compared to the control group that contained FO directly in the gel. Microstructural analysis revealed that the emulsion with peanut protein uniformly filled the empty spaces in the gel matrix, forming a firmer network structure, presenting a sustainable alternative to replace fat in food products [130].

Another interesting application of nut proteins is their ability to form emulsions to carry hydrophobic compounds, effectively incorporating substances like antioxidants, fat-soluble vitamins, and other nutrients in food and pharmaceutical systems. This technique can lead to the increased bioavailability and bioaccessibility of these substances, providing effective protection for oxidation-sensitive compounds, improving their solubility, and consequently enhancing their absorption in the body [60].

5.4. Other Applications

An underexplored application involves the reuse of protein-rich bio-waste derived from major walnut oil processing industries. Such waste exhibits high protein, carbohydrate, and fiber content, offering the potential to produce bioactive peptides, protein isolates, and ingredients for protein-rich preparations, among other uses [131,132]. Functional beverages containing 2% and 4% hazelnut meal (HMP) showed higher sensory acceptance compared to a commercial reference beverage. With a protein content of approximately 80% and significant concentrations of essential and branched-chain amino acids, the results suggest that hazelnut meal obtained from industrial waste could emerge as a promising protein source for functional foods, providing a sustainable and economically viable alternative to traditional protein sources [58].

Proteins from nuts, such as peanuts, almonds, and other varieties, have emerged as alternative sources of plant-based protein, offering a range of nutritional and sensory benefits. The application of peanut protein isolate (PPI) was tested in the preparation of pork meatballs, which showed greater acceptance in terms of color, texture, and overall acceptability compared to control meatballs. This indicates that PPI has potential in pork meatball production [133].

6. Protein Digestibility

The nutritional value of nuts is inherently linked to their protein-rich composition, encompassing various types such as albumin, globulin, prolamin, glutenin, and scleroprotein. These proteins stand out for their high digestibility and solubility in different solvents, giving nuts superior nutritional quality compared to other sources of plant protein [134,135]. It is worth noting that the protein content and composition may vary depending on the species, variety, and processing method of the nuts (Table 2).

The digestibility, bioavailability, and bioaccessibility of proteins from nuts have garnered considerable attention in scientific research due to their potential impact on human nutrition. Digestibility refers to the body’s ability to break down and absorb proteins from food, while bioavailability focuses on the actual availability of proteins for use by the body. Bioaccessibility, in turn, is related to the ability of proteins to be released from their food matrices and be readily available for absorption.

Protein digestion is a complex process unfolding in various stages, starting with chewing, involving enzyme action, pH variations, and usually culminating in the breakdown of proteins into smaller fragments called peptides. Throughout the process, especially in the intestine, the gut microbiota plays a crucial role, aiding in the further degradation of peptides and the final absorption of amino acids. Peptides formed during digestion have been the subject of studies and research regarding their potential therapeutic and nutritional benefits. They are generally termed bioactive peptides when they exhibit beneficial health effects, such as regulating physiological processes and promoting antioxidant, anti-hypertensive, and anti-inflammatory functions, among others. The digestion of defatted cashew nut flour resulted in a peptide-rich soluble fraction that demonstrated an increased antioxidant capacity during the digestive process. Additionally, this protein fraction protected DNA from oxidative damage and exhibited potential prebiotic effects for Bifidobacterium lactis [136].

Although nut proteins are often associated with allergic reactions, it is relevant to highlight that raw nuts have a substantial crude protein content. For example, peanut protein content can reach around 30%, and there are up to 18 types of amino acids, including all eight essential amino acids (isoleucine, leucine, lysine, methionine, phenylalanine, threonine, tryptophan, and valine), except for asparagine and glutamine [135]. Thirteen free amino acids were identified in a nut mixture, including cashews and pistachios, of which six are considered essential amino acids, six are non-essential amino acids, and one is classified as a semi-essential amino acid. The total amino acid content in the nut samples varied in the following order: pistachio (1469.06 ± 43.56 µg·g⁻¹), nut mix (1002.46 ± 11.34 µg·g⁻¹), walnut (848.06 ± 3.66 µg·g⁻¹), and cashew nut (790.23 ± 2.60 µg·g⁻¹). Compared to individual nuts, the bioaccessibility of amino acids in the mix group significantly increased after in vitro digestion, particularly for Gly (2.43–7.04 times), Lys (2.02–4.30 times), Arg (1.38–1.90 times), and Leu (1.45–1.75 times). In the same study, a significant increase was observed in the concentrations of Ala, Thr, Lys, and Leu in the small intestine of rats in response to the mixed diet. The authors highlighted that these concentrations of amino acids in mouse tissues were correlated with an increase in simulated protein digestibility. Due to the higher protein digestibility in the mix group, there was also an improvement in the absorption of smaller molecules, such as amino acids and fatty acids, in the tissues [137]. Overall, studies have demonstrated that the digestion of proteins found in nuts results in an increased antioxidant capacity due to the rise in bioactive amino acid content. This suggests that consuming nuts may contribute not only to high-quality proteins but also to potential antioxidant benefits [136,138].

7. Conclusions

In conclusion, this review underscores the potential of nut proteins as valuable contributors to the evolving landscape of plant-based and sustainable food options. The nutritional richness, functional versatility, and technological applications discussed in this review position nut proteins as key players in addressing the demand for alternative protein sources. The emphasis on amino acid complementation and the exploration of various nuts offer a comprehensive approach to harnessing the full nutritional potential of these proteins. As the food industry continues to innovate, incorporating nut proteins not only aligns with current dietary trends but also promotes environmentally conscious choices. Future research and development in this field hold promising prospects for creating healthier, more sustainable, and palatable food products, catering to the diverse needs of consumers worldwide.