Advancing Sustainable Nutrition: Enhancing Physical and Nutritional Qualities of Cookies with Apple Pomace Extrudates

Abstract

Apple pomace is a by-product of the apple processing industry and can be used for various uses such as animal feed, for composting, or to extract valuable compounds such as pectin or antioxidants. In recent years, it has also gained attention as a potential food ingredient due to its fibre content and antioxidant properties. The aim of this study was to examine the effect of three parameters: the percentage of wheat flour replaced by extrudate (5%, 10%, and 15%), the percentage of apple pomace in the extrudate (15%, 30%, and 45% based on the mass of corn grits), and the particle size of the extrudate (<250 µm; 250–1000 µm; 1000–2000 µm) on the properties of cookies, using the Box–Behnken experimental design. The addition of extrudates enriched with apple pomace significantly increased the total fibre and ash content of the cookies. The hydroxymethylfurfural content also increased, but not above the permitted limits (25 mg/kg). The sensory quality was strongly influenced by the particle size, especially the hardness, graininess, and appearance of the cookies. The addition of extrudate led to a darker colour of the cookies and a significant increase in the proportion of red tones, but generally had no negative influence on the acceptability of the cookies and their microbiological stability during the 6-month storage period.

1. Introduction

Apple pomace makes up almost 25% of the total weight of the apple fruit left behind after processing. It is primarily rich in dietary fibre (35–60%). However, unlike cereals, which contain a larger portion of insoluble dietary fibres (cellulose and hemicellulose), apple pomace contains significant amounts of soluble dietary fibres, primarily pectin, which is used as a gelling agent and binder and can be used as a stabilizer in food products [1]. These properties are due to the porous matrix structure formed by polysaccharide chains, which incorporate a large amount of water through hydrogen bonds. The main components of these fibres are pectin (5.5–11.7%), cellulose (7.2–43.6%), hemicellulose (4.2–24.4%), lignin (15.3–23.5%), and gums [2]. In addition to dietary fibre, the apple pomace is also a significant source of minerals, carbohydrates, phenolics, and antioxidant compounds [3]. The apple pomace has found a special place in the production of biscuits and related products. An example of the use of this by-product is the powder obtained from apple peel, which can be used as a substitute for wheat flour in the production of muffins in an amount of up to 16%. This substitution increases the proportion of dietary fibre and polyphenolic compounds without worsening the sensory properties. [4]. Usman et al. [5] came to the same conclusion when they investigated the addition of apple pomace in the production of cookies. Based on the sensory and compositional properties, they concluded that good quality cookies with improved organoleptic properties can be prepared using 10% apple pomace powder. By adding apple pomace, the products acquire a characteristic, fruity aroma and a darker colour. Due to the pleasant aroma of this fruit pomace, it is possible to reduce the amount of sugar in the dough. However, if added in larger quantities during the production of biscuits, apple pomace can cause an unpleasant aroma, a colour that is too dark, and a lower volume of the product. Sudha et al. [6] concluded that the sensory quality of cookies (colour, grain, texture, and eating quality) deteriorates significantly when more than 30% apple pomace is added. Usman et al. [5] stated that no more than 10% of apple pomace should be used in the production of cookies, while according to Oh et al. [7] and Jung et al. [8], the optimal amount is 20%. With up to 20% apple pomace, the cookies had a pleasant fruit aroma and were softer, chewier, and moister than the control. The addition of apple pomace can significantly affect the width and thickness of the cookies. According to several different studies, the addition of apple pomace can cause a reduction in width and thickness of the cookies. Usman et al. [5], García et al. [9], Meilgaard et al. [10], and Zlatanović et al. [11] showed that the size of the apple pomace particles used in cookies is also very important. They concluded that coarse apple pomace particles performed better, both regarding functional properties and sensorial attributes in comparison to fine apple pomace particles. According to their opinion, even 50% of apple pomace can be added to the cookies without compromising products’ acceptability. Their cookies retained a pleasant apple taste and crunchy texture throughout 12 months of storage. In addition to cookies, the apple pomace has been successfully used in the production of other bakery products such as bread [12,13,14,15], pastry products [16], muffins [17,18,19,20,21], and cake [22,23,24,25].

Apple pomace contains a significant amount of moisture, and it is necessary to process it to make it easier to handle and store for a longer period. Compared to other methods of raw material preparation, the extrusion process has numerous advantages. By simply changing the process parameters or the raw materials during extrusion, a varied range of products can be produced on very easy-to-use equipment. Compared to other heat treatment procedures, energy losses and operating costs are lower and large investments are not required. High productivity, continuity of the process, and quick quality control are possible, and significantly, high-quality products are obtained due to the increase in the digestibility of proteins and starch and the reduction in the number of microorganisms. The amount of waste (by-products) is very small and can even be used in the process, including materials labelled as waste in another process [26]. In addition, the extrusion process deactivates unwanted enzymes, inactivates some anti-nutritional factors (trypsin inhibitors, hemagglutinins, tannins, and phytates), sterilises the final product, and retains the natural colours and flavours of the food [27,28].

The aim of this work was to examine the influence of substituting part of the wheat flour with extruded snack products enriched with apple pomace on the properties of the cookies. Extruded corn meals with added apple pomace were prepared at ratios of corn meal to by-product of 85:15, 70:30, and 55:45. The resulting snack products were milled and sieved to obtain three fractions with different particle sizes (<250 μm, 250–1000 μm and 1000–2000 μm). These extrudates were then used to replace wheat flour in cookie production at quantities of 5%, 10%, and 15%.

