1. Introduction
The production of vegetable oils generates significant amounts of residues, some of which are already recognised as by-products with specific applications, such as their use in animal feed production [
1]. However, a portion of these residues continues to be discarded and contributes to environmental problems and economic losses in the plant oil industry. Exploring innovative uses and technologies for these underutilised residues can contribute to a more sustainable and circular economy [
2]. Several studies have highlighted the potential benefits of a circular economy approach in the plant oil industry, including the reduction in waste and greenhouse gas emissions, as well as the creation of new business opportunities [
3,
4].
Flaxseed oil cake (FOC), a by-product of flaxseed oil extraction [
5], has great potential to improve the nutritional and functional properties of food products. It offers a significant content of essential nutrients, including proteins (~32–36%) and fat (~12–21%), and is additionally abundant in dietary fibre (~9–10%) [
6]. FOC proteins are characterised by a high content of essential amino acids, prominently arginine and leucine, and non-essential amino acids, with glutamic acid and aspartic acid being the major constituents [
7]. Regarding the fat profile, FOC is particularly rich in polyunsaturated fatty acids, specifically linoleic acid (17%) and α-linolenic acid (52%) [
7,
8]. Moreover, FOC is also a good source of minerals (K, P, Mg, Zn), and bioactive compounds, such as lignans [
6,
7,
9]. On the other hand, the potential benefits of FOC application in foods may be hindered by the presence of phytotoxic compounds, including phytic acids, cyanogenic glycosides, and linatine [
10], which may reduce the bioavailability of nutrients or pose a health risk for consumers [
11]. To make flaxseed derivatives safe for consumption, the antinutritive components must be removed or inactivated to undetectable limits. Phytic acid is a low molecular antioxidant [
12]; however, it is considered antinutrient due to its ability to chelate with divalent cations (calcium, zinc, magnesium, copper, iron) and render them insoluble and unavailable for absorption [
13]. Sourdough fermentation and leavening using yeast can help break down phytic acid in breadmaking due to the activation of native phytases, which release inorganic phosphate and a series of inositol phosphate intermediates. Baca et al. [
14] indicated that the elevation of the temperature (up to 30 °C) and elongation of the time of yeast fermentation caused an increase in phytic acids hydrolysis due to an increase in phytase activity at the higher temperature. Contrary to phytic acid, cyanogenic glycosides are heat-labile and can be reduced or removed via thermal processing, solvent extraction, extrusion [
15], and enzyme (β-glycosidases) application [
16]. Thus, the combined techniques applied during breadmaking could be sufficient to reduce the content of antinutrients in FOC, enabling it to be used as a safe and valuable food ingredient.
The use of FOC in different foods, along with its derivatives such as FOC extract [
17] and FOC flour [
18], has gained increasing attention due to its high nutritional value and potential applications as a functional ingredient in various food products. Many studies determined its effect on the physical, chemical, and sensory properties of the products, as well as the dietetic and potential health benefits associated with FOC consumption [
17,
18,
19,
20,
21]. Łopusiewicz et al. [
17] focused on the development and characterisation of a non-dairy kefir-like fermented beverage using FOC as subtract. The authors concluded that the FOC-based kefir-like beverage had a similar composition to traditional kefir and a higher content of probiotic bacteria, indicating its potential health-promoting effects. In addition, the authors showed a 94.05% reduction in cyanogenic compounds in FOC (from the primary amount of 187.35 ± 8.34 mg/kg to 11.15 ± 4.41 mg/kg) after incubating for 1 h at 90 °C, which is seen as a safe level for consumers [
22]. Zarzycki et al. [
19] developed an FOC-enriched pasta and assessed its nutritional value, antioxidant capacity, and cooking quality. The results showed a significant increase in all measured parameters, indicating the beneficial effects of incorporating FOC in pasta production. Regarding breadmaking, Taglieri et al. [
20] conducted a study to examine the impact of using different leavening agents (sourdough and baker’s yeast) on the characteristics of bread that is fortified with FOC. Similarly, Sanmartin et al. [
21] explored the use of FOC as an ingredient improving the nutraceutical and sensory features of sourdough bread.
FOC extract has also been applied to ameliorate the GFB quality. Krupa-Kozak et al. [
23] investigated the impact of the level of FOC extract on the nutrient content, antioxidant properties, and sensory quality of GFB. The authors found that increasing the level of FOC extract in a formulation resulted in higher antioxidant activity, improved nutritional properties, and better sensory quality of the developed GFB. Meanwhile, Łopusiewicz et al. [
24] evaluated the effect of FOC extract on the texture and shelf life of GFB, finding an improvement in these features. Moreover, FOC extended the shelf life of GFB and successfully delayed microbial growth, which could potentially increase the safety of GFB. All the above-mentioned studies have shown promising results relating to the applications of FOC in the food industry in terms of improving the nutritional value and sensory attributes of various food products. However, further research could be helpful to better understand the effects of FOC on GFB processing and quality characteristics.
