Upcycled Apple Pomace as an Innovative Ingredient in High-Moisture Meat Analogs: Sustainable Valorization for Food Production

Abstract

This study evaluated the effects of apple pomace (AP) incorporation on the physicochemical, structural, and functional properties of high-moisture meat analogs from a sustainability perspective. AP, an abundant agro-industrial by-product rich in dietary fiber and polyphenols, was upcycled at inclusion levels of 0–20%. By transforming this food waste into a value-added ingredient, the study aligns with circular bioeconomy principles to reduce environmental footprints. Increasing AP content led to reductions in textural attributes such as hardness, cohesiveness, springiness, and chewiness. Integrity index and cutting strength also declined, particularly beyond 15%, due to the disruption of the protein-starch matrix by dietary fibers. In contrast, antioxidant activities (DPPH and ABTS) improved significantly with higher AP levels, reflecting polyphenol release during extrusion. These findings highlight a trade-off between enhanced nutritional functionality and reduced structural integrity. Moderate inclusion below 10–15% may therefore offer a balance between antioxidant benefits and acceptable texture. Overall, this research demonstrates the potential of sustainable AP valorization in developing senior-friendly and plant-based meat analog products. The outcomes not only provide practical formulation guidance but also contribute to resource efficiency, waste reduction, responsible consumption, and sustainable food production systems, and the advancement of a circular bioeconomy.

1. Introduction

With the global increase in per capita income and population growth, both per capita meat consumption and total meat consumption have risen [1]. While meat is an important source of essential nutrients, excessive intake has been associated with an increased risk of various chronic diseases, including colorectal cancer and cardiovascular disorders [2,3]. Furthermore, an ecological study by You et al. [4] examining the relationship between global meat supply and disease prevalence reported a positive association between global meat supply and the prevalence of cardiovascular diseases. The livestock industry significantly contributes to global environment challenges, including climate change and resource depletion. Livestock production is recognized as a major source of anthropogenic greenhouse gas emissions and accounts for a substantial portion of global arable land and freshwater usage [5]. Furthermore, nitrogen and phosphorus compounds released from livestock operations cause water pollution and eutrophication, negatively affecting both human and animal health [5]. Against this backdrop, the interest in plant-based diets has surged significantly; for instance, the number of vegans in certain regions has reportedly increased by approximately 350% over the past decade [6]. In Brazil alone, the vegetarian population reached approximately 30 million people in 2018, reflecting a broader global shift toward alternative protein sources [6]. Vegans obtain and supplement protein primarily from plant-based sources such as cereals, legumes, nuts, seeds, tofu, and tempeh [7]. In this context, the concept of “meat analogs” based on plant proteins has emerged. Meat analogs are designed to mimic the sensory properties and nutritional components of conventional meat products. Key manufacturing techniques include extrusion processing and 3D printing [8,9].

Among these, extrusion processing is considered an efficient and cost-effective thermal treatment method that integrates multiple unit operations, mixing, grinding, heating, shaping, and drying, into a single process [10]. During extrusion, starch gelatinization, protein denaturation, and microbial sterilization occur rapidly [11], and various product characteristics can be achieved by adjusting process parameters such as feed rate, moisture content, barrel temperature, screw speed, and die shape [12]. Apple pomace (AP), a by-product generated during apple processing, typically constitutes about 20% of the raw apple mass [13], with over 4 million tons produced annually worldwide [14]. A significant portion of this by-product remains unrecycled and is discarded, leading to environmental burdens. Most AP is directly landfilled, but due to its high organic matter content, rapid microbial decomposition causes odor, leachate generation, and consequent soil and water pollution [14]. Furthermore, management of organic wastes like AP can contribute to release of greenhouse gases, such as methane and carbon dioxide, depending on the treatment and operating conditions. For instance, a life cycle assessment (LCA) focused on apple pomace management in Québec demonstrated that specific landfilling and composting scenarios can significantly contribute to environmental burdens under certain study assumptions [15]. For these reasons, extensive research is being conducted to repurpose AP into safe and sustainable resources [16]. AP contains various beneficial compounds suitable for use as food ingredients. In particular, it comprises 35–65% dietary fiber and 0.2–0.8% polyphenols [17]. The main phenolic compounds in AP include quercetin glycosides, cyanidin glycosides, epicatechin, and procyanidins, all of which exhibit strong antioxidant activity [18]. AP also contains organic acids such as malic acid, citric acid, succinic acid, and quinic acid, along with ursolic acid, a triterpenoid noted for its anti-inflammatory and anticancer activities [18]. These compounds have been associated with lowering blood glucose and cholesterol levels, exerting anticancer effects, and reducing the risk of arteriosclerosis and other cardiovascular diseases, osteoporosis, diabetes, and neurodegenerative disorders, including Alzheimer’s disease [17].

Given its rich nutritional profile, upcycling AP into high-value-added resources rather than discarding it offers considerable environmental and economic benefits. Especially in the food sector, AP has high potential as a functional food ingredient, prompting active research [19,20]. Such upcycling applications help address disposal challenges, reduce environmental pollution, and contribute to the development of health-oriented food products-driving growing academic and industrial interest [21]. Based on this rationale, this study hypothesized that the incorporation of apple pomace (AP) would enhance the antioxidant potential of meat analogs, while its high dietary fiber content would induce structural modifications by interfering with the protein-starch matrix. Accordingly, this study aimed to upcycle AP and evaluate its effects on the physicochemical properties of high-moisture extruded meat analogs.