2. Materials and Methods

2.1. Materials

The apple pomace was produced using a hand press. The chemical composition of the obtained apple pomace was as follows: moisture 11.93%, protein 2.48%, fat 0.00%, ash 1.86%, and total dietary fibre 40.47%. Corn grits were obtained from “Žito” company Ltd., Osijek (Croatia), and wheat flour for cookie production was obtained from the milling company “Ratar“ Pančevo (Serbia). Vegetable fat (palm oil) was obtained from the oil factory “Dijamant” Zrenjanin (Serbia). Powdered sugar, salt, sodium bicarbonate, and ammonium bicarbonate were obtained from a local store. Apple pomace extrudates (APEs) are produced as was described in our previous work [29]. Three types of extrudates were obtained with ratios of corn grits to apple pomace of 85:15, 70:30, and 55:45. After grinding and sieving, three fractions of extrudates were obtained: fraction 1: particles < 250 μm; fraction 2: particles from 250 to 1000 μm; and fraction 3: particles from 1000 to 2000 μm (Figure 1).

2.2. Methods

2.2.1. Chemical Composition

The chemical composition of wheat flour, apple pomace extrudates, and prepared cookies was determined according to the methods described in the AOAC (protein (No. 950.36), fat (No. 935.38), total, soluble, and insoluble dietary fibre (No. 958.29), ash (No. 930.22), and moisture (No. 926.5)) [30]. The factors, n = 5.70 (for wheat flour) and n = 6.25 (for extrudates and cookies) were used for the conversion of nitrogen to crude protein. The analyses were carried out in triplicates.

2.2.2. Colour of Apple Pomace Extrudates

The colour of apple pomace extrudates was measured in triplicate using a MINOLTA Chroma Meter CR-400 (Minolta Co., Ltd., Osaka, Japan). The CIE L*, a*, b* colour coordinates (where L* represents lightness, a* indicates redness to greenness, and b* signifies yellowness to blueness*) [31] were determined using D-65 lighting, a 2-standard observer angle, and an 8-mm aperture in the measuring head.

2.2.3. Particle Size Distribution of Apple Pomace Extrudates

Particle size distribution analysis has been conducted using a Mastersizer 2000 laser diffraction particle size analyser (Malvern Instruments, Malvern, England), and the Scirocco unit was used for dispersing extrudates in the air. The results were processed using Mastersizer 2000 software and presented as volume mean diameter D[4,3] and the parameters d(0.1), d(0.5), d(0.9) that represent the particle sizes where 10%, 50%, and 90% of the total particle volume that include particles that are smaller than that size.

2.2.4. Experiment Planning and Statistical Analysis

The Box–Behnken experimental design was used to evaluate the impact of three parameters. The input factors were set on three levels: factor F—particle size of the extrudate (<250 µm—granulation 1; 250–1000 µm—granulation 2; 1000–2000 µm—granulation 3), factor T—proportion of by-product in the extrudate (15%, 30%, and 45% based on the mass of corn grits), and factor E—the percentage of wheat flour replaced with extrudate (5%, 10%, and 15% based on the mass of the flour). All input factors are distributed across three levels, and the measured variables are presented in Table S1. The chosen design recommended 15 combinations of factors (15 experiments) with 3 central points, which give sufficient information for testing the lack of fit. The quadratic model obtained from the Box–Behnken experimental design is given as:

R = β₀ + β₁A + β₂B + β₃C + β₁₂AB + β₁₃AC + β₂₃BC + β₁₁A² + β₂₂B^{2 +} β₃₃C²

(1)

where R is a dependent variable; β₀ is an intercept; β₁ to β₃₃ are regression coefficients; and A, B, and C are independent variables. The terms AB, AC, and BC represent interactions of input factors, while A², B², and C² are quadratic terms. The sufficiency of the devised model and statistical significance in the regression coefficients were evaluated by applying the analysis of variance (ANOVA). A positive (negative) sign of the coefficient of the regression equation indicates that the output quantity increases (decreases) as the factor increases. The experimental values of the dependent variables were analysed using Statistica 12 and Design Expert 7 (trial version) [32].

2.2.5. Preparation of Cookies

Mixtures of wheat flour and APE were combined using the F-6-RVC agitator (Forberg International AS, Oslo, Norway). Wheat flour in the mixtures was substituted with APE in the amount of −5, 10, and 15%. Each cookie sample consisted of 200 g of wheat flour/APE mixtures, 42.00 g vegetable fat, 70 g sugar, 0.6 g sodium bicarbonate (NaHCO3), 0.4 g ammonium bicarbonate (NH₄HCO₃), and 1.1 g sodium chloride (NaCl). The amount of water was adjusted to achieve a dough with 22% moisture content. After mixing, the dough was rolled out to a thickness of 5 mm and cut using a 6 cm diameter mould. Cookies were baked for 10 min at 180 °C in a laboratory oven, packed in aluminium foil and a polyethylene bag and stored at room temperature for further analysis [33]. Sample names and factor combinations are given in

Table 1. Cookie sample names and factor combinations.

Cookie Sample		Factor
	F	T	E
J-3.45.10	3	45	10
J-1.30.5	1	30	5
J-1.30.15	1	30	15
J-2.45.15	2	45	15
J-2.15.15	2	15	15
J-2.30.10	2	30	10
J-2.45.5	2	45	5
J-3.15.10	3	15	10
J-3.30.15	3	30	15
J-2.30.10	2	30	10
J-1.45.10	1	45	10
J-1.15.10	1	15	10
J-3.30.5	3	30	5
J-2.15.5	2	15	5
J-2.30.10	2	30	10
Control	/	/	/

F—particle size of the extrudate (<250 µm—granulation 1; 250–1000 µm—granulation 2; 1000–2000 µm—granulation 3); T—the proportion of by-products in the APE; E—the percentage of wheat flour replaced with APE (5%, 10%, and 15% based on the mass of the flour).

