Evaluation of Selected Properties of Corn Snacks Enriched with Pomace from Common Flaxseed (Linum usitatissimum L.) and Golden Flaxseed (Linum flavum L.) with the Addition of Cocoa

Abstract

Numerous authors have tried to develop crispy snacks enriched with flaxseed pomace. Unfortunately, its dark color was completely unacceptable to consumers and eliminated these products from the market despite their very good health-promoting properties. This study attempted to produce corn snacks enriched with pomace from various flax varieties with the addition of cocoa, which could mitigate the poor perception of their dark color by consumers. The snacks produced with the addition of common flax and cocoa received high scores given by panelists, not diverging from those given to the snacks with golden flax pomace addition. In particular, the addition of cocoa contributed to a significant improvement in the sensory evaluation of the color of the snacks produced with dark pomace addition, while not deteriorating their other assessed parameters, compared to the snacks prepared without cocoa. There were also no differences in the mechanical properties between the snacks with different pomace types added. Only the high additive dose (ca. 10%) resulted in deterioration of these properties.

1. Introduction

Flaxseed is known for its health-promoting properties. Its consumption exerts a beneficial effect on lowering blood levels of cholesterol and glucose and prevents atherosclerosis and other cardiovascular diseases (CVD) [1,2]. Until recently, this effect has been ascribed to the consumption of oil contained in flaxseeds, in particular to the properties of its essential unsaturated fatty acids (EFAs). Currently, however, many authors have confirmed that the best health-promoting effects are achieved upon the intake of lignans contained in the flaxseed fiber, including mainly secoisolariciresinol occurring in the form of secoisolariciresinol diglycoside (SDG) [2,3]. Administering flax pomace to persons with hypertension at a dose of 30 g/day for a few months has been shown not only to reduce blood cholesterol level and blood pressure but also to minimize the risk of CVD development. A similar effect was obtained when consuming whole flaxseeds [1,4,5]. At the same time, patients consuming flaxseed pomace provided themselves with a lower amount of calories in their diet compared to the patients consuming whole seeds, which additionally allowed for an effective fight against obesity. Golden flaxseed pomace, which is the residue of industrial oil extraction, is no longer an unmanaged waste product. Due to its valuable nutritional value, it has been used as an enriching additive to many food products (e.g., as an ingredient in breadcrumbs and additive to bread, cookies, and muffins) [1,3,6,7,8,9]. Different is the case of pomace obtained from common flaxseed. Although it has more fiber and health-promoting components compared to golden flaxseed pomace, it is not used in the industry and is still treated as waste, at best used as animal feedstuff [10,11]. The few studies that have attempted to use this waste as a component for enriching food products with fiber have clearly indicated that the dark color imparted to food products by even a small addition of flaxseed pomace discouraged potential customers and completely disqualified products on the market [6,12]. It should be noted that consumer acceptance surveys of functional foods have shown that consumers are generally not willing to compromise on the taste and appearance of foods despite their health benefits [9,13,14,15]. This has been confirmed by our previous research, which proved that the organoleptic assessment of products with common flaxseed pomace (dark color) was lower than of those with the addition of golden flaxseed pomace [6]. Analyzing in detail the data obtained in previous works, it was found that the disqualifying effect was due to the change of color of these products, as there were no statistically significant differences between any of the examined characteristics of products made with the addition of common flaxseed and golden flaxseed, except for their color. The color of the snacks with the addition of common flaxseed was rated much lower compared to the golden flaxseed snacks. This effect was clear even with small additions of the pomace [6].

In this study, it was hypothesized that it is possible to change the consumer’s views on the color of products with the addition of common flaxseed pomace. Consumer surveys have confirmed that the dark color disqualifies a food product only if the consumer expects it to have a bright color. However, there is a range of products whose dark color does not deter customers, e.g., cocoa. Consumers accept the darker color of chocolate-flavored corn snacks. The dark color is even desired by consumers in such products because—in their opinion—it is tantamount to a high cocoa content. It can, therefore, be assumed that the color of extrudates produced with the addition of common flaxseed pomace will not disqualify them when presented to consumers as a chocolate-flavored snack. It is likely that the color of products with the addition of common flaxseed will be effectively masked with dark color of the cocoa. It should also be remembered that melanoidin isolated from the cocoa infusion is not only responsible for the dark color of cocoa but can also inhibit the action of α-glycosidase, which significantly supports the treatment of diabetes [16,17,18]. Previous studies have shown that cocoa consumption is directly linked to a reduced risk of development of coronary heart disease, depression, and certain types of cancer. Theobromine and polyphenols of cocoa have a very beneficial effect on the nervous system, whereas its potassium facilitates the maintenance of normal blood pressure. It also supports the proper functioning of the heart and regulates blood pressure, expanding blood vessels. In addition, the antioxidant properties of polyphenols contained in cocoa powder may have a protective effect against the residues of EFAs contained in flaxseed pomace [16,19].