2. Materials and Methods

2.1. Experimental Materials

In this study, the formulation consisted of isolated soy protein (Pingdingshan TianJing Plant Albumnen Co., Ltd., Pingdingshan, China), wheat gluten (Roquette Freres, Lestrem, France), corn starch (Samyang, Ulsan, Republic of Korea), and apple pomace (AP) obtained from Chusawine (Yesan, Republic of Korea). Before processing, the AP was dried in a convection oven at 50 °C for 24 h to obtain a stable and reproducible moisture status. The dried AP was subsequently ground and sieved to collect particles in the range of 50–70, thereby minimizing variability in particle size. These controlled moisture and particle size conditions were applied to ensure homogeneous mixing with the soy protein and starch matrix, enabling uniform texturization behavior and reproducibility during high-moisture extrusion.

2.2. Extrusion Processing

High-moisture extrusion was performed using a co-rotating twin-screw system (THK31-No. 5, Incheon Machinery Co., Incheon, Republic of Korea). The extruder was equipped with 30 mm screws and had an overall length-to-diameter ratio of 23:1. The arrangement of screw elements utilized in this study is presented in Figure 1. During operation, the process parameters were maintained at a feed rate of 100 g/min, an inlet moisture level of 65%, a screw rotation speed of 200 rpm, and a barrel temperature setting of 150 °C. These specific processing conditions were determined through preliminary experiments and optimized to achieve a stable fibrous structure. In particular, the selected moisture level, screw speed, and barrel temperature were designed to accommodate the high water-binding capacity of AP by ensuring sufficient hydration and fiber dispersion for stable flowability, while minimizing thermal degradation of its bioactive compounds during the texturization process. Isolated soy protein, wheat gluten, and corn starch were mixed at a ratio of 5:4:1, with apple pomace added at varying levels. The specific formulation for each treatment is shown in

Table 1. Formulations of high-moisture extruded meat analogs incorporating varying levels of apple pomace (AP).

AP Content (%)	ISP (1)	WG	Corn Starch
0	50	40	10
5	47.5	38	9.5
10	45	36	9
15	42.5	34	8.5
20	40	32	8

⁽¹⁾ ISP, isolated soy protein; WG, wheat gluten.

.

2.3. Color Measurement

The color attributes of the high-moisture meat analogs, which were freeze-dried and subsequently ground, were evaluated using a chroma meter (CR-300, Minolta Co., Ltd., Osaka, Japan). Measurements of lightness (L*), redness (a*), and yellowness (b*) were taken in triplicate, and the mean values were used for analysis. The overall color difference (∆E) was determined according to Equation (1).

∆E = {(L* − L)² + (a* − a)² + (b* − b)²}^1/2

(1)

2.4. Water Holding Capacity (WHC) and Water Solubility Index (WSI)

To evaluate the water retention properties of high-moisture meat analogs with apple pomace (AP), measurements were conducted separately for whole and powdered forms.

The water holding capacity (WHC) of the whole samples was determined using a modified method based on Gu and Ryu [22]. Approximately 2 g (dry basis) of the sample, cut into cubes measuring 1 cm × 1 cm, was submerged in 100 mL of distilled water and allowed to hydrate in a 60 °C water bath for 16 h. After hydration, excess water was drained for 15 min, and the samples were weighed. WHC was calculated using Equation (2). The water solubility index (WSI) of the powdered samples was evaluated by mixing 0.5 g of powder with 5 mL of distilled water, followed by centrifugation at 3000 rpm for 30 min. The supernatant was collected and dried at 105 °C for 12 h, after which the residue was weighed. The WSI value was determined according to Equation (3).

WHC (g/g) = (Wet sample weight − Dry sample weight)/Dry sample weight

(2)

WSI (%) = Wt. of dissolved solid in supernatant/Dry sample weight × 100 (%)

(3)

2.5. Expansion Ratio (ER) and Bulk Density (BD)

The expansion ratio (ER) and bulk density (BD) of high-moisture meat analogs containing apple pomace were measured. The ER was calculated using Equation (4) by measuring the width and height of the cross-sectional surface (front face) of the extrudates with a digital vernier caliper (942000 IP67, Brütsch/Rüegger Tools Ltd., Urdorf, Switzerland) and relating these values to the width and height of the extrusion die. The BD was calculated using Equation (5) after measuring the width, depth, and height of the extrudates and weighing their mass using an electronic balance (BA310, DAIHAN Co., Ltd., Wonju, Republic of Korea).

ER = A_product/A_die

(4)

A_product: cross-sectional area of the extrudate

A_die: cross-sectional area of the extrusion die

BD (g/cm³) = Mass of the meat analog/Volume of the meat analog

(5)

2.6. Texture Profile Analysis (TPA)

The texture characteristics of the high-moisture meat analogs were evaluated using a texture analyzer (Z0.5 TS, Zwick Roell, Ulm, Germany). Each sample was cut into 1 cm × 1 cm cubes and compressed with a 10 cm diameter probe at a maximum load of 600 N, with six replicates conducted for each treatment. The first hardness (HD1), second hardness (HD2), hardness degradation ratio (HDR), springiness (SP), cohesiveness (COH), and chewiness (CHE) were calculated using Equations (6)–(11), as modified from Trinh and Glasgow [23]. Shear force was measured using a 70 mm × 3 mm probe, applied to the samples in both the parallel and perpendicular directions at a maximum load of 600 N, with measurements taken six times. The shear force values were obtained using Equation (12), and the extent of fibrous structure was evaluated according to Equation (13).

