**1. Introduction**

The global dynamics of the production and consumption of meat and meat products has evolved rapidly as the result of changing lifestyles and nutritional ideologies among part of the population. As a result, it is important to address several different aspects regarding the quality of meat and meat products, particularly nutritional (as relates to health), safety and sustainability aspects.

Regarding the nutritional aspects of meat and meat products, different strategies have been explored to optimize the composition of these products to make them healthier and to bring them in line with health recommendations and nutritional guidelines promoted by public bodies in response to new consumer demands.

Of the different strategies to improve meat and meat product composition, lipid content optimization has attracted the most attention owing to health recommendations [1]. This typically entails the partial or nearly complete substitution of fat with other healthier lipids by means of different technological procedures [1]. However, the most appropriate procedure in each case depends on the type of product, lipid material used, and the nature and magnitude of the proposed change. In the last few years, there has been

**Citation:** Ruiz-Capillas, C.; Herrero,A.M. Development of Meat Products with Healthier Lipid Content: Vibrational Spectroscopy. *Foods* **2021**,*10*, 341. https://doi.org/10.3390/ foods10020341

Academic Editor: Thierry Astruc

Received: 19 December 2020 Accepted: 2 February 2021 Published: 5 February 2021

**Publisher's Note:** MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations.

**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

growing attention in developing lipid materials to reduce or even completely replace fat in order to develop health-enhanced meat products. However, in most cases it is difficult to completely replace fat in food products. The physical and thermal properties of these products should be similar to those of animal fat but with fewer calories and an improved lipid profile [2,3]. A recent novel development is the formation and use of structured lipids such as emulsion gels (EGs) or oil-bulking agents (OBA) [4,5] that provide remarkable applications in the reformulation of health-enhanced meat products [2,6–10]. Although in some isolated cases, oil-structuring procedures applied to the reformulation of meat products as animal fat replacers could affect some of the properties of the final product, they have been successful in creating food that is viable in terms of technology, microbiology, sensorial properties and safety [2,3,6,7,9], with a shelf life similar to that of its standard meat counterpart. These animal fat replacers can improve the technological properties of meat products (water and fat content), and can also be employed as carriers of different nutrients (fiber, minerals, phenolic compounds, etc.), some with biological activity offering health benefits [6,7,9,11,12].

Successful development of structured lipid materials with healthier characteristics requires in-depth study of their nutritional, sensorial and technological characteristics, and a broader understanding of the complex relationship between the structure of their components and their properties. A new approach to the understanding of these interrelationships based on an analysis of the conformational modifications taking place within the components of lipid material and reformulated new meat products that affect their properties, merits consideration. In these connections, vibrational spectroscopic techniques such as Raman and infrared spectroscopy are direct and non-invasive techniques which offer an extensive range of possibilities in meat and meat products [6,7,13] and show grea<sup>t</sup> potential in supplementing structural information relating to fat-replacer production processes and the reformulation of health-enhanced meat products into which they are ultimately incorporated [4–7]. These techniques are also useful for quality control as it has been shown that their structure correlates with traditional ways of assessing meat quality (water retention capacity, texture, etc.) [13]

Based on the above, this review features a brief description of the novel strategies used to develop meat products with a healthier lipid content based mainly on the use of structured lipids (emulsion gels and oil bulking agents). It also proposes the employment of vibrational spectroscopy (infrared and Raman spectroscopy) to enhance the development of both structured lipids and the meat products which are included as animal fat substitutes. Moreover, the basic concepts of infrared and Raman spectroscopy are reviewed to aid in the understanding of how these spectroscopic techniques are applied.

#### **2. Meat and Meat Products with a Healthier Lipid Content**

Meat and meat products form part of the diet of many consumers in the world and are a significant source of an extensive range of nutrients essential for healthy development. These foods provide many nutrients such as high-value protein, fatty acids such as conjugated linoleic acid, minerals (iron, zinc and selenium) and B-complex vitamins. However, meat products also contain processing additives (sodium, nitrites, high fat content, etc.) with negative health implications. This has sparked the development of meat products more in line with health recommendations [14] using different procedures to enhance the composition of meat and especially meat products [1–3,15]. These procedures are mostly based on animal production (genetics and nutrition) and/or technological procedures focused on reducing or minimizing compounds with negative health implications and increasing compounds with positive health implications [1–3]. From among the technological strategies which aim to improve the global composition of meat products, special mention should be made to meat product reformulation processes which aim to remove, reduce, increase, add and/or replace certain components to develop healthier meat products [1–3,15].