In this context, it seems necessary to determine the effect of cocoa addition to snacks produced with common flaxseed pomace on their color and other characteristics. Therefore, the aim of the present study was to produce snacks with the addition of pomace from various varieties of flax and cocoa and to determine the influence of cocoa addition on selected properties of the snacks, including primarily on the organoleptic assessment of their color. The additional aim was to determine the maximum share of pomace from common flaxseed at which it would be possible to cover the color of the snacks with cocoa.

2. Materials and Methods

2.1. Materials

The experimental material included:

• Pomace left after oil pressing from seeds of common flax (Linum usitatissimum L.) (polysaccharides 3.4 g/100 g; dietary fiber 41.0 g/100 g; protein 36.6 g/100 g; lipids 9.0 g/100 g) and golden flax (Linum flavum L.) (polysaccharides 3.4 g/100 g; dietary fiber 36.7 g/100 g; protein 40.1 g/100 g; lipids 9.0 g/100 g) obtained from Oleofram Sp. z o.o. company (Wroclaw, Poland). The pomace is a residue from the production of two types of linseed oil. The oil was produced using a single cold pressing method. No chemical extraction was used during the oil extraction process and the process temperature did not exceed 40 °C.
• Maize semolina (polysaccharides 70.0 g/100 g; dietary fiber 2.5 g/100 g; protein 8.6 g/100 g; lipids 1.5 g/100; 12% moisture content; granulation—1 mm ± 0.2 mm) produced by Sante (Sobolew, Poland);
• Cocoa powder (2.2% moisture content) obtained from Maspex (Warsaw, Poland), and sodium chloride (produced by P.P.H. “StanLab” Sp.J. Lublin, Poland).

Data on the composition of the materials used come from the manufacturers’ declarations.

2.2. Preparation of Samples Used for Extrusion

Two types of flaxseed pomace were crushed in a mill (Retsch GM 200; Haan, Germany) to granulate 1 mm ± 0.2 mm. Pomace from golden flaxseed or common flaxseed was added to maize semolina in doses of 0%, 5%, 10% or 15% of mixture dry matter. The range of pomace additives used was selected in previous studies [6]. The mixtures were divided in half, and 1.5% cocoa and 0.5% sodium chloride were added to one half, whereas only 0.5% sodium chloride was added to the second half. The dry matter content of the mixtures prepared in this way was determined and, based on calculations, water was added to the mixture in such a way as to obtain a moisture content of 14%. Afterwards, all samples were closed in polyethylene bags and conditioned at room temperature for 24 h (Figure 1).

2.3. Extrusion Process

The experiment was carried out in two technological replications.

The extrusion process was carried out in a Brabender DN 20 single-screw extruder (Duisburg, Germany). The following parameters were used:

• Temperature: section 1; 2; 3—respectively: 130 °C; 150 °C; 170 °C;
• Screw rotational speed; 200 rpm (the screw rotational speed was selected so that the load of the screw driving motor was within the range 7–8 A);
• Feeder speed: 25–30 rpm;
• Screw with a compression ratio of: 4:1;
• Nozzle: round, diameter 4 mm.

The samples obtained in the extrusion process were conditioned for 48 h at room temperature.

2.4. Color Measurement

Color measurements were performed using a Minolta CM-5 (Osaka, Japan) spectrophotometer on the ground extrudates. Samples were placed in a glass cuvette, and the color was recorded using L*, a*, and b* color spaces, where L* indicates lightness (which varies in the range of 0–100), a* the share of green or red (positive value—shades of red, negative value—shades of green), and b* the share of blue or yellow (positive value—shades of yellow, negative value—shades of blue). Chroma describes the intensity or purity of the color (0—neutral, C > 0—more intense, saturated). Chroma was calculated using Equation (1) [20]

C = (a*² + b*²)^½,

(1)

Hue determines the shades of different samples. Hue angle (h) was calculated using Equation (2):

h = tan⁻¹(b*/a*),

(2)

Total color difference (ΔE) was calculated using Equation (3):

ΔE = (ΔL*² + Δa*² + Δb*²)^½,

(3)

Data were reported as the mean value of 5 measurements.

2.5. Expansion Ratio Measurement

The ratio of expansion was determined as the ratio of extrudate diameter to nozzle diameter. Data were reported as the mean value of 10 measurements [6].

2.6. Measurement of Mechanical Properties

Mechanical properties were measured using an Instron 5544 Tensile Tester (Norwood, MA, USA) with a bend fixture attachment and a cross-head. The following measurement parameters were used:

• Cutting blade load: 2 kN
• Cutting blade speed: 4.5 mm/min
• Distance between support points: 40 mm
• Measurement recording frequency: 16.7 s⁻¹

Based on the average obtained from 30 measurements, the minimum force required to break the extrudates and the deformation were calculated [20].