HD1 (N) = Maximum force of load application in the first cycle

(6)

HD2 (N) = Maximum force of load application in the second cycle

(7)

HDR = HD1/HD2

(8)

SP (%) = (S₂ − S₀)/S₁ × 100 (%)

(9)

S₀: Travel at the starting force in the second cycle

S₁: Travel at the maximum force of load application in the first cycle

S₂: Travel at the maximum force of load application in the second cycle

COH (%) = (E₂₁ + E₂₂)/(E₁₁ + E₁₂) × 100 (%)

(10)

E₁₁: Area below the load application curve up to the maximum force in the cycle

E₁₂: Area below the load application curve as from the maximum force in the cycle

E₂₁: Area below the load application curve up to the maximum force in the second cycle

E₂₂: Area below the load application curve up as from the maximum force in the second cycle

CHE (g) = SP × COH × HD1

(11)

For the calculation of CHE, SP and COH values expressed as percentages were converted to dimensionless ratios (0–1)

Cutting strength (g/mm²) = Maximum force/Cross-sectional area

(12)

Texturization degree of Cutting strength (TD) = CS_V/CS_P

(13)

CS_V: Cutting strength in the vertical direction

CS_P: Cutting strength in the parallel direction

2.7. Integrity Index (I-Index)

To assess the structural integrity of the high-moisture meat analogs containing apple pomace under elevated temperature and pressure, the I-index was determined according to the procedure described by Gu and Ryu [22]. Approximately 3.0 g (dry basis) of sample was mixed with 100 mL of distilled water and autoclaved at 121 °C for 15 min. After being cooled under running water, the sample was homogenized at 15,000 rpm for 30 s using a homogenizer (IKA-T10B, IKA Co., Seoul, Republic of Korea). The homogenate was filtered using a 20-mesh sieve, and the residue was dried at 105 °C for 12 h. The I-index was calculated using Equation (14), based on the dry weight of the residue and the original sample.

I-index (%) = Dry residue wt./Sample wt. × 100

(14)

2.8. DPPH Radical Scavenging Activity

The antioxidant activity of the high-moisture meat analogs formulated with apple pomace was analyzed using the DPPH radical scavenging method as outlined by Brand-Williams et al. [24], with modifications. One gram of the sample (dry basis) was mixed with 10 mL of absolute ethanol and extracted in a shaking incubator (SI-300R, Jeiotech, Daejeon, Republic of Korea) at 200 rpm for 2 h. The mixture was then centrifuged at 3000 rpm for 30 min. Next, 1 mL of the supernatant was combined with 0.5 mL of a 0.2 mM DPPH solution prepared in ethanol and allowed to react for 30 min in the dark. The absorbance was recorded at 515 nm, and the DPPH radical scavenging activity was determined using Equation (15).

DPPH radical scavenging activity (%) = (A₀ − A)/A₀ × 100

(15)

A₀: Absorbance value of the control solution

A: Absorbance measured for the sample extract

2.9. ABTS Radical Scavenging Activity

The ABTS radical scavenging capacity was determined following the procedure of Re et al. [25] with slight modifications. Samples (1 g, dry basis) were extracted with 10 mL of ethanol in a shaking incubator (SI-300R, Jeiotech) operated at 200 rpm for 2 h. After extraction, the mixture was centrifuged at 3000 rpm for 30 min, and the resulting supernatant was used for analysis. The ABTS working solution was generated by mixing 7 mM ABTS with 140 mM potassium persulfate (Sigma Aldrich Co., St. Louis, MO, USA) and allowing the reaction to proceed in the dark. This mixture was then diluted with distilled water until the absorbance reached 0.6–0.8 at 760 nm. The diluted ABTS solution was combined with the sample extract and allowed to react for 15 min. Absorbance was recorded at 760 nm using a microplate reader (BioTek, Winooski, VT, USA). The ABTS scavenging percentage was calculated using Equation (16), where A_sample denotes the absorbance of the reacted sample solution.

ABTS radical scavenging activity (%) = 1 − (A_sample/A_blind) × 100

(16)

2.10. Statistical Analysis

All statistical analyses were performed using IBM SPSS Statistics (version 27.0). Descriptive statistics are reported as means ± standard deviations. Group means were compared using one-way ANOVA, and significant differences were determined using Tukey’s HSD test at a significance threshold of p < 0.05, after confirming normality (Shapiro–Wilk) and homogeneity of variances (Levene’s test). Multivariate tests (Pillai’s Trace, Wilks’ Lambda) with Partial η² as effect size were performed. Curve estimation with quadratic terms (y = β0 + β1x + β2x²) was applied. Model fit was evaluated using R² and overall F-tests. Standard errors for regression coefficients were estimated using the HC3 robust method.

Each measurement was conducted using independently prepared extrudate samples. Unless otherwise specified, all analyses were performed in triplicate (n = 3). Texture Profile Analysis (TPA) was conducted using six independent replicates (n = 6). The experimental unit was defined as the extruded meat analog sample at each apple pomace inclusion level. Quadratic regression was applied to capture potential non-linear response patterns across the five formulation levels (0–20%), particularly in view of threshold-like behavior observed in several quality traits. To account for possible heteroscedasticity and small sample effects, HC3 robust standard errors were used for regression coefficient estimation.

3. Results and Discussion

3.1. Appearances

Figure 2 illustrates the appearance of meat analogs produced with different levels of apple pomace (AP). As the AP content increased from 0% to 20%, the fibrous structure of the extrudates became progressively less distinct. The control sample (0% AP) exhibited the characteristic well-aligned, layered fibrous network of high-moisture meat analogs, with clearly defined fiber bundles and strong internal cohesion. In contrast, samples with higher AP contents showed visibly disrupted structures. At 5% and 10% AP, fibrous layers were still present but appeared noticeably loosened, while at 15% and 20% AP the structure was severely degraded. This trend suggests that excessive AP interferes with the formation of the desirable meat-like fibrous architecture, a phenomenon similarly reported with other high-fiber additives [26].

The reduction in structural integrity observed at high AP levels can be attributed to the inhibitory effect of dietary fiber on protein-starch matrix formation. AP is rich in insoluble fibers, which primarily act as passive fillers within the dough and physically interrupt the continuity of the protein-starch network [27]. Additionally, dietary fibers can influence water distribution during extrusion. Previous studies have reported that the inclusion of AP induces milder extrusion conditions with reduced starch gelatinization [28]. This indicates that starch granules in high-AP formulations may fail to fully swell or disperse into the protein phase, thereby contributing to structural weakness. In other words, the dietary fibers in AP, particularly hydrophilic polysaccharides like pectin and cellulose, exhibit a high affinity for water, leading to intense competitive water absorption with the protein phase. This competition restricts the amount of water available for protein hydration, which is a critical prerequisite for protein unfolding and the subsequent exposure of reactive groups during extrusion. Consequently, the limited hydration hinders the formation of a cohesive protein-starch matrix and interferes with the development of strong intermolecular networks, such as disulfide bonds and hydrophobic interactions, resulting in a more fragmented matrix [29]. Such disrupted fiber-protein interactions create discontinuous domains within the extrudate, making the structure more friable and mechanically weaker. Although increasing AP content weakens the fibrous structure, the resulting softer and more easily fragmentable texture may have implications for the development of texture-modified food systems, including senior-oriented plant-based meat analogs.

3.2. Multivariate Analysis

These visual observations were statistically confirmed by MANOVA (

Table 2. Multivariate test results for the effect of apple pomace addition.

Effect	Value	F	Hypothesis df	Error df	Partial η2
Pillai’s Trace	0.906	16.619 ***	40.000	16.000	0.976
Wilks’ lambda	0.000	298.476 ***	40.000	5.647	0.999

*** indicate significance at p < 0.001.

), which provides an overall summary of the effects of apple pomace (AP) addition. Both Pillai’s Trace and Wilks’ Lambda were highly significant (p < 0.001), indicating a robust multivariate effect across all measured traits. Most quality-related attributes, including color parameters (L*, a*, b*, ∆E), water-holding and swelling properties (WHC, WSI), BD, ER, texture parameters, and antioxidant activities (DPPH, ABTS), showed significant differences (p < 0.001) across AP levels.

While Table 2 summarizes the overall multivariate differences,

Table 3. Tests of between-subjects effects for apple pomace (AP) addition on quality traits.

Independent Variable	Dependent Variable		Sum of Type III Squares	df	Mean Square	F	Partial η2
AP content (%)	L* (1)		503.792	4	125.948	174.764 ***	0.986
	a*		8.185	4	2.046	10.544 **	0.808
	b*		13.864	4	3.466	41.286 ***	0.943
	∆E		499.980	4	124.995	163.970 ***	0.985
	WHC		4.626	4	1.157	12.088 ***	0.829
	WSI		47.935	4	11.984	43.940 ***	0.946
	BD		0.0150	4	0.004	40.245 ***	0.942
	ER		0.0280	4	0.007	43.138 ***	0.945
	HD	HD1	1315.972	4	328.993	150.925 ***	0.984
		HD2	1169.077	4	292.269	123.891 ***	0.980
		HDR	0.196	4	0.049	14.588 ***	0.854
	TPA	SP	1227.214	4	306.804	56.487 ***	0.958
		CHE	3,658,620.324	4	914,655.081	209.012 ***	0.988
		COH	872.936	4	218.234	40.699 ***	0.942
	CS	CSP	38.440	4	9.610	75.638 ***	0.968
		CSV	132.600	4	33.150	253.094 ***	0.990
	TD		0.234	4	0.059	58.369 ***	0.959
	I-index		501.445	4	125.361	17.758 ***	0.877
	DPPH		508.119	4	127.030	66.970 ***	0.964
	ABTS		241.891	4	60.473	20.860 ***	0.893

⁽¹⁾ L*, lightness; a*, redness; b*, yellowness; ∆E, color difference; WHC, water holding capacity; WSI, water solubility index; BD, bulk density; ER, expansion ratio; HD, hardness; HD1, first hardness; HD2, second hardness; HDR, hardness degradation ratio; TPA, texture profile analysis; SP, springiness; CHE, chewiness; COH, cohesiveness; CS, cutting strength; CS_P, Cutting strength of parallel direction; CS_V, cutting strength of vertical direction; TD, Texturization degree of cutting strength; I-index, integrity index. ***, and ** indicate significance at p < 0.001, and p < 0.01, respectively.

reports the univariate between-subjects effects for each trait. Most quality attributes showed highly significant F values along with large effect sizes (Partial η²), including color, texture, antioxidant activity, and other physicochemical parameters. These findings confirm that AP incorporation exerted strong influences on both physicochemical and functional properties at the individual trait level.

To provide a clearer understanding of these effects, the subsequent sections present detailed results for each attribute group, beginning with color properties.

3.3. Color Properties

The color values of the meat analogs are presented in

Table 4. CIE L* a* b* values of raw ingredients and high-moisture extruded meat analogs formulated with different apple pomace (AP) levels.

AP Content (%)	L* (1)	a*	b*	∆E
0	66.68 ± 0.00 a	2.39 ± 0.01 c	20.99 ± 0.03 a	20.59 ± 0.00 e
5	61.58 ± 0.04 b	2.84 ± 0.01 bc	21.04 ± 0.01 a	25.68 ± 0.04 d
10	57.43 ± 0.15 c	3.73 ± 0.07 ab	21.07 ± 0.06 a	29.88 ± 0.13 c
15	54.78 ± 1.42 d	3.96 ± 0.11 ab	19.92 ± 0.21 b	32.44 ± 1.40 b
20	49.69 ± 1.25 e	4.39 ± 0.98 a	18.61 ± 0.61 c	37.52 ± 1.35 a

⁽¹⁾ L*, lightness, a*, redness; b*, yellowness; ∆E, color difference. Superscript letters (a–e) denote groups that differ significantly at p < 0.05, as determined by Tukey’s HSD test.

. As the apple pomace (AP) content increased, the lightness (L*) and yellowness (b*) of the meat analogs tended to decrease, while the redness (a*) increased. In addition, the color difference (∆E) was calculated by comparing each sample to the control containing 0% AP before the extrusion process. The ∆E values increased with higher levels of AP, indicating a greater color shift. A higher content of AP generally resulted in a darker appearance of the meat analogs, as reflected by the decreased L* values. AP contains sugars, amino acids, and proteins, which undergo non-enzymatic browning reactions during extrusion [30]. Melanoidins, brown-red pigments formed through the Maillard reaction, contribute to the decrease in lightness. Moreover, AP is rich in polyphenols, which can induce enzymatic browning [30]. These polyphenols can also bind to proteins during extrusion, forming dark-colored complexes. Regarding the a* values, redness increased with higher AP content. This trend aligns with previous findings by Yadav et al. [31], who observed a significant increase in a* values in chicken sausages containing 3–9% AP due to browning pigments formed during heating, compared to control sausages without AP.

In contrast, b* values, representing yellowness, decreased as the AP content increased. Overall, the addition of AP caused browning in the meat analogs, resulting in decreased L* values and shifts in a* and b* values, leading to a generally darker, brown-toned color. ∆E values also increased proportionally with the AP content. In the case of meat analogs, a brownish hue is often perceived by consumers as more similar to cooked meat products. In fact, sausages containing 3–6% AP were shown to have decreased lightness and increased redness, which improved their color attributes and received higher scores in sensory evaluations compared to sausages without AP [31]. In conclusion, the addition of AP alters the color characteristics of meat analogs through both enzymatic and non-enzymatic reactions and contributes to increased ∆E values.

3.4. Water Retention Capacity

The water holding capacity (WHC) and water solubility index (WSI) of the meat analogs were measured to evaluate their water retention properties, and the results are presented in

Table 5. Water holding capacity (WHC), water solubility index (WSI), bulk density (BD), and expansion ratio (ER) of high-moisture extruded meat analog as affected by apple pomace (AP) content.

AP Content (%)	WHC (g/g) (1)	WSI (%)	BD (g/cm3)	ER
0	4.67 ± 0.19 a	7.84 ± 0.65 c	0.92 ± 0.01 bc	1.14 ± 0.00 a
5	3.93 ± 0.05 ab	9.12 ± 0.89 bc	0.94 ± 0.01 bc	1.09 ± 0.02 b
10	4.17 ± 0.08 ab	9.98 ± 0.31 b	0.95 ± 0.01 b	1.08 ± 0.01 bc
15	3.68 ± 0.36 bc	11.73 ± 0.22 a	0.96 ± 0.01 b	1.05 ± 0.02 c
20	2.99 ± 0.55 c	12.82 ± 0.11 a	1.01 ± 0.01 a	1.01 ± 0.01 d

Superscript letters (a–d) denote groups that differ significantly at p < 0.05, as determined by Tukey’s HSD test. ⁽¹⁾ WHC, water holding capacity; WSI, water solubility index; BD, bulk density; ER, expansion ratio.

. The WHC of the meat analogs decreased as the apple pomace (AP) content increased, showing a value of 4.67 ± 0.19 g/g at 0% AP and 2.99 ± 0.55 g/g at 20% AP. During extrusion, plant proteins such as soy protein and wheat gluten undergo denaturation due to heat and shear forces. These proteins align into fibrous structures and form cross-linked networks resembling the texture of animal meat [32]. This protein network plays a key role in retaining water. However, with increasing AP content, the formation of this protein structure was hindered [27]. The dietary fibers in AP physically interfered with protein–protein interactions, and the relative decrease in protein content resulted in a weaker fibrous network that was less capable of trapping water [33].

The WSI increased with higher AP content, from 7.84 ± 0.65 at 0% to 12.82 ± 0.11 at 20% AP. When dietary fibers are incorporated into the protein matrix, the interactions between proteins are reduced, and the continuity of the starch gel is disrupted, leading to an overall weakening of the structure [27]. Consequently, the internal cohesiveness of the meat analog is weakened and more components are released into water, which explains the higher WSI values observed with increasing AP content.

3.5. Expansion Ratio (ER) and Bulk Density (BD)

The bulk density (BD) and expansion ratio (ER) of the meat analogs containing apple pomace (AP) were measured and are presented in Table 5. As the level of AP increased, the BD of the meat analogs significantly increased, while ER showed a decreasing trend. Specifically, the sample without AP exhibited a BD of 0.92 ± 0.01 g/cm³ and an ER of 1.14 ± 0.00. In contrast, when 20% AP was added, the BD increased to 1.01 ± 0.01 g/cm³ and the ER significantly decreased to 1.01 ± 0.01.

Approximately 35–60% of the total solids in AP consist of dietary fiber, including insoluble components such as cellulose, hemicellulose, and lignin [34]. These insoluble fibers interfere with pore formation and growth during the expansion process [35]. Furthermore, the addition of AP reduces the relative content of starch and protein, which are key macromolecules responsible for expansion [35]. The high fiber content in AP also disrupts the formation of a starch matrix and restricts air incorporation during extrusion, thereby inhibiting expansion [36]. In general, ER and BD exhibit an inverse relationship: well-expanded meat analogs contain more internal pores, resulting in lower mass per unit volume and, consequently, lower density. Conversely, poorly expanded products tend to have limited pore formation and thus higher density [35]. This observation aligns with previous extrusion studies using AP, which similarly reported increased product density with higher levels of AP addition [37].

3.6. Texture Properties

3.6.1. Hardness (HD)

The hardness (HD) of extruded meat analogs with varying levels of apple pomace (AP) was measured and presented in

Table 6. Hardness (HD), hardness degradation ratio (HDR) and texture profile analysis (TPA) of high-moisture extruded meat analogs with varying apple pomace (AP) content.

AP Content (%)	HD (N)		HDR	TPA
	HD1 (1)	HD2		SP (%)	CHE (g)	COH (%)
0	35.97 ± 2.30 a	30.63 ± 1.79 a	1.17 ± 0.01 c	83.31 ± 1.25 a	1598.31 ± 107.84 a	52.33 ± 0.68 a
5	28.77 ± 1.38 b	22.92 ± 1.24 b	1.26 ± 0.02 bc	80.22 ± 1.98 a	1002.86 ± 97.92 b	42.56 ± 2.53 b
10	19.68 ± 2.31 c	15.55 ± 2.21 c	1.27 ± 0.04 bc	75.10 ± 1.26 b	628.02 ± 123.01 c	41.45 ± 4.51 b
15	13.32 ± 0.71 d	9.64 ± 1.36 d	1.40 ± 0.14 a	63.17 ± 1.94 c	290.59 ± 38.16 d	33.93 ± 3.95 c
20	10.27 ± 1.47 e	7.59 ± 1.05 d	1.35 ± 0.05 ab	60.61 ± 3.10 c	212.77 ± 33.45 d	33.51 ± 1.41 c

⁽¹⁾ HD1, first hardness; HD2, second hardness; SP, springiness; CHE, chewiness; COH, cohesiveness. Superscript letters (a–e) denote groups that differ significantly at p < 0.05, as determined by Tukey’s HSD test.

. The results showed a general decrease in both the first hardness (HD1) and second hardness (HD2) values as the AP content increased. The control group without AP (0%) exhibited the highest HD values, with a HD1 of 35.97 ± 2.30 N and a HD2 of 30.63 ± 1.79 N. In contrast, the sample containing 20% AP showed the lowest values, with a HD1 of 10.27 ± 1.47 N and a HD2 of 7.59 ± 1.05 N. This reduction in HD1 can be attributed to the high dietary fiber content in AP, which interferes with the original protein-starch matrix, thereby hindering the formation of a firm structure. A previous study also reported that the addition of apple fiber to burger patties reduced their HD and contributed to a softer texture [38]. The Hardness degradation ratio (HDR), calculated as the ratio of HD1 to HD2, represents the extent of structural breakdown that occurs after the first compression. Higher HDR values therefore indicate greater loss of structural integrity and poorer elastic recovery. The 0% group showed the lowest HDR (1.17 ± 0.01), whereas the 15% and 20% groups exhibited higher ratios (1.40 ± 0.14 and 1.35 ± 1.05, respectively), confirming that increasing AP content accelerates hardness degradation during repeated compression. Previous studies have similarly shown that the addition of dietary fiber weakens the structural matrix of extruded products, making them more fragile [36]. In conclusion, bioactive compounds such as pectin and polyphenols in AP likely interacted with proteins and starch, weakening the binding ability of the meat analog and disrupting gel network formation, which negatively affected its textural properties.

3.6.2. Springiness (SP), Chewiness (CHE) and Cohesiveness (COH)

The effects of apple pomace (AP) content on the springiness (SP), chewiness (CHE), and cohesiveness (COH) of meat analogs are presented in Table 6. SP showed a significant decreasing trend as the AP content increased. The control group (0%) exhibited the highest SP value of 83.31 ± 1.25%, while the 20% group showed a significantly lower value of 60.61 ± 3.10%. This decrease is attributed to the disruption of the protein-starch matrix by dietary fiber in the AP, which reduced the sample’s ability to recover its original shape after compression. This finding is consistent with a previous study by Marczak and Mendes [33], which demonstrated that the addition of dietary fiber to pea protein-based extruded meat analogs reduced mechanical strength, leading to decreased SP and CHE. Similarly, Aslam et al. [34] reported a reduction in SP when more than 6% AP was incorporated into meat patties. These results suggest that the internal structure of the meat analogs became looser, contributing to a softer texture. CHE also decreased significantly with increasing AP content. The CHE of the 0% group was 1598.41 ± 107.84 g, while the 20% group recorded the lowest value of 212.77 ± 33.45 g. This decline can be explained by the ability of dietary fiber to interrupt the protein matrix and create voids within the product, making the structure easier to break down with less force [39]. These findings align with the study by Marczak and Mendes [33], which also reported a significant reduction in CHE in meat analogs containing dietary fiber. COH also declined with increasing AP levels. The control group showed the highest COH (52.33 ± 0.68%), while the 20% group had the lowest value (33.51 ± 1.41%). This reduction is likely due to the hydrophobic interactions between fibers and proteins that limit intermolecular bonding among proteins, thereby weakening the gel network structure [40]. Marczak and Mendes [33] similarly observed that the addition of dietary fiber to pea protein-based patties reduced COH by interfering with the protein network and softening the overall structure. In addition, the high water-binding capacity of dietary fiber in AP likely competed with proteins for available moisture during extrusion, limiting protein hydration and molecular mobility. This water competition may have further hindered protein unfolding and re-association, thereby weakening the elastic and cohesive properties of the extruded structure.

3.6.3. Cutting Strength (CS) and Texturization Degree (TD)

The cutting strength (CS) and texturization degree (TD) of meat analogs with varying AP content are presented in

Table 7. Cutting strength (CS), texturization degree (TD), and integrity index (I-index) of high-moisture extruded meat analogs with varying apple pomace (AP) content.

AP Content (%)	CS (g/mm2)		TD	I-Index (%)
	CSP (1)	CSV
0	10.01 ± 0.74 a	15.19 ± 1.34 a	1.52 ± 0.04 a	23.47 ± 0.82 a
5	8.93 ± 0.24 b	12.53 ± 0.83 b	1.40 ± 0.06 b	18.89 ± 0.86 b
10	8.09 ± 0.64 b	9.47 ± 0.31 c	1.17 ± 0.06 c	12.57 ± 2.28 c
15	6.34 ± 0.25 c	7.69 ± 0.53 d	1.21 ± 0.05 c	6.73 ± 0.69 d
20	5.40 ± 0.38 d	6.19 ± 0.46 e	1.15 ± 0.04 c	8.34 ± 1.85 d

⁽¹⁾ CS_P, cutting strength measured in the parallel direction; CS_V, cutting strength measured in the vertical direction. Superscript letters (a–e) denote groups that differ significantly at p < 0.05, as determined by Tukey’s HSD test.

. CS in both the parallel and vertical directions decreased with increasing AP content. The control group (0%) showed the highest CS values in both directions, with 10.01 ± 0.74 g/mm² in the parallel direction and 15.19 ± 1.34 g/mm² in the vertical direction. In contrast, the 20% group showed significantly lower values, with 5.40 ± 0.38 g/mm² in parallel direction and 6.19 ± 0.46 g/mm² in vertical direction. This decrease is attributed to the disruption of the continuous starch-protein network caused by the incorporation of dietary fiber. When fiber is embedded within the protein matrix, it interferes with protein–protein interactions and disrupts the continuity of starch gels, leading to weakened structural integrity [27]. Moreover, dietary fiber has relatively low adhesiveness and tends not to bind strongly to the protein-starch matrix, resulting in decreased cohesiveness and structural stability of the meat analog. From a structural perspective, fiber particles may act as physical discontinuities within the protein-starch continuous phase, preventing the formation of a well-aligned fibrous network under shear flow conditions.

Texturization degree (TD), which indicates the extent of fiber structure formation in meat analogs, also decreased as AP content increased. A lower TD suggests poor fiber alignment, where CS in both directions become similar, resulting in a value close to 1. In the control group, the TD was 1.52 ± 0.04, whereas the 20% group showed a significantly lower value of 1.14 ± 0.04, approaching 1. These results indicate that the internal structure of the extruded meat analogs became more disorganized with the addition of AP. The weakening of internal cohesion likely allowed gaps to form between fiber bundles, and the structure became more prone to unraveling [27].

3.7. Integrity Index (I-Index)

Table 7 summarizes the integrity index (I-index) of the meat analogs formulated with different amounts of apple pomace (AP). The control sample containing 0% AP exhibited the highest I-index at 23.47 ± 0.82%. As the AP content increased, the I-index significantly decreased. Specifically, the values were 18.89 ± 0.86% for the 5% group, 12.57 ± 2.28% for the 10% group, and reached the lowest value of 6.73 ± 0.69% for the 15% group. In the 20% group, the I-index slightly increased to 8.34 ± 1.85%; however, the difference was not statistically significant compared to the 15% group. This suggests that once the AP content exceeds 15%, the weakening effect on structural integrity plateaus, and the I-index remains at a minimal level. These results indicate that the fibrous structure of the meat analogs was progressively weakened by the addition of AP. During extrusion processing, protein ingredients undergo melting and alignment through heating and shear forces, resulting in the formation of fibrous structures similar to muscle fibers [41]. However, the addition of AP appears to have disrupted this continuous protein bonding. AP contains high amounts of dietary fiber, including pectin and cellulose, which interfere with protein–protein interactions and lead to the distribution of non-protein particles within the matrix of the extrudate. This hinders the formation of a uniform fibrous network [27]. Consequently, as the AP content increased, the extruded samples exhibited reduced structural integrity during heat and homogenization treatments, resulting in easier fragmentation and lower proportions of large fibrous residues.

Notably, when the AP content reached 15% or more, the I-index reached its lowest level and showed no further significant change. This suggests that excessive dietary fiber addition had reached a threshold beyond which further structural degradation did not occur. According to Samard et al. [42], meat analogs with higher integrity indices tend to exhibit better elasticity and cutting strength. In this study, the decrease in I-index with increasing AP content corresponded with reductions in cutting strength and chewiness observed in previous experiments, indicating a decline in internal cohesiveness and textural properties. This threshold behavior at 15% inclusion likely indicates a saturation point where the protein-starch matrix can no longer effectively accommodate additional non-protein particles. According to [43], apple pomace consists predominantly of insoluble fibers, which can account for up to 59.92% of its total fiber content. These insoluble particles act as physical defects within the extruded matrix, causing mechanical disruption of the continuous protein network and hindering the formation of a cohesive fibrous structure. Similar structural shifts have been reported in other meat systems; for instance, inclusion levels of apple pomace reaching 14% were found to significantly increase instrumental hardness while reducing scores for juiciness and chewiness [43]. This suggests that 15% represents a critical limit for AP in this specific extrusion system, beyond which the mechanical interference of the fibers becomes the dominant factor governing structural integrity, rather than further progressive matrix degradation.

3.8. Antioxidant Activity

To evaluate the antioxidant capacity of meat analogs with varying levels of apple pomace (AP), DPPH and ABTS radical scavenging activities were measured. The results are presented in Figure 3 and Figure 4. DPPH radical scavenging activity increased significantly with AP content, from 40.99 ± 0.33% in the 0% group to 58.02 ± 2.34% in the 20% group. Similarly, ABTS radical scavenging activity increased from 60.5 ± 2.38% in the control to 71.9 ± 2.17% in the 20% group. These results are consistent with a previous study by Antonic et al. [30], which reported increased phenolic content and antioxidant activity when AP was added to extruded snacks. AP contains a wide range of antioxidant compounds, including phenolic acids (such as caffeic and ferulic acid), flavonoid glycosides, catechin, procyanidin, and phloridzin [44]. These polyphenols exert antioxidant activity by donating hydrogen atoms from their phenolic -OH groups or through resonance stabilization of free radicals. In particular, quercetin present in AP exhibits strong antioxidant activity due to the dihydroxy structure on its B-ring [44]. Since the vitamin content in AP is relatively low, polyphenols are considered the primary contributors to its antioxidant effects. Additionally, during high-temperature, high-pressure extrusion processing, the cell wall of AP is disrupted, facilitating the release of polyphenols. This likely contributed to the observed increase in DPPH and ABTS values with higher AP content. While some loss of polyphenols may occur due to protein denaturation during extrusion, polyphenol-protein interactions can contribute to the retention of antioxidant activity by stabilizing bioactive compounds within the protein-starch matrix. The thermal and mechanical energy applied during extrusion promotes the formation of non-covalent polyphenol-polymer complexes, which may protect polyphenols from excessive degradation [45]. However, it should be noted that the antioxidant activities reported in this study are based on in vitro assays. Polyphenol-protein interactions formed during extrusion may influence bio accessibility during gastrointestinal digestion; therefore, further studies using in vitro digestion models and in vivo assessments are required to clarify the bioavailability and physiological relevance of these antioxidant compounds. In this context, this polyphenol-protein binding behavior during extrusion may partially explain the observed trade-off between enhanced antioxidant activity and reduced structural integrity, as non-covalent interactions between polyphenols and proteins can alter protein aggregation and network formation. Nevertheless, the retention of DPPH and ABTS radical scavenging activities should be interpreted as in vitro potential. Future studies focusing on the bioavailability of AP polyphenols post-digestion are necessary to confirm their in vivo relevance.

3.9. Quadratic Regression Analysis

Having established significant effects across multiple physicochemical and functional attributes, the next step was to model these responses in order to identify both linear and curvilinear trends. Accordingly, quadratic regression models were fitted based on the univariate and descriptive analyses above (

Table 8. Quadratic regression results of apple pomace (AP) addition level on selected quality traits.

	Constant		AP		AP (Quadratic)		F	p-Value	R2
	Coef. (1)	S.E	Coef.	S.E	Coef.	S.E
L*	66.406 ***	0.532	−0.903 ***	0.126	0.004	0.006	260.199	<0.001	0.977
a*	2.338 ***	0.229	0.143 **	0.054	−0.002	0.003	22.592	<0.001	0.790
b*	20.945 ***	0.161	0.105 **	0.038	−0.011 ***	0.002	77.638	<0.001	0.928
∆E	20.861 ***	0.546	0.907 ***	0.129	−0.005	0.006	245.710	<0.001	0.976
WHC	4.521 ***	0.198	−0.037	0.047	−0.002	0.002	14.950	<0.001	0.714
WSI	7.853 ***	0.276	0.223 ***	0.065	0.001	0.003	92.210	<0.001	0.939
BD	0.921 ***	0.007	0.001	0.002	0.000 *	0.000	45.347	<0.001	0.883
ER	1.133 ***	0.008	−0.006 ***	0.002	0.000	0.000	71.411	<0.001	0.922
HD1	36.544 ***	0.715	−1.967 ***	0.169	0.032 ***	0.008	394.604	<0.001	0.967
HD2	30.965 ***	0.620	−1.918 ***	0.147	0.037 ***	0.007	419.133	<0.001	0.969
HDR	1.170 ***	0.029	0.018 **	0.007	0.000	0.000	14.447	<0.001	0.517
SP	84.149 ***	1.092	−0.920 ***	0.259	−0.016	0.012	145.667	<0.001	0.915
CHE	1596.418 ***	33.855	−130.962 ***	8.021	3.065 ***	0.385	500.590	<0.001	0.974
COH	51.767 ***	1.272	−1.628 ***	0.301	0.035 **	0.014	61.576	<0.001	0.820
CSP	10.022 ***	0.197	−0.200 ***	0.047	−0.002	0.002	159.919	<0.001	0.922
CSV	15.299 ***	0.298	−0.663 ***	0.071	0.010 ***	0.003	264.997	<0.001	0.952
TD	1.529 ***	0.024	−0.039 ***	0.006	0.001 ***	0.000	70.999	<0.001	0.840
I-index	24.679 ***	1.774	−1.894 ***	0.420	0.061 **	0.020	20.838	<0.001	0.776
DPPH	42.016 ***	1.094	1.658 ***	0.259	−0.045 ***	0.012	59.044	<0.001	0.908
ABTS	61.058 ***	1.008	0.954 ***	0.239	−0.021 *	0.011	33.326	<0.001	0.847

⁽¹⁾ Coef., coefficient; S.E, standard error; L*, lightness; a*, redness; b*, yellowness; ∆E, color difference; WHC, water holding capacity; WSI, water solubility index; BD, bulk density; ER, expansion ratio; HD1, first hardness; HD2, second hardness; HDR, hardness degradation ratio; SP, springiness; CHE, chewiness; COH, cohesiveness; CS_P, cutting strength of parallel direction; CS_V, cutting strength of vertical direction; TD, texturization degree; I-index, integrity index. ***, **, and * indicate significance at p < 0.001, p < 0.01, and p < 0.05, respectively.

). Most quality traits exhibited statistically significant linear and, in several cases, quadratic effects of apple pomace (AP) addition. In terms of color, L* and ∆E showed strong linear decreases with higher AP levels, while a* increased significantly, and b* followed a concave trend (R² > 0.90). Water-related properties responded differently: Water holding capacity decreased without a significant quadratic term, whereas Water