.

2.2.6. Cookie Colour

The colour of cookie samples was measured in triplicate as was described in Section 2.2.2, 24 h after baking. The total colour change was calculated according to Formula (2):

ΔE = [(L* − L*₀)² + (a* − a*₀)² + (b* − b*₀)²]^1/2

(2)

The parameters with the index “0” indicate the colour values for the control sample made from wheat flour without APE. The colour difference is not visible to the human eye if the value of ΔE is less than 0.2; very little visibility exists if the value of ΔE is between 0.2 and 1; a value between 1 and 3 indicates a slightly visible difference; values between 3 and 6 indicate an average visible colour difference; ΔE values above 6 indicate high visibility of the difference between the samples [34].

2.2.7. Cookie Hardness

The hardness was determined using a texture analyser TA.HD Plus (Stable Micro systems, Godalming, UK), utilizing the cutting method for hardness measurement (hardness measurement of biscuit by cutting, BIS2/KB, Texture Exponent 32 software version 4.0.11.0 Stable Micro Systems, Godalming, Surrey, UK). The measurements were conducted in three replicates using a knife edge with a slotted insert (HDP/BS) and 25 kg load cell with the following operating parameters: 1.5 mm/s pre-test speed, test speed of 2 mm/s, 10 mm/s post-test speed, and 5 mm distance. The resulting force–deformation curve was analysed with the Texture Exponent 32 software version 4.0.11.0 (Stable Micro Systems, Godalming, Surrey, UK), and hardness values (mean maximum force values) were automatically obtained.

2.2.8. Determination of Hydroxymethylfurfural (HMF) in Cookies

The extraction was carried out according to the method described by Rufian-Henares et al. [35] with some modifications [36]. Ten grams of the sample were dissolved in 5 mL of a mixture of water:methanol (70:30, v/v). The mixture was stirred for 1 min, then 2.0 mL of Carrez I and Carrez II solutions were added, and the mixture was centrifuged at 5000 rpm (4 °C) for 15 min. The supernatant was separated into a 15-mL volumetric flask. The extraction procedure was repeated two more times with 2 mL of a water-methanol mixture (70:30) until 10 mL of supernatant were collected. Two millilitres of the supernatant were centrifuged at 8000 rpm for 15 min before analysis.

2.2.9. Microbiological Analysis

Cookie samples were analysed 24 h after baking and then monthly for a period of 6 months, to examine the influence of adding by-product extrudate on the durability of cookies. Cookies were stored at room conditions. For the microbiological analysis, twenty grams of each sample were homogenized for 10 min at 200 revolutions (Unimax 1010, Heidolph, Germany) in 180 mL of 1 g/L buffered peptone water (Merck, Darmstadt, Germany), and then prepared sequences of decimal dilution (up to 10⁻³). One millilitre of each dilution was placed in a sterile Petri plate and covered with the appropriate medium depending on the type of microorganism tested. The total number of aerobic mesophilic bacteria was determined on Plate Count Agar (PCA) (Merck, Darmstadt, Germany) after incubation at 30 °C for 72 h [37]; the total number of aerobic mesophilic sporogenic bacteria were determined on PCA after thermal treatment (90 °C, 5 min) of basic dilution and incubation at 30 °C for 72 h; total yeast and mould counts were determined on Dichloran 18% Glycerol (DG18) agar (Merck, Darmstadt, Germany) after incubation at 25 °C for 5 days [38]; the total number of Escherichia coli was determined on Tryptone blue glucuronic (TBX) agar (Merck, Darmstadt) after incubation at 37 °C for 24–48 h [39]; the total number of Clostridium perfringens was determined on Sulfite cycloserine (SC) agar (Merck, Darmstadt) after incubation under anaerobic conditions at 37 °C for 24–48 h [40]; the total number of Enterobacteriaceae was determined in Violet Glucose Agar (VRBGA) (Merck, Darmstadt, Germany) after incubation at 37 °C for 24–48 h [41]. After incubation, typical and atypical grown colonies were identified by microscopic observation of cell morphology and also by using biochemical tests. The results are expressed as the number of colony-forming units per gram (cfu/g).

2.2.10. Sensory Analysis

Cookie samples were evaluated by a panel of 10 trained panellists who were trained according to the ISO 8586 [42] standard. Each training session lasted 2 h. Initially, the panellists were introduced to the descriptors that can be used to evaluate cookies [43]. Subsequently they were trained to recognise extremely unfavourable sensory attributes. In the last section, the evaluators developed and finalized a list of six descriptors to be used in the test. The intensity of each attribute is rated on a 7-point intensity scale (1 = lowest intensity—7 = highest intensity) [44]. The evaluated attributes and their corresponding intensity scales were as follows: colour (intensity of colour on the surface: from 1—extremely light to 7—extremely dark); appearance of the surface (number of cracks on the surface of the cookies: from 1—many to 7—none); hardness (the ease with which the samples can be broken into two parts: from 1—extremely soft to 7—extremely hard); granularity (proportion of small solid particles between the teeth during chewing: from 1—a lot to 7—none); shape (from 1—regular to 7—deformed); taste (from 1—bad, foreign to 7—characteristic, aromatic). The evaluation of the cookies was carried out 24 h after baking in the sensory laboratory of the Faculty of Technology, University of Novi Sad, which was designed according to the ISO 8589 [45] standard. Cookie samples were placed on white plastic plates, coded with three-digit codes generated from a random number table, and each panellist rated five samples per session.