From among the different reformulation processes, improved lipid content (in qualitative (lipid profile) and quantitative terms) is one of the most relevant processes in the development of healthier meat products [1–3,15] because there is an increasing indication of the connection between dietary fat intake and chronic disorders such as ischemic heart disease, some forms of cancer, obesity, and other ailments [14,16–18]. More specifically, some studies have shown a correlation between the intake of saturated fatty acids (SFAs) and an increase in total cholesterol levels in the blood. An increase in High-density lipoprotein (HDL) (ratio HDL/total cholesterol) is generally considered to be positive, whereas an increase in Low-density lipoprotein (LDL) is detrimental to health.

The consumption of SFA has been related to the development of cardiovascular disease, obesity, hypertension and some types of cancer [19]. The consumption of monounsaturated fatty acids (MUFAs), among which oleic acid stands out due to its high prevalence, has also been related to a reduction in LDL cholesterol and triglycerides in the blood. However, the role played by SFAs and MUFAs in human health is a controversial topic. Other studies question the health effects of SFAs and certain unsaturated fatty acids [20,21].

Despite this controversy, general consumption recommendations of total and unsaturated fatty acids can be made [14,22]. Briefly, dietary fat intake should preferably account for between 20–35% of total daily calories consumed [14,22]. According to dietary recommendations for the intake of specific fatty acids as a proportion of total diet, no more than 10% of calorie intake should be from SFAs, 6–10% from poly-unsaturated fatty acids (PUFAs) (n-6 and n-3, 6–11%), between 16–19% from MUFAs, and less than 1% from total fatty acids (TFAs) [14,22].

To meet these recommendations and improve the lipid content of meat products, technological strategies generally replace animal fat with different lipids, mainly from plant and marine sources, more in line with health recommendations (e.g., lower proportion of saturated fatty acids (SFA), higher in monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA), especially from the omega-3 or n-3 family of fats). Efforts are also being made to improve PUFA content, the n-6/n-3 and PUFA/SFA ratios and, where possible, to reduce cholesterol and trans-fatty acids) [1]. This will predictably reduce the risk of developing the diseases discussed above.

Different procedures have been used to substitute animal fat with healthier lipids, ranging from the most conventional (direct addition, encapsulated, emulsified form, etc.) to the most novel recently developed techniques such as lipid structuring with EGs or the oilbulking agents referred to above [1–3]. To that end, different vegetable oils (olive, soybean, etc.), marine oils (fish and algae), or mixtures of these have been employed to partially or completely substitute animal fat in various types of meat derivates (fresh, cooked, and dry cured). However, studies have shown that meat products reformulated in this way have different physicochemical characteristics which could impact negatively on the preferred quality parameters of the reformulated product [1–3]. Nevertheless, novel strategies such as lipid restructuring can be employed to enhance the quality of the reformulated meat products since these lipid materials and generate a solid fat which maintains solid-like properties. This could be a better way to develop meat products with a healthy lipid profile without negatively impacting their characteristics (Figure 1).

**Figure 1.** Example of healthy meat products with enhanced lipid content based on used structured lipids (as an animal fat replacer: (**a**) emulsion gels) and (**b**) oil bulking agent.

#### *Lipid Materials Based on Structured Lipid: Emulsion Gels and Oil Bulking Agents*

Lipid materials created using lipid structuring procedures have obtained a grea<sup>t</sup> deal of attention in the context of meat products. These lipid constituents can be employed as fat substitutes in meat products thanks to their solid-like properties simulating animal fat [1–3]. Their characteristics are of grea<sup>t</sup> interest for their application in meat products from a nutritional (healthy composition) and technological point of view. Of the various types of structured lipids EGs and OBAs are the most interesting due to their singular properties and possibilities as animal fat substitutes in meat products [1–3].

EGs are lipid materials in which emulsions and gels (hydrogels) concur. Formulation of these EGs fundamentally includes a lipid phase (olive, microalgal and chia oils, among others), an aqueous phase, an emulsifier (soy and whey protein, among others) and a gelling process [1–3]. Different compounds can be used in their formulation to give them particular technological characteristics, comprising specific bioactive compounds that provide nutritional benefits [2]. There are various procedures that can be used to create the gelling process such as heating, acidification, addition of divalent cations (Ca2+), and enzymes with hydrocolloids [15,23]. Particular attention is being paid to cold gelling strategies employing binding agents such as alginates which form cold-set EGs [2,4].

Other interesting structured lipids are OBAs based on the dispersion of oil droplets in a continuous aqueous matrix-forming gel [2]. In these OBAs, liquid oil such as olive oil is usually enclosed in a hydrogel network structure. The formation of these OBAs includes mostly an early phase where the oil is distributed in the aqueous phase and lastly the gelation of the aqueous phase is induced by a gelling agen<sup>t</sup> such as alginate. This procedure provides the OBAs with a solid structure, permitting it to be incorporate, for example, as an optimal fat replacer in meat products. Different gelling agents such as hydrocolloids (alginate) have been employed independently or in a mixture, causing diverse textures and structures in the final OBA [15].

Many studies have been conducted in recent years concentrating on the creation and characterization of EGs and their application in food products (yoghurt, cheese, sauces), particularly in the elaboration of cooked and fresh meat products (frankfurter type cooked sausages, fresh sausages, hamburgers, etc.) with a healthier lipid content [2,15,24,25]. However, OBAs have not been extensively employed in food science but their application as animal fat substitutes in healthier meat products is especially remarkable. These works have been focused mainly in the composition and technological characterization of EGS and OBAs. However, a further study of their structure and of the interactions between their diverse components, and their technological characterization, is required to gain

greater insight into the different essential aspects necessary for their practical application as animal fat substitutes in meat products for the purpose of creating health-enhanced meat products. We likewise require a better interpretation of the changes in the structure of the main components found in meat products (proteins, lipids, etc.) that take place when structured lipids are incorporated as animal fat replacers, and how these affect the end product's technological properties. This knowledge will help to improve and optimize the development of health-enhanced meat products with specific technological characteristics. A useful approach could be to use vibrational spectroscopy (infrared and Raman spectroscopy) to analyze structural modifications in proteins and lipids in the formation of lipid materials and in the health-enhanced meat products in which these materials are incorporated as animal fat replacers.

To better understand the use of vibrational spectroscopy in the development of EGs and OBAs and in meat products with a healthier lipid content, it is necessary to adequately know the basic concepts of these techniques and the interpretation of the spectral results obtained with them.

#### **3. Basic Concepts of Vibrational Spectroscopic Techniques: Infrared and Raman Spectroscopy**

Vibrational spectroscopy, which involves infrared (IR) and Raman spectroscopies, is founded on the transitions concerning quantized vibrational energy states of molecules. In IR spectroscopy, the energy for these transitions is supplied by radiation in the IR regions (mid-IR and near-infrared) of the electromagnetic spectrum [26]. In Raman spectroscopy, samples are excited with monochromatic incident radiation that may be in the ultraviolet (UV), visible (VIS) or near-infrared (NIR) regions of the electromagnetic spectrum [26]. Complementary data on fundamental vibrational modes can be found from mid-IR and Raman spectra, as some vibrational motions are perceived mainly with IR radiation and others mostly by Raman scattering. In relation to protein structure, both vibrational spectroscopic techniques provide information about the secondary structure of proteins ( α-helix, β-sheet, unordered), but Raman spectroscopy can also provide more detailed information about the tertiary structure of proteins [13,27–29]. Regarding structural changes in lipids, both provide relevant information (particularly in meat products with improved lipid content) and both have been used to provide information about the changes in their lipid structure. However, it should be noted that IR is faster and requires a smaller sample size than Raman spectroscopy [13,27–29].

IR and Raman spectroscopy provide many special benefits in food researches [13,27–29]. These techniques can be used to condensed-phase samples in several physical states, whether liquid or solid, clear or opaque. In many situations, insignificant or no sample pre-treatment is needed, and a spectrum can habitually be obtained quite fast [13,27–29]. There has been a remarkable increase in mid-infrared applications resulting from the progress of mid-infrared Fourier transform (FT-IR) spectrometers in combination with sampling methods comprising attenuated total reflection (ATR), for solids, semisolids and liquids, due to the benefits they offer [13,27–29]. Additionally, FT-IR microscopy has paved the way for novel uses of in situ microspectroscopic food mapping and imaging. In the last few years, hyperspectral imaging (HSI) has appeared as a hopeful analytical method for quality control and has involved a lot of attention in the non-destructive analysis of food products. Similarly, Fourier transform Raman spectroscopy (FT-Raman), using NIR excitation from a Nd: YAG laser at 1064 nm, can usually solve the drawback of fluorescence in foods [29]. Methods such as surface-enhanced Raman spectroscopy (SERS), confocal Raman microspectroscopy, and Raman imaging spectroscopy are established for their possibilities and special benefits in examining food components at very low amounts, and for in situ multi-component determination [26,30].

#### *3.1. Analysis of Infrared and Raman Spectra Data*

Infrared and Raman spectroscopy offer relevant data on the structure of the components of meat and meat products (proteins, lipids, water, etc.) non-invasively and in

situ [26,30]. The reformulation processes used to develop meat products with an improved lipid content can modify Raman and infrared spectral bands due to structural changes in meat components (proteins, lipids and water). Basic information is therefore needed about how these changes are analyzed separately in proteins and lipids. For qualitative analysis, modifications can be visualized by comparing the spectrum in question with the characteristic bands of proteins, lipids or water. This requires an analysis of changes in the intensity, frequency, and half-widths of the Raman and infrared bands of chemical groups of proteins, lipids and water which are indicative of qualitative structural changes [13,27,29]. For quantitative analysis, curve-fitting of these bands is often used [13,27,29]. All this structural information is briefly described in the following sections.