2.7. Descriptive Sensory Analysis

The sensory properties of extruded snacks were assessed in accordance with the Polish Standard [21].

The study involved 10 trained panelists—employees of the Faculty of Food Sciences at the University of Environmental and Life Sciences in Wroclaw (6 women, 4 men, aged 25–29 years). The assessment was carried out in a room adapted for sensory assessment, with natural lighting and at room temperature. The samples were given to the assessors in a random order. Each sample was coded with a three-digit random number. In order to cleanse the palate between the samples, the assessors were given water.

The following sensory attributes were assessed: taste, aroma, color, texture and consistency, assigning them values from 1 (worst) to 7 (best) [22].

Data were reported as the mean value of 10 measurements.

2.8. Statistical Analysis

The results were analyzed using Statistica 13.1 (Statsoft, Krakow, Poland). The experiment was conducted in two technological repetitions. The obtained results were consistent with the normal distribution, which confirms the lack of influence of other factors (e.g., air temperature and humidity, atmospheric pressure) on the obtained results. In all obtained data, the variance was calculated for the three variables. After confirming their homogeneity, the standard deviation was calculated in all analyzed groups, then the values of the means in the groups were analyzed using Duncan’s test depending on the type of additive, pomace type, and pomace addition level. The result groups in which the differences were statistically significant at the level of 0.05 were marked with different letters. An analysis of the correlation between the size and type of pomace addition and between the size of pomace addition and cocoa addition for selected mechanical properties and expansion ratio was also performed.

3. Results and Discussion

Table 1. The influence of the type and amount of pomace from flaxseed added and cocoa added on the color of snacks.

Type of Addition	Type of Pomace	Pomace Addition Level	Lightness L*	Hue Angle	Color Saturation Chroma	Total Color Difference ΔE
Without cocoa powder	Golden flax	0	77.26 ± 0.41	87.93 ± 6.25	26.40 ± 4.57	0.00 ± 0
		5	75.52 ± 0.53	85.55 ± 6.21	24.11 ± 4.70	0.86 ± 0.5
		10	73.03 ± 0.58	84.69 ± 5.77	20.01 ± 5.33	5.64 ± 0.8
		15	70.92 ± 0.72	84.03 ± 6.34	17.83 ± 3.45	8.79 ± 0.8
	Common flax	0	77.26 ± 0.41	87.93 ± 6.25	26.40 ± 4.57	0.00 ± 0
		5	73.31 ± 0.60	85.26 ± 4.86	21.56 ± 3.59	2.73 ± 0.4
		10	69.83 ± 0.55	84.52 ± 8.07	18.09 ± 4.08	9.11 ± 0.6
		15	66.82 ± 0.67	82.60 ± 5.50	16.27 ± 3.65	17.12 ± 0.9
With cocoa powder	Golden flax	0	67.06 ± 0.44	80.13 ± 6.36	25.38 ± 3.74	0.00 ± 0
		5	65.75 ± 0.52	80.11 ± 7.54	24.82 ± 2.68	1.83 ± 0.5
		10	65.13 ± 0.41	79.43 ± 8.11	21.13 ± 4.36	5.77 ± 0.4
		15	64.40 ± 0.39	78.52 ± 8.20	17.90 ± 4.05	8.94 ± 1.4
	Common flax	0	67.06 ± 0.44	80.13 ± 6.36	25.38 ± 3.74	0.00 ± 0
		5	64.52 ± 0.61	78.26 ± 4.97	22.12 ± 4.11	4.36 ± 0.3
		10	63.84 ± 0.38	78.01 ± 6.56	19.08 ± 3.27	11.12 ± 0.9
		15	62.27 ± 0.45	77.84 ± 6.72	16.88 ± 4.80	22.03 ± 1.1
LSD Multiple Range test
Type of addition
Without cocoa powder			72.99 a	85.31 a	21.33 a	5.53 a
With cocoa powder			65.00 b	79.05 b	21.59 a	6.76 b
Type of pomace
Golden flax			69.88 a	82.55 a	22.20 a	3.98 a
Common flax			68.11 b	81.82 a	20.72 b	8.31 b
Pomace addition level
0			72.16 a	84.03 a	25.89 a	0.00 a
5			69.78 b	82.30 b	23.15 b	2.45 b
10			67.96 c	81.66 c	19.58 c	7.91 c
15			66.10 d	80.75 d	17.22 d	14.22 d

For each column and per treatment (main effect), mean values with different letters are statistically different at p < 0.05.

presents selected color coordinates measured colorimetrically in the CIELab system in the products made based on maize semolina with different contents of pomace from two varieties of flax: golden flax and common flax, with and without added cocoa.

The color coordinates depended to a statistically significantly extent on all statistical factors analyzed (type of flax pomace, its content in the product, and cocoa addition). The highest brightness value (L) was measured in the corn snacks produced without the addition of cocoa and without the addition of flaxseed pomace (77.26). The addition of each type of flaxseed pomace led to a decrease in the brightness value, whereas the snacks produced with the addition of golden flaxseed pomace had a statistically higher L value (69.88) compared to those with common flaxseed pomace (68.11). Similar results were obtained in the analysis of the chroma value, which was 22.20 in the snacks with the addition of golden flaxseed pomace and 20.72 in those with common flaxseed pomace. In turn, when analyzing the color hue angle, there were no statistically significant differences in its values as affected by the type of pomace used (Table 1). Analyzing the effect of only the content of pomace on snack brightness, it was found that the highest brightness value was recorded in the case of the products without the pomace (72.16), and the lowest in those with 15% content of the pomace (66.10). However, each of the analyzed pomace addition levels caused a statistically significant decrease in the value of this color coordinate (Table 1). The same correlations were noted in the case of chroma and hue angle values. Other authors also achieved similar values in their works with snacks [6,23,24,25], pasta [8], cookies [26], muffins [27], bread or breadcrumbs. The decrease in the color coordinates recorded in the products with flaxseed pomace is mainly due to the dark color of flaxseed itself [9]. Wójtowicz et al. in their work with extruded products based on maize semolina also obtained products with a darker color after adding flaxseed pomace compared to the snacks produced without pomace [23]. However, the reduction in color parameters of products reported by these authors was much lower than in the products manufactured by other authors, despite similar proportions of flaxseed pomace in the snack formulas. These differences were due to the fact that Wójtowicz et al. [23] used only golden flaxseed pomace, whose color is much lighter (yellow) than that of pomace obtained from common flax pressing (dark brown). The darker color of common flaxseed pomace is due to its chemical composition as the common flax variety was found to contain significantly more fiber, lignans, phytosterols and associated pigments than the golden flax variety [6].

When comparing the values describing the color of products produced without and with cocoa, it was found that cocoa addition caused a statistically significant decrease in their brightness (65.00) and shifted the hue angle value toward red colors (79.05) compared to the snacks produced without cocoa addition in which these values were 72.99 and 85.31, respectively. These results are due to the dark color of cocoa used in the study (L = 63.14 ± 0.03; a = 1.95 ± 0.02; b = 25.33 ± 0.02), which is ascribed to its phenolic and flavonoid pigments. Other authors also achieved similar or even greater reductions in color brightness of products extruded with cocoa addition [28,29]. In their work, Ondo and others obtained up to 50% reduction in brightness of the products with a 5% cocoa addition (2013). This was, undoubtedly, due to the use of a larger cocoa addition, but also a lower extrusion temperature (130 °C), which caused lesser degradation of the cocoa pigments [28], and higher moisture content of the extruded mixture [12].

The analysis of the total color difference between the snacks produced with common flaxseed pomace (8.31) and those with golden flaxseed pomace (3.98) showed that their ∆E differed significantly compared to the control snacks. The difference between these values was 4.33 and hence fitted within the range of 3.5 < ∆E < 5, which means that the color of the snacks produced from both varieties of flaxseed is perceived by the observer as significantly different [6]. This difference is also reflected in the previously described reduction in brightness (L) caused by hue angle shift toward red color. This effect is caused by Maillard reactions occurring during the extrusion process. Many authors have pointed to fiber content as a feature that can also cause a significant change in the color of extruded products. The higher fiber content increases viscosity of the extruded material and contributes to higher friction forces, which consequently leads to an increase in temperature of the extruded mass and intensification of Maillard reactions, caramelization reactions, or pigment degradation processes [20].

Also, the proportions of flaxseed pomace in the snack formula had a significant impact on the objective difference in the colors of the snacks, with each increase in the pomace proportion in the extruded mixture resulting in an increase in the ∆E value. The snacks produced with 5% pomace content differed from the snacks made without pomace by only ∆E = 2.45. However, in the snacks with a 10% addition of pomace, this difference was ∆E = 7.91; i.e., it was higher than the expected value (resulting from the doubling of the flaxseed pomace addition) by 3.01, i.e., by 61%. In the case of snacks with the highest analyzed pomace addition (15%), the value of ∆E = 14.22 was higher than the expected value (resulting from the tripling of the flaxseed pomace addition) by 6.87, namely by as much as 93%. The described increase in the total color difference clearly indicates that it is not only due to the increase in the content of dark pomace in the material but also due to the synergistic action of other factors, including the caramelization and Maillard reactions. It should also be noted that only in the case of the snacks with the lowest addition of flaxseed pomace was the total color difference less than 5. In the case of the snacks with 10% and 15% addition of flaxseed pomace, the ∆E value significantly exceeded the threshold value of 5, after which the observer assesses the samples as having a completely different color [6,25].