3. Results

3.1. Chemical Composition of Apple Pomace Extrudates

The results of the chemical analysis of apple pomace extrudates are shown in

Table 2. Chemical composition of apple pomace extrudates and wheat flour.

Apple Pomace Extrudate	Particle Size (µm)	By-Product Share (%)	Moisture %	Fat %	Protein %	Ash %	Total Fibre %	Insoluble Fibre %
APE - 1 - 15	1	15	10.3 ± 0.22 c	0.51 ± 0.05 d	5.47 ± 0.12 c	0.61 ± 0.0.1 b	7.64 ± 0.32 b	5.53 ± 0.04 b
APE - 2 - 15	2	15	10.2 ± 0.31 c	0.18 ± 0.02 a	6.23 ± 0.10 d	0.62 ± 0.05 b	7.76 ± 0.12 b	5.62 ± 0.11 b,c
APE - 3 - 15	3	15	9.50 ± 0.05 b	0.21 ± 0.03 a	6.05 ± 0.08 d	0.51 ± 0.02 a	8.58 ± 0.22 c	5.90 ± 0.25 c
APE - 1 - 30	1	30	8.98 ± 0.15 a	0.50 ± 0.07 d	5.27 ± 0.04 b	0.89 ± 0.07 c	16.37 ± 0.13 d	9.38 ± 0.25 d
APE - 2 - 30	2	30	8.89 ± 0.21 a	0.26 ± 0.04 b	5.19 ± 0.11 b	0.93 ± 0.05 c	16.92 ± 0.24 d	9.82 ± 0.11 e
APE - 3 - 30	3	30	8.44 ± 0.29 a	0.26 ± 0.02 b	5.27 ± 0.06 b	0.93 ± 0.02 c	18.33 ± 0.44 e	11.97 ± 0.33 f
APE - 1 - 45	1	45	10.4 ± 0.07 c	0.86 ± 0.08 e	4.42 ± 0.05 a	1.15 ± 0.06 d,e	18.18 ± 0.23 e	12.21 ± 0.09 g
APE - 2 - 45	2	45	10.6 ± 0.11 c	0.49 ± 0.03 c	4.49 ± 0.02 a	1.02 ± 0.09 d	19.03 ± 0.16 f	12.50 ± 0.27 h
APE - 3 - 45	3	45	10.6 ± 0.10 c	0.39 ± 0.05 c	4.45 ± 0.06 a	1.36 ± 0.07 e	19.11 ± 0.11 f	12.42 ± 0.44 h
Wheat flour	/	/	10.8 ± 0.25 c	0.94 ± 0.02 e	10.0 ± 0.12 e	0.54 ± 0.04 b	3.57 ± 0.17 a	1.89 ± 0.08 a

Means ± SD with different superscript letters (a–h) in the same column differ significantly (p < 0.05).

. The results clearly show that all extrudates contain a statistically significant higher amount of dietary fibre compared to wheat flour. As the proportion of the by-product increased, the content of total and insoluble fibre increased. All samples with the addition of 45% apple pomace have a statistically significantly higher dietary fibre content compared to samples with the addition of 15% apple pomace. Additionally, it has been observed that the particle size of the extrudate has a significant impact on the dietary fibre content in samples with the same amount of apple pomace added. Samples with the largest particles (fraction 3) had statistically significantly higher values (p < 0.05) for dietary fibre content compared to samples with smaller particles (fraction 1 and 2).

The ratio of soluble to insoluble fibres is very important from a nutritional point of view, as both soluble and insoluble fibres have important physiological functions. The dietary intake of insoluble fibre is important for water absorption and proper intestinal function, while the intake of soluble fibre helps reduce blood cholesterol levels and glucose absorption. In their effect on human health, these two fibre fractions complement each other, and it is generally assumed that the proportion of soluble fibre should be 30–50%, and the proportion of insoluble fibre 50–70% of the total fibre mass, i.e., their ratio should be approximately 1:2 [46]. As can be seen in Table 2, the proportion of insoluble fibre in the total fibre mass of apple pomace extrudates is between 57.29% and 72.42%, and it can be said that apple pomace extrudates have the most favourable ratio of soluble to insoluble fibres.

The protein content decreases as the proportion of apple pomace in the extrudate increases, as apple pomace contains a lower protein content than corn grits [47]. It was observed that the fat content is statistically significantly higher (p < 0.05) in extrudates with smaller particle sizes (with particles smaller than 250 µm) and decreases as the proportion of apple pomace increases. Samples with the addition of 45% apple pomace are statistically significantly different in ash content from samples with the addition of 15% and 30% apple pomace. In all extrudates, the amount of minerals increases significantly with the increase in the proportion of this by-product.

3.2. Physical Characteristics of Apple Pomace Extrudates

When apple pomace is added, the L* value, i.e., the lightness of the extrudate, decreases significantly with the increase in the proportion of apple pomace and with the increase in particle size (

Table 3. Colour and particle size distribution parameters of apple pomace extrudates and wheat flour.

Apple Pomace Extrudate	Particle Size (µm)	By-Product (%)	L*	a*	b*	d(0.1) (µm)	d(0.5) (µm)	d(0.9) (µm)	D[4,3] (µm)
APE - 1 - 15	1	15	45.14 ± 0.85 g	9.60 ± 0.77 e	20.73 ± 0.58 h	52.22 ± 0.11 c	154.6 ± 1.55 d	303.81 ± 2.44 c	168.21 ± 2.55 c
APE - 2 - 15	2	15	33.15 ± 0.44 c	8.50 ± 0.84 d	10.27 ± 0.22 c	350.1 ± 0.45 f	689.2 ± 1.87 g	1231.6 ± 2.46 d	744.45 ± 1.25 f
APE - 3 - 15	3	15	29.61 ± 0.54 a	4.50 ± 0.21 b	5.31 ± 0.45 a	620.6 ± 1.44 h	1019.6 ± 2.47 i	1572.2 ± 1.23 f	1061.3 ± 1.74 i
APE - 1 - 30	1	30	44.71 ± 1.12 g	9.42 ± 0.16 e	20.40 ± 0.32 h	33.17 ± 0.12 b	122.6 ± 2.15 b	261.7 ± 2.35 b	137.27 ± 4.11 b
APE - 2 - 30	2	30	36.78 ± 0.25 e	8.91 ± 0.41 d	14.17 ± 0.08 e	243.6 ± 0.45 d	505.5 ± 0.89 e	951.15 ± 4.74 c	555.01 ± 3.12 d
APE - 3 - 30	3	30	32.85 ± 0.56 b,c	5.30 ± 0.25 c	8.28 ± 0.36 b	626.75 ± 0.87 i	1033.5 ± 4.22 j	1558.5 ± 2.55 e	1059.1 ± 2.11 h
APE - 1 - 45	1	45	42.68 ± 0.98 f	9.53 ± 0.20 e	19.16 ± 0.22 g	33.03 ± 0.05 b	126.9 ± 1.15 c	263.37 ± 1.85 b	139.85 ± 3.52 b
APE - 2 - 45	2	45	35.97 ± 0.12 d	8.35 ± 0.55 d	13.29 ± 0.27 d	256.7 ± 0.64 e	535.0 ± 0.35 f	995.39 ± 5.78 c	583.74 ± 0.98 e
APE - 3 - 45	3	45	31.78 ± 0.75 b	5.16 ± 0.54 c	8.70 ± 0.21 b	607.04 ± 1.15 f	1002.4 ± 5.44 h	1559.4 ± 3.11 e	1046.3 ± 1.25 g
Wheat flour	/	/	89.20 ± 0.74 h	-2.54 ± 0.31 a	18.26 ± 0.12 f	18.03 ± 0.09 a	81.19 ± 0.15 a	182.89 ± 1.25 a	92.36 ± 0.54 a

L*: lightness; a*: proportion of red/green tone; b*: proportion of yellow/blue tone; D[4,3]: volume mean diameter; d(0.1), d(0.5), d(0.9) represent the particle sizes where 10%, 50% and 90% of the total particle volume include particles that are smaller than that size. Means ± SD with different superscript letters (a–j) in the same column differ significantly (p < 0.05).

). The proportion of red tone significantly decreases with increasing particle size of apple pomace extrudate. The apple pomace extrudates with the largest particles have the lowest b* values. All samples of apple pomace extrudate have significantly lower L* values and significantly higher proportions of red tone compared to wheat flour.

The values of the parameters d(0.1), d(0.5), d(0.9), and D[4,3] of wheat flour and all extrudate samples used in the experiment are presented in Table 3. The value d(0.5) represents the median mass diameter of the volume distribution of particles, expressed in microns, indicating that 50% of the sample particles are smaller than this value. The d(0.1) indicates that 10% of the sample particles are smaller than this value, while d(0.9) indicates that 90% of the particles are smaller and 10% are larger than this value. The D[4,3] value represents the volume mean diameter and signifies the average diameter of all particles [48]. The volume mean diameter (D[4,3]) value of the particle size distribution of wheat flour, which is 92.36 µm, is significantly smaller than the D[4,3] values of the particle size distributions of all extrudate samples. Generally, regardless of the type of by-product, there is a gradual decrease in the D[4,3] values with increasing proportion of the by-product in the extrudate (15–45%).

3.3. Chemical Composition of Cookies

The addition of APEs significantly increased the total fibre content in the cookies as shown in

Table 4. Box–Behnken experimental design and chemical characteristics of cookies.

Sample	Factor Combinations			Responses
	F	T	E	JH1 %	JH2 %	JH3 %	JH4 %	JH5 %	JH6 %	HMF mg/kg
J-3.45.10	3	45	10	4.42	4.84	13.20	0.60	4.56	2.11	4.18
J-1.30.5	1	30	5	4.61	5.29	13.21	0.50	3.91	1.65	4.10
J-1.30.15	1	30	15	4.86	4.05	13.42	0.51	4.80	2.10	14.6
J-2.45.15	2	45	15	4.76	4.98	13.33	0.46	5.42	3.56	15.2
J-2.15.15	2	15	15	4.80	4.40	13.12	0.43	4.06	2.80	3.54
J-2.30.10	2	30	10	4.49	4.75	13.35	0.47	4.36	2.20	3.91
J-2.45.5	2	45	5	4.85	5.23	13.16	0.51	4.31	2.82	7.89
J-3.15.10	3	15	10	4.37	4.74	13.18	0.43	3.37	2.49	10.1
J-3.30.15	3	30	15	4.64	5.04	12.87	0.42	4.58	3.03	2.21
J-2.30.10	2	30	10	4.54	4.79	13.27	0.45	4.30	2.10	6.90
J-1.45.10	1	45	10	5.10	4.69	13.48	0.50	4.60	2.80	4.02
J-1.15.10	1	15	10	4.50	4.86	13.36	0.49	3.82	2.70	4.63
J-3.30.5	3	30	5	4.58	5.19	13.26	0.46	3.96	1.64	7.43
J-2.15.5	2	15	5	4.58	5.24	13.23	0.41	3.51	1.31	7.69
J-2.30.10	2	30	10	4.59	4.72	13.22	0.49	4.45	2.10	7.32
Control	/	/	/	5.02	7.50	13.98	0.45	2.02	1.19	1.47

F—particle size of the extrudate (<250 µm—granulation 1; 250–1000 µm—granulation 2; 1000–2000 µm—granulation 3); T—proportion of apple pomace in the APE; E—the percentage of wheat flour replaced with APE (5%, 10%, and 15% based on the mass of the flour); JH1: Moisture; JH2: Protein; JH3: Fat; JH4: Ash; JH5: Total fibre; JH6: Insoluble fibre; HMF: hydroxymethylfurfural (mg/kg).

. According to the Regulation on health claims [49] and the Rulebook on declaration and labelling of packaged foods [50], a food product can carry the statement “source of fibre” if it contains more than 3 g of dietary fibre per 100 g of product or more than 1.5 g of dietary fibre per 100 kcal of product. It can be labelled as “high fibre” if it contains at least 6 g of fibre per 100 g of product. According to the results of this research, all samples with the addition of APE can carry the statement “source of fibre” based on their dietary fibre content.

Generally, apple pomace contains a well-balanced ratio of insoluble and soluble dietary fibres (2:1) as already stated in the Section 3.1. The extrusion process itself favourably affects this ratio [47], and the optimal ratio of insoluble to soluble fibres was maintained in cookie samples containing extrudates of apple pomace (Table 2). The increase in fibre content in cookie samples is significantly influenced by factors T and E; that is, the fibre content increases with the higher proportion of extrudates in the cookies and with the higher proportion of apple pomace in the extrudate. This is indicated by the positive values of coefficients β₂ and β₃ for responses JH5 and JH6 in Table S2.

The ash content in cookies increased proportionally with the higher proportion of apple pomace in the extrudate (factor T) (Table 4 and Table S2). According to O’Shea et al. [1], apple pomace is a rich source of minerals such as calcium, zinc, iron, copper, and magnesium. Regarding protein content, the model did not prove to be statistically significant, meaning that the experimental design did not identify which factors have a statistically significant influence on the protein content in the cookie samples. Based on the values obtained for the chemical composition in Table 4, it is evident that replacing wheat flour with APE (5–15%) resulted in a decrease in the protein content compared to the control sample made only from wheat flour, which has a protein content of 5.49% (Table 2). This reduction is due to the lower protein content in the apple pomace (Table 2).

During the baking of cookies, one of the key products in the browning process, HMF, is formed, which can be converted into the cytotoxic and mutagenic compound 5-sulfoxymethyfurfural [51]. With the addition of APE, there was a significant increase in the HMF content in the samples (up to 15.29 mg/kg) compared to the control (1.47 mg/kg). However, all samples had HMF content of less than 25 mg/kg, which is the maximum allowed amount indicated by the European Food Safety Organization (EFSA). The content of HMF increased with an increase in the proportion of APEs in the cookies and with an increase in the proportion of apple pomace in the extrudate. Apple pomace is rich in carbohydrates, so the reactions that lead to the generation of HMF are more intense. Jozinović [52], in an analysis of extrudates enriched with apple pomace, demonstrated a significant increase in HMF content with higher proportions of this by-product in the extrudate.

3.4. Cookie Colour and Hardness

As can be seen from the results shown in

Table 5. Box–Behnken experimental design and cookies colour and hardness.

Sample	Factor Combinations			Responses
	F	T	E	JF1	JF2	JF3	JF4	Δ E
J-3.45.10	3	45	10	168.93	56.88	8.99	25.25	16.94
J-1.30.5	1	30	5	196.30	58.11	9.77	26.77	16.52
J-1.30.15	1	30	15	244.86	41.10	13.33	22.29	33.01
J-2.45.15	2	45	15	199.39	49.49	11.35	23.84	24.52
J-2.15.15	2	15	15	155.24	61.82	3.24	20.71	10.59
J-2.30.10	2	30	10	172.75	60.68	7.70	25.31	13.07
J-2.45.5	2	45	5	163.34	65.08	5.21	24.90	8.05
J-3.15.10	3	15	10	148.77	63.74	1.89	21.85	8.12
J-3.30.15	3	30	15	93.00	62.98	4.80	25.01	9.56
J-2.30.10	2	30	10	160.54	64.92	7.02	24.26	9.39
J-1.45.10	1	45	10	203.95	43.76	13.2	23.97	30.50
J-1.15.10	1	15	10	189.38	59.88	8.99	25.25	14.48
J-3.30.5	3	30	5	134.94	76.01	3.03	23.57	5.62
J-2.15.5	2	15	5	161.13	77.25	0.71	19.06	7.98
J-2.30.10	2	30	10	165.99	62.83	7.39	25.00	11.17
Control	/	/	/	135.86	83.05	0.20	21.34	/

F—particle size of the extrudate (<250 µm—granulation 1; 250–1000 µm—granulation 2; 1000–2000 µm—granulation 3); T—the proportion of by-products in the APE; E—the percentage of wheat flour replaced with APE (5%, 10%, and 15% based on the mass of the flour); JF1: hardness (N); JF2: L* (lightness); JF3: a* (proportion of red/green tone); JF4: b* (proportion of yellow/blue tone).

and Table S3, as well as in Figures S1 and S2, particle size (factor F) had the most significant negative effect on the cookie hardness (response JF1). Increasing the particle size of the added extrudate resulted in decreased hardness (negative values of coefficient β₁ for response JF1 in Table S3), whereas increasing the proportion of extrudate (factor E) increased the hardness (positive values of coefficients β₃ for the same response).

The highest value for cookies hardness (244 N) was obtained for the sample with the smallest particle size and the highest proportion of extrudate (sample J-1.30.15). In contrast, the lowest hardness (93.0 N) was recorded for the sample containing 15% APE with the largest particles (sample J-3.30.15). Factor T (the proportion of apple pomace in the extrudate) did not have a statistically significant effect on the cookie’s hardness (p > 0.05), as can be seen in Table S3. The higher hardness values of cookie samples containing smaller extrudate particles stem from a denser packing of these particles. The cookie samples with larger APE particles (fraction 3) had larger air-filled spaces in their structure. Additionally, larger particles hinder the formation of a continuous gluten network during mixing, further contributing to brittle structure and cookie hardness. The increase in hardness with an increase in the proportion of APE (Factor E) may be attributed to the higher fibre content in the apple pomace. These fibres absorb water during kneading, leading to increased dough hardness and subsequently affecting the final product’s hardness. The same results were obtained by Kruczek et al. [53] who observed increased cookie hardness following the incorporation of apple pomace from 15% to 60%.

Colour is a parameter strictly related to thermally treated products’ quality and safety. The colour on the surface of cookies is an essential characteristic influencing product acceptability. Substituting wheat flour with apple pomace extrudates reduced the cookies’ lightness and increased the proportion of red tones, while the impact on the proportion of yellow tones was minimal.

The lightness of cookies, (L* value), significantly depends on both the particle size (factor F) and the proportion of APEs in the cookies (factor E). The effects of these factors on the L* value exceed 30%, as shown in Figure S1. Lightness decreased significantly when extrudate with a smaller particle size replaced wheat flour (positive sign of the coefficient β₁ for the response JF2). Additionally, the proportion of apple pomace in the extrudate (factor T) also had a significant effect on the lightness of the colour. The lightness decreased with the increase in the proportion of APE in cookies and with the increase in the apple pomace content in the APE, as indicated by the negative coefficients β₂ and β₃ for the response JF2, as shown in Table S3. With an increase in the proportion of apple pomace in the extrudate, the proportion of the red tone in the cookie’s colour increased (positive value of coefficient β₂ for response JF3 in Table S3). Factors F and E significantly influenced the a* value of the colour in these samples - the proportion of red tone also increased with higher levels of factor E (positive coefficient β₃ for response JF3 in Table S3), while factor F had a negative impact on this parameter (negative values of coefficient β₁ for response JF3 in Table S3).

The ΔE values, indicating the colour difference visible to the human eye between samples with the addition of extrudate and the control sample made of wheat flour, exceed 6 for all samples indicating a very significant difference compared to the control sample. The content of HMF in the samples is significantly correlated with the colour of the cookies because surface darkening results from caramelisation and the Maillard reaction, both processes in which HMF is formed [54]. Additionally, the results indicate that the L* values were lowest in samples with the highest HMF content.

3.5. Sensory Analysis

The results of the sensory analysis are shown in

Table 6. Box–Behnken experimental design and sensory characteristics of cookies.

Sample	Factor Combinations			Responses
	F	T	E	JS1	JS2	JS3	JS4	JS5	JS6
J-3.45.10	3	45	10	6.08	1.56	4.68	4.56	2.10	6.05
J-1.30.5	1	30	5	6.56	6.32	6.43	5.41	6.62	6.45
J-1.30.15	1	30	15	6.85	5.41	6.08	6.01	6.48	6.52
J-2.45.15	2	45	15	6.25	3.00	5.33	5.33	5.02	6.65
J-2.15.15	2	15	15	6.07	2.75	5.09	4.11	5.23	6.21
J-2.30.10	2	30	10	5.92	3.90	5.88	4.70	5.41	6.68
J-2.45.5	2	45	5	5.81	5.98	6.18	4.39	5.58	6.51
J-3.15.10	3	15	10	5.84	2.83	5.29	3.98	2.45	6.11
J-3.30.10	3	30	15	6.15	1.64	5.11	2.91	2.06	5.96
J-2.30.10	2	30	10	6.17	4.10	5.76	4.31	5.48	6.64
J-1.45.10	1	45	10	6.71	5.32	5.96	5.47	6.54	6.60
J-1.15.10	1	15	10	6.63	5.86	5.88	5.06	6.50	6.57
J-3.30.5	3	30	5	5.61	3.75	4.94	3.78	3.86	6.23
J-2.15.5	2	15	5	5.72	5.81	6.21	4.29	6.04	6.75
J-2.30.10	2	30	10	5.95	4.21	5.81	4.38	5.82	6.59

F—particle size of the extrudate (<250 µm—granulation 1; 250–1000 µm—granulation 2; 1000–2000 µm—granulation 3); T—proportion of by-products in the APE; E—the percentage of wheat flour replaced with APE (5%, 10%, and 15% based on the mass of the flour); JS1: Colour; JS2: Surface appearance; JS3: Shape; JS4: Hardness; JS5: Graininess; JS6: Taste.

, and the appearance of the samples is shown in Figure 2. Factor F, representing the particle size of the added extrudate, significantly influenced all sensory characteristics (Figures S3 and S4). This factor had a negative impact on all parameters (negative sign of coefficient β₁ for all responses JS1–JS6 in Table S4).

With the increase in the particle size of APEs, several changes were observed in the cookies: the colour of the cookies become lighter, surface cracks and granularity were more pronounced, and hardness decreased. These results for colour and hardness align with the results of instrumental measurements of these parameters shown in Table 5. In Rocha Parra et al. [55] research, many panellists noted that cookies with the smallest particle size of apple pomace were perceived as harder and more compact. When extrudates with a particle size below 250 µm were used, the colour of the cookies was uniform. However, the addition of extrudate with larger particles was visible on the cookies in the form of black spots.

The addition of apple pomace extrudate significantly influenced the taste of the samples, and the apple flavour becoming more intense as the proportion of extrudate increased (factor E effect for response JS6 in Figure S3 is 58.56%). Table S4 indicates that factors F and E had a statistically significant impact (p < 0.05) on the taste of cookie samples.

3.6. Microbiological Analysis

Microbiological analysis was conducted initially 24 h after baking and subsequently every month over the 6-month storage period. As the results remained consistent across each month with no significant changes in the microbiological quality of the samples, only the results from the analysis performed after the sixth month of storage are presented in

Table 7. Results of microbiological analysis of cookies with added APE, 6 months after baking.

Sample	Factor Combinations			AMB	UK	UP	EC	CP	EB	SB
	F	T	E	(cfu/g)	(cfu/g)	(cfu/g)	(cfu/g)	(cfu/g)	(cfu/g)	(cfu/g)
J-3.45.10	3	45	10	<10	<10	<10	<10	<10	<10	<10
J-1.30.5	1	30	5	<10	<10	<10	<10	<10	<10	<10
J-1.30.15	1	30	15	<10	<10	<10	<10	<10	<10	<10
J-2.45.15	2	45	15	40	<10	<10	<10	<10	<10	<10
J-2.15.15	2	15	15	20	<10	<10	<10	<10	<10	<10
J-2.30.10	2	30	10	<10	<10	<10	<10	<10	<10	<10
J-2.45.5	2	45	5	10	<10	<10	<10	<10	<10	<10
J-3.15.10	3	15	10	20	<10	<10	<10	<10	<10	<10
J-3.30.10	3	30	15	20	<10	<10	<10	<10	<10	10
J-2.30.10	2	30	10	40	<10	<10	<10	<10	<10	<10
J-1.45.10	1	45	10	<10	<10	<10	<10	<10	<10	<10
J-1.15.10	1	15	10	<10	<10	<10	<10	<10	<10	<10
J-3.30.5	3	30	5	<10	<10	<10	<10	<10	<10	<10
J-2.15.5	2	15	5	<10	<10	<10	<10	<10	<10	<10
J-2.30.10	2	30	10	<10	<10	<10	<10	<10	<10	<10

F—particle size of the extrudate (<250 µm—granulation 1; 250–1000 µm—granulation 2; 1000–2000 µm—granulation 3); T—proportion of by-products in the APE; E—the percentage of wheat flour replaced with APE (5%, 10%, and 15% based on the mass of the flour): AMB: Total number of aerobic mesophilic bacteria; UK: Total number of yeasts; UP: Total number of moulds; EC: Total number of E.coli; CP: Total number of Clostridium perfringens; EB: Total number of Enterobacteriaceae; SB: Total number of aerobic mesophilic sporogenic bacteria.

. Cookies are generally microbiologically stable products, capable of maintaining quality for extended periods, typically ranging from 6 to 8 months [56]. However, if cookies have a slightly higher moisture content (above 6%), precautions should be taken during packaging and storage to prevent the growth of mould. Regarding pathogenic microorganisms, with proper production management, their development in cookies is uncommon, contributing to their long shelf life. For this reason, the addition of any raw material should not affect durability of these products. Based on the results, it can be observed that none of the tested samples of cookies showed the presence of enterobacteria E. coli, sulfite-reducing clostridia and lipolytic bacteria 24 h after baking, and during the entire 6-month storage period.

Based on the results of the microbiological analysis, it can be concluded that the addition of extrudates enriched with apple pomace did not have a negative impact on the microbiological stability of the cookies during storage for 6 months. The absence of enterobacteria, E. coli, sulfite-reducing clostridia, and lipolytic bacteria in the tested samples throughout the storage period supports the conclusion that these cookies maintained microbiological safety over time.

4. Conclusions

The addition of extrudates enriched with apple pomace significantly increased the total fibre content in the cookies. All samples with the addition of apple pomace extrudates (APE) can carry the statement “source of fibre”. Additionally, samples with a higher proportion of extrudate can carry the statement “high in fibre” depending on their specific fibre content per 100 g or per 100 kcal, as regulated by health claim guidelines. The content of ash, i.e., mineral content in the cookies, increased proportionally with the increase in the proportion of apple pomace in the extrudate used in the cookies. The content of hydroxymethylfurfural (HMF) increased with higher proportions of extrudate in the cookies and with increased proportion of apple pomace in the extrudate. Importantly, all cookie samples had an HMF content of less than 25 mg/kg, which is considered safe according to health standards.

The replacement of wheat flour with the apple pomace extrudates significantly increased the intensity of the darker colour of the cookie samples and also increased the proportion of red tone. The size of the extrudate particles had the most significant influence on all sensory characteristics of the samples. With an increase in the size of the extrudate particles, the cookies exhibited a brighter appearance, more pronounced cracks on the surface, lower hardness, and greater graininess. Panellists liked the taste influenced by the addition of apple pomace in the cookies. Even the samples with the highest amount of extrudate (15%) received very high ratings. The samples with the highest hardness and darkest colour were those with the addition of APE that had the smallest particle size. Therefore, it would be possible to add APE in an amount greater than 15%, but with coarser particles, in order to minimize the negative impact that the addition of APE has on the colour and hardness of the cookies.

Importantly, no pathogenic microorganisms developed in any of the samples over the 6-month storage period. This indicates that the addition of extrudate did not compromise the microbiological stability of the final product, confirming that all samples were safe for consumption.