Based on the literature, it is evident that commercial GFBs have nutritional limitations. In particular, these bakery products are recognised as being low in proteins and deficient in minerals (calcium, iron, and zinc) and vitamins (folate, niacin, thiamin, and riboflavin) that are needed in a healthy and balanced diet, in addition to being excessive in fat and simple sugars [
25]. Therefore, it is important to enhance its nutritive value while simultaneously balancing this with technological and sensory benefits. On the other hand, the valorisation of plant-origin waste and by-products that are rich in nutrients, dietary fibre, and bioactive compounds into food recipes is a current trend in the development of value-added food products [
23,
24,
26,
27]. The main purpose of this study was to design and produce a new high-quality FOC-enriched GFB, characterised by improved nutritional features and enhanced sensory attributes. To examine the developed GFBs, the physical parameters, texture profile, proximal chemical composition, and sensory features were determined.
2. Materials and Methods
2.1. Flaxseed Oil Cake
In the present study, FOC, produced and donated by ACS Sp. z o. o. (Bydgoszcz, Poland), was used. Our preliminary analysis (data not published) of FOC’s nutritional composition showed that it was a valuable source of proteins (30 g/100 g DM), carbohydrates (32 g/100 g DM), and fat (2.5 g/100 g DM). FOC was also rich in dietary fibre (7.9 g/100 g DM).
2.2. Composition of Experimental Gluten-Free Breads
The GFB used as the control was based on a previously optimised formulation [
28] and was composed of corn starch (HORTIMEX, Konin, Poland), potato starch (PPZ “Trzemeszno” Sp. z o. o., Trzemeszno, Poland), pectin (E 440(i), ZPOW Pektowin, Jasło, Poland), sugar (Diamant, Pfeifer & Langen Polska S.A., Poznań, Poland), salt (Cenos Sp. z o. o., Września, Poland), fresh yeast (Lesaffre Polska S.A., Wołczyn, Poland), rapeseed oil “Wielkopolski” (EOL Polska Sp. z o. o., Szamotuły, Poland), and deionised water. FOC was added to the experimental formulation as a substitute for starches (
Table 1).
2.3. Preparation of Experimental Gluten-Free Breads
A straight dough method was used to prepare the experimental GFBs [
28]. To make the GFBs, the main ingredients (starches, pectin, FOC) were mixed (5 min; t min. speed) using Kenwood Chef XL Titanium P-9878 (Kenwood Limited, Havant, UK). Subsequently, sugar, salt, and yeast dissolved in the deionised water were added to the mixture along with the oil. The batter was mixed at low speed (speed 2) for 12 min. Then, the batter was divided into 240 g samples and placed into the square pans (10 cm × 10 cm × 9 cm) and proofed for 40 min at 35 °C and 70% humidity. Afterwards, samples were baked in the oven (ZBPP, Bydgoszcz, Poland) for 30 min at 220 °C. Baked loaves were cooled for 2 h at room temperature and then stored (24 h) in the dark at room temperature in clip-seal plastic bags for further analysis. The products of four batches were analysed.
2.4. Sample Preparation for Further Analysis
To determine the moisture content, texture properties, and sensory analysis, fresh (24 h after baking) GFBs were used. On the other hand, the chemical composition and acrylamide content was determined in freeze-dried GFB samples. Briefly, a whole loaf of each type of GFB was manually crushed, packed in a paper envelope, and placed in the ultra-freezer at −80 °C for at least 24 h. Then, the frozen samples were placed in a freeze-dryer (Labconco Corporation, Kansas City, MO, USA) for about 40 h. The freeze-dried samples were ground with a laboratory mill (WZ-1 type, Zakład Badawczy Przemysłu Piekarskiego Sp. z o. o., Poland) for 12 s and sieved through a 0.40 mm mesh. The obtained homogenous powder was packed in polyurethane string bags and kept in the dark at 4 °C for further analysis.
2.5. Characteristics of Experimental Gluten-Free Breads
2.5.1. Analysis of Nutritional Composition and Energy Value
Moisture [
29], proteins [
30], fat [
31], ash [
32], and dietary fibre [
33] content were determined according to the standard methods. The content of carbohydrates was calculated by subtracting the values in percentage of moisture, fat, protein, and ash from 100. Energy values (kJ) were calculated as previously described [
23]. The conversion factor for calorie calculation was considered to be 1 kJ = 0.239 kcal [
34].
2.5.2. Determination of Acrylamide Content
The acrylamide was extracted from gluten-free bread using the procedure of Ciesarová et al. [
35] without modifications. Then, the micro-HPLC (LC-200, Eksigent) system coupled with a mass spectrometer (QTRAP 5500, AB Sciex, Vaughan, ON, Canada) consisting of a triple quadrupole and ion trap was used to analyse samples. The chromatographic separation was conducted on a HALO C
18 column (0.5 mm × 50 mm × 2.7 μm, Eksigent, Vaughan, ON, Canada) at 45 °C at 25 μL/min flow rate. The elution solvents were A (H
2O/formic acid; 99.9:0.1;
v/v) and B (acetonitrile/formic acid; 99.9:0.1;
v/v). The gradient elution was used as follows: 0–0.7 min (1% B), 0.7–3.2 min (1–90% B), 3.2–4.2 min (90% B), 4.2–4.4 min (90–1% B), and 4.4–5 min (1% B). A calibration curve with R
2 = 0.998 was plotted for acrylamide using the external standard (17.4–1740 × 10
−1 ng/g). The LOD and LOQ were established at the level of 2.54 × 10
−4 µg/g and 0.77 × 10
−3 µg/g, respectively. Acrylamide was identified and quantified by comparing its retention time and the presence of respective parent and daughter ion pairs (multiple reaction monitoring, MRM). Acrylamide (≥99%), acetonitrile, formic acid, water of MS grade, potassium hexacyanoferrate (II) trihydrate (K
4[Fe(CN)
6]
•3H
2O), zinc sulfate heptahydrate (ZnSO
4•7H
2O), and ethyl acetate were bought from Sigma Chemicals Co. (St. Louis, MO, USA).