Also, the use of cocoa in the snack formula had a significant impact on the objective difference in the color of the snacks. It was found to increase the ∆E value (6.76) compared to the snacks produced without cocoa (5.53). Technological operations (such as cooking, baking, drying, roasting, extrusion, etc.), in which cocoa is exposed to high temperatures, are crucial in imparting desired organoleptic attributes to food products, including color. As a result of the high temperatures applied, these processes result not only in the degradation of enzymes and polyphenolic and flavonoid pigments but also trigger Maillard reactions, whose products are the major players in ensuring the right quality of cocoa [29,30]. Analyzing the effect of pH on the color of extruded products, it was observed that products with a darker color were obtained in an alkaline environment because the degradation of polyphenols occurred to a lesser extent. There are also reports that the fermentation process of cocoa beans can stabilize the amounts of polyphenols and tannins when subjected to various high-temperature culinary treatments [19]. It is also likely that a change in pH could contribute to a reduction in the degree of degradation of the polyphenols contained in snacks made from maize semolina with added cocoa and flaxseed pomace, which would increase their antioxidant activity and contribute to a reduced risk of development of heart disease and certain types of cancer among consumers of these products [31].

Another important feature describing the quality of snacks, which significantly affects their assessment by consumers, is their expansion ratio. Customers search for products with a light and delicate structure and therefore a large and even expansion. The highest expansion ratio was determined in the snacks produced without cocoa and without flaxseed pomace (3.09), while the lowest one was recorded in the snacks with cocoa and with 15% of golden flaxseed pomace (

Table 2. The influence of the type and amount of pomace from flaxseed added and cocoa added on the expansion ratio and mechanical properties of snacks.

Type of Addition	Type of Pomace	Pomace Addition Level	Expansion Ratio	Deformation [mm]	Force [mN]
Without cocoa powder	Golden flax	0	3.09 ± 0.17	2.01 ± 0.16	11.41 ± 1.89
		5	3.01 ± 0.11	1.90 ± 0.10	13.18 ± 2.26
		10	2.82 ± 0.13	1.63 ± 0.13	16.73 ± 2.17
		15	2.67 ± 0.19	1.36 ± 0.14	20.85 ± 2.95
	Common flax	0	3.09 ± 0.17	2.01 ± 0.16	11.41 ± 1.89
		5	3.01 ± 0.20	1.88 ± 0.13	12.97 ± 2.44
		10	2.84 ± 0.20	1.58 ± 0.17	16.52 ± 1.98
		15	2.67 ± 0.11	1.35 ± 0.20	21.19 ± 2.56
With cocoa powder	Golden flax	0	3.05 ± 0.14	1.95 ± 0.12	10.11 ± 1.15
		5	3.01 ± 0.15	1.86 ± 0.11	9.87 ± 1.66
		10	2.95 ± 0.18	1.50 ± 0.14	12.53 ± 1.27
		15	2.62 ± 0.13	1.13 ± 0.10	17.20 ± 1.85
	Common flax	0	3.05 ± 0.14	1.95 ± 0.12	10.11 ± 1.15
		5	3.03 ± 0.20	1.78 ± 0.09	11.51 ± 2.22
		10	2.85 ± 0.19	1.50 ± 0.13	14.19 ± 1.78
		15	2.70 ± 0.12	1.22 ± 0.11	18.02 ± 2.10
LSD Multiple Range test
Type of addition
Without cocoa powder			2.90 a	1.72 a	15.53 a
With cocoa powder			2.91 a	1.61 a	12.94 b
Type of pomace
Golden flax			2.90 a	1.67 a	13.98 a
Common flax			2.91 a	1.66 a	14.49 a
Pomace addition level
0			3.06 a	1.98 a	10.76 a
5			3.02 a	1.86 a	11.88 a
10			2.86 b	1.55 b	14.99 b
15			2.67 c	1.27 c	19.32 c

For each column and per treatment (main effect), mean values with different letters are statistically different at p < 0.05.

, Figure 2a,b).

Comparing the results obtained, it was found that the only factor that had a statistically significant impact on the expansion ratio of the products was the level of flaxseed pomace addition, whereas such an effect was not confirmed for flaxseed variety. With the increasing amount of pomace in the formula of the extruded mixture, the expansion ratio of the snacks decreased, but this difference was statistically significant only in the products with at least 10% pomace addition (Table 2, Figure 2a,b). Similar results were obtained by other authors [6,25]. The decrease in the expansion ratio noted in the sample containing flaxseed pomace is due to an increase in the amount of fiber (provided with the pomace) [6,32,33,34]. Many authors who studied the effect of fiber content on the expansion ratio concluded that the addition of flaxseed pomace significantly increased the contents of all fiber fractions, which during the extrusion process forms a highly viscous and complex network with starch, reducing the stretching ability of air bubbles in the extruded material, which consequently led to a decrease in the expansion ratio [6]. Flaxseed pomace also has a high content of non-extruded oil, which reduces the friction forces inside the extruder, thereby contributing to an additional reduction in the expansion ratio [6,12,35,36]. Zhang and others also point to the formation of lipid complexes with starch and protein as another factor that reduces the expansion ratio of snacks [35]. This fact is confirmed by the results achieved in the present study (Table 2), where the expansion ratios of the snacks were lower than the results reported by other authors analyzing the expansion of products with high-fiber additives but without a lipid component [37]. There were no statistically significant differences in the expansion ratio of the snacks produced with the pomace from the studied flaxseed varieties, despite the fact that the pomace of common flaxseed contains more ballast substances compared to golden flax pomace (Figure 2a). The reduction in the expansion ratio of the common flaxseed products having a higher fiber content is probably comparable to the decrease in this ratio caused by the higher protein content of products made with the addition of pomace from golden flax varieties [2,6].

Table 2 presents values of the selected mechanical properties of the produced snacks (minimum force needed to break the extrudate and deformation determined at the time of snack breaking). The analysis of the influence of the factors examined in the study on the mechanical properties of the snacks showed no significant correlation between these parameters and the variety of the pomace used. In the case of snack deformation at breaking, there was no statistically significant effect of cocoa addition on this parameter. On the other hand, a statistically significant decrease in the value of the minimal breaking force was noted in the cocoa-added snacks compared to the snacks produced without cocoa. The relationship between the size of the force and the amount of added pomace in the case of snacks produced with the addition of cocoa was also a quadratic function, but its course was more gentle in relation to the course of this function describing snacks produced without cocoa (Figure 2f). The minimal breaking force determined for the cocoa-free snacks was 15.33 [mN] and was 17% higher than that determined for the snacks made with cocoa (12.94 [mN]). Other authors have also noted an increase in the force necessary to break products resulting from cocoa addition [28,30,38]. Similarly, Ondo and co-authors reported a significant increase in the breaking force after increasing cocoa addition in the extruded mixture from 5% to 20% [28]. Analyzing the morphology of the cross-section of the extrudates, they found that the snacks without the addition of cocoa were characterized by the presence of very large pores. With the increase in cocoa proportion in the extruded material, the number of pores increased, whereas their size significantly decreased, resulting in the lack of any change in the expansion ratio of the snacks but at the same time contributing to the formation of a more rigid structure, which translated into a significant increase in the force necessary to break the products [28]. However, the increase noted in the force necessary to break the snacks (which is tantamount to the force needed to squeeze the product between the teeth) would not be a drawback in the case of products used as breakfast cereals because this higher force translates into greater crispness, which is a very desirable feature in the case of products served in milk, as it extends the time of their softening after soaking [38].

The factor that significantly affected all the studied mechanical properties of the snacks was the dose of flaxseed pomace addition. It was found that the higher the addition of pomace was, the greater was the force needed to break the product (Figure 2e,f) and the smaller was its deformation at breaking (Figure 2c,d). The lowest force values were obtained for the snacks produced without the addition of flaxseed pomace (10.76 mN) and in those with the smallest pomace addition studied, i.e., 5% (11.76 mN). Statistically significantly higher force values were noted in the case of the snacks with 10% of pomace (14.99 mN) and the highest value of this parameter was determined in the snacks with 15% addition of flaxseed pomace (19.32 mN). The highest deformation values were recorded in the snacks without the addition of pomace (1.98 mm) and with 5% of flaxseed pomace (1.86 mm), and the lowest ones in the products with 15% of pomace (19.32 mm). Other authors reported similar results [6,12,25,36,39]. Analyzing the effect of the addition of various waste materials from the food industry on the mechanical properties of extruded products based on maize semolina, they found that these additives increased their hardness mainly due to the high fiber content. Fiber forms a compact structure with starch, which increases density of the extruded mass and hinders the growth of air bubbles, thereby makes the structure of the formed bubbles more rigid [20,36,40]. The observed effect is particularly pronounced when the added fiber consists in major part from water-insoluble fractions, which account for more than half of the fiber found in flaxseed pomace [40].

Also, fat increases the force necessary to break the extrudates and their hardness as a result of reducing friction and reducing the temperature inside the extruder during extrusion. Many authors have confirmed that starch complexes formed during extrusion with free fatty acids found in linseed oil can form a fraction of resistant starch, which, depending on the moisture content of the extruded mixture, can increase the mechanical strength of the snacks [35]. In addition, Ondo et al. [28] concluded that the interactions of proteins and lipids occurring inside the extruder also contributed to changes in the mechanical properties of the snacks. They demonstrated that the average size of the air bubbles in the extruded snacks decreased along with the increasing content of protein in the extruded material.

Probably, the synergistic effect of all the phenomena described above is the reason for the significant increase in the mechanical strength of the snacks observed in this study, and in particular in the breaking force, which in the case of the snacks with 15% flaxseed pomace addition increased almost twice as compared to the snacks produced without flaxseed pomace (Table 2). As already mentioned, the increase in the force necessary to break the snacks is not a drawback in the case of products classified as breakfast goods. The greater force translates into greater crispness, which extends the time of their softening in milk [38]. In addition, the fat contained in the extruded products will create a hydrophobic protective barrier and will further extend the time during which the snacks will retain the appropriate crispness in milk.

Table 3. The influence of the type and amount of pomace from flaxseed added and cocoa added on the results of the sensory analysis of snacks.

Type of Addition	Type of Pomace	Pomace Addition Level	Taste	Color	Consistance	Texture	Aroma
without cocoa powder	Golden flax	0	5.9 ± 0.4	6.1 ± 0.4	5.8 ± 0.8	5.3 ± 0.4	6.1 ± 0.7
		5	5.8 ± 0.6	5.3 ± 0.5	5.4 ± 0.7	5.4 ± 0.6	5.9 ± 0.6
		10	5.4 ± 0.5	4.4 ± 0.4	4.7 ± 0.7	4.5 ± 0.5	4.9 ± 0.7
		15	4.6 ± 0.4	3.7 ± 0.5	3.9 ± 0.6	3.9 ± 0.5	4.6 ± 0.8
	Common flax	0	5.9 ± 0.4	6.1 ± 0.4	5.8 ± 0.8	5.3 ± 0.4	6.1 ± 0.7
		5	6.0 ± 0.6	5.0 ± 0.5	5.4 ± 0.6	5.2 ± 0.5	6.0 ± 0.8
		10	5.0 ± 0.5	3.6 ± 0.6	5.0 ± 0.7	4.5 ± 0.6	4.8 ± 0.5
		15	4.3 ± 0.5	2.8 ± 0.3	4.2 ± 0.5	4.0 ± 0.8	4.6 ± 0.6
with cocoa powder	Golden flax	0	5.5 ± 0.5	6.1 ± 0.4	6.1 ± 0.4	5.8 ± 0.4	6.1 ± 0.4
		5	5.6 ± 0.6	5.8 ± 0.4	5.3 ± 0.6	5.9 ± 0.5	6.2 ± 0.5
		10	5.6 ± 0.6	5.4 ± 0.5	4.9 ± 0.6	5.1 ± 0.4	5.8 ± 0.4
		15	5.1 ± 0.7	5.0 ± 0.6	4.3 ± 0.8	4.8 ± 0.9	5.44 ± 0.5
	Common flax	0	5.5 ± 0.5	6.1 ± 0.4	6.1 ± 0.4	5.8 ± 0.4	6.1 ± 0.4
		5	5.7 ± 0.5	5.6 ± 0.3	5.7 ± 0.6	5.5 ± 0.8	6.1 ± 0.6
		10	5.5 ± 0.8	5.5 ± 0.6	5.2 ± 0.5	4.9 ± 0.8	5.7 ± 0.8
		15	5.0 ± 0.6	4.8 ± 0.7	4.7 ± 0.9	4.0 ± 0.8	5.2 ± 0.6
LSD Multiple Range test
Type of addition
without cocoa powder			5.4 a	4.6 a	5.0 a	4.8 a	5.4 a
with cocoa powder			5.4 a	5.5 a	5.3 a	5.2 a	5.8 a
Type of pomace
Golden flax			5.4 a	5.1 a	5.0 a	5.1 a	5.6 a
Common flax			5.4 a	4.9 a	5.2 a	4.9 a	5.6 a
Pomace addition level
0			5.7 a	6.1 a	5.9 a	5.6 a	6.1 a
5			5.8 a	5.4 b	5.4 b	5.5 a	6.1 a
10			5.4 b	4.7 c	4.9 c	4.8 b	5.3 b
15			4.8 c	4.1 d	4.3 d	4.2 c	5.0 b

For each column and per treatment (main effect), mean values with different letters are statistically different at p < 0.05.

presents the results of sensory evaluation of the snacks made from maize semolina with different contents of pomace from two varieties of flaxseed with and without added cocoa.

Analyzing the impact of only the type of pomace used on the sensory evaluation of the snacks, no differences were found in any of the evaluated parameters between the samples produced with pomace from golden and common flaxseed (Table 3). In other works which investigated the influence of flaxseed pomace types as additives to snacks, there were no differences in the assessment of attributes related to taste, consistency, texture or aroma, but the differences in color were very noticeable. Common flaxseed pomace contains more health-promoting compounds (polyphenols, fiber, etc.) than golden flaxseed pomace. Many of these health-promoting components are dark-colored, which means that products made with common flaxseed pomace addition are darker than those produced with golden flaxseed pomace. This effect is particularly noticeable when pomace is used as an additive to products usually characterized by a light color, e.g., corn snacks [6,12] or wheat bread [9]. The lack of significant differences in color scoring during sensory evaluation in this study results from the use of cocoa, whose dark color superimposed on the color of the snacks and worked as a camouflage, eliminating the difference between the snacks made with golden flax and common flax pomace. Since the panelists were informed that they were evaluating cocoa-added snacks, the dark color was expected and desired by them (even in those samples made without cocoa in which the dark color was owed to flaxseed pomace). A statistical analysis of the snacks produced with and without added cocoa showed differences between these products only in the assessment of their color, with snacks with added cocoa rated significantly higher (5.5) than those without added cocoa (4.6).

Statistically, the highest values of sensory quality attributes describing taste, texture, and aroma were obtained for the snacks produced without flaxseed pomace and in those with 5% pomace addition. In the case of the snacks with 15% of flaxseed pomace, statistically lower results compared to the snacks with 10% of pomace were obtained only in terms of taste and texture. Analyzing the color and consistency of the snacks, it was found that each of the levels of additives used had a significant impact on the results of their evaluation. The scores given by panelists for these parameters were inversely proportional to the dose of the additive used. Investigations conducted by other authors confirms these correlations [6,23]. The lower scores for consistency and texture of the snacks observed with the increasing addition of pomace were associated with the modified structure of the extrudates. Numerous but fine bubbles with thick walls were formed in the snacks with a higher proportion of flaxseed pomace, which increased the sensation of hardness [6,20,25,36]. Many authors associate this effect with the increase in fiber content [25,36]. The sensory quality and the level of acceptance of snacks depend mainly on their expansion ratio as well as their texture and color, but also worthy of notice is the statistically significant reduction in the scores given by panelists to the snacks with flaxseed pomace content exceeding 5%.

In the samples made with 10% and 15% of flaxseed pomace, the panelists noticed an unpleasant oily aroma. Probably, masking the oily aroma by adding extra aromas to the produced snacks (e.g., through the addition of spices, dried fruits, herbs, vanilla, spicy aromas, or coffee aromas) may improve their overall sensory evaluation and make them more desirable products on the food market [7].

Extrusion is perceived by many people as a process in which highly processed food is created, which may pose certain health risks. However, it should be emphasized that extrusion is a process involving the simultaneous action of temperature, pressure, and mechanical force on the raw material. Even taking into account the synergistic effect of these physical factors, they do not cause dangerous changes in extruded food. However, the extrusion process is used to produce crisps or breakfast cereals that contain preservatives, synthetic dyes, or refined sugar. These products are desired by society as convenience foods. Currently, the awareness of consumers is growing, and they are looking for both easy-to-prepare and low-processed food characterized by a simple composition. In order to meet these expectations, only high-quality, health-promoting, and natural ingredients that have a high health potential can be used in the composition of extruded mixtures. Replacing part of the cornmeal in the recipe of extruded products with ingredients rich in fiber (for example, flaxseed pomace) will not only contribute to improving the structure of dietary fiber consumption in society but will also reduce the glycemic index of these products, contributing to the reduction of the number of civilization diseases. Also, replacing unhealthy synthetic dyes with raw materials that can naturally modify the color (e.g., cocoa) can increase the health-promoting potential of the food produced and create a new range of products that are “easy to prepare but healthy”, which are not at the same time classified as ultra-processed food. The extruded snack products produced by the authors, containing flaxseed pomace and cocoa, can perfectly meet the requirements of conscious consumers looking for healthy and sensorially attractive food (Table 3).

4. Conclusions

The snacks produced with the addition of common flaxseed pomace and cocoa received high scores given by panelists, not diverging from those given to the snacks with golden flaxseed addition. In particular, the addition of cocoa contributed to a significant improvement in the sensory evaluation of the color of the snacks produced with dark pomace addition. However, in the opinion of the panelists, the addition of cocoa did not cause any deterioration of other parameters assessed compared to the snacks prepared without its addition. There were also no differences in the mechanical properties between the snacks with different pomace types added. Only the high additive dose (ca. 10%) resulted in deterioration of these properties. To sum up, the optimal formula of the extruded snacks with flaxseed pomace that would ensure high consumer scores and very good mechanical properties of products should include cocoa and up to 10% addition of common flax pomace.