2.5.3. Determination of Physical Parameters
The loaf weight was determined using a digital balance (0.01 g accuracy), and its volume was determined using the standard rapeseed displacement method [
36]. Three loaves of each GFB type were analysed.
Other physical parameters of experimental GFBs, in particular the specific volume (SV; cm
3/g), density (D; g/mL), and the ratio of height to width (H/W), were determined as previously described [
23], whereas the bake loss was calculated through Equation (1):
where:
a is the weight of batter (g),
b is the weight of baked and cooled GFBs (g).
A middle slice of GFBs was scanned using a flatbed scanner (Epson Scan GT-1500, Epson Europe, Warsaw, Poland), supported by Epson Creativity Suite Software Images.
2.5.4. Instrumental Colour Determination
Due to the irregularity of the crust surface of experimental GFBs, colour was analysed only in the crumb samples at the middle point of a central slice (of 20 mm thickness) using a Hunter Lab ColorFlex 45/0 (Hunter Associates Laboratory, Inc., Reston, VA, USA). The results were expressed following the CIELab system: lightness
L* (=0 to black; =100 to white) and chromatic components
a* (−
a to greenness; +
a to redness) and
b* (−
b to blueness; +
b to yellowness). The whiteness index (WI) was calculated according to Hsu et al. [
37]. The difference in colours (Δ
ELab), expressed as metric distances among the chromatic coordinates values [
38], were calculated through Equation (2):
where
The crumb colour values for each kind of GFB were the mean of fifteen replications.
2.5.5. Instrumental Textural Profile Analysis (TPA)
To analyse the texture of the crumb of GFBs, a TA.HD Plus Texture Analyser (Stable Micro Systems Ltd., Godalming, UK), equipped with a 30 kg load cell, was used. A 25 mm thick central slice was exposed to a double compression cycle up to 40% deformation of its original height with a 35 mm flat-end aluminium compression disc (probe P/35). The selected settings were as follows: pretest/test/post-test speed, 2.0 mm/s, force, 10 g, relaxation time, 5 s, trigger, and auto mode [
39]. The textural parameters that were determined were as follows: hardness, springiness, cohesiveness, chewiness, and resilience. The texture profile was analysed in six replicates.
2.5.6. Sensory Analysis
The trained and monitored according to the ISO standard [
40] expert panel (five women and one man), acquainted with gluten-free products, performed the sensory analysis of experimental GFBs using quantitative descriptive analysis (QDA) [
41]. The vocabulary for sensory attributes was determined in a round-table session, following the standardised procedure [
42]. Twenty established attributes were defined, and the scale edges are shown in
Table 2.
GFBs were evaluated using the QDA, which was performed in a sensory laboratory room [
43] at room temperature and under normal lighting conditions. A three-digit number was assigned to each sample and given to the assessors all together in a random order. To minimise residual effects, water was available to drink between each sample evaluation. The panellists evaluated the intensity of attributes through unstructured graphical scales. Results were converted into numerical values (from 0 to 10 arbitrary units) via the ANALSENS system (IAR&FR PAS, Olsztyn, Poland). GFBs were tested in duplicate at different time points.
2.6. Statistical Analysis
In this study, unless specified otherwise, the results are shown as the mean of triplicate observations and standard deviation. The differences between experimental GFBs were analysed using one-way ANOVA, followed by Tukey’s multiple comparison test (p ≤ 0.05). The statistical analysis was conducted using GraphPad Prism version 8.0.0 for Windows (GraphPad Software; San Diego, CA, USA).
4. Conclusions
The conducted study showed that FOC, due to its valuable characteristics, augmented the nutritional value of developed GFBs. The physico-technological parameters, colour, and texture of GFBs were beneficially modified by the incorporation of FOC. At low-to-moderate levels (5% and 15%), FOC improved the specific volume, texture characteristics (reduced crumb hardness, gumminess, and chewiness), and appearance of GFBs, which allowed us to ameliorate its sensory features, although a seed-like aroma and taste were noticed. However, as an increase in crumb hardness was detected with an increased FOC percentage, the concentration of FOC needs careful regulation to achieve the desired textural characteristics. Among the obtained experimental formulations, FOCE15% can be perceived as the most appreciated product due to its improved quality, providing an opportunity to meet the nutritional needs and sensory expectations of individuals following a gluten-free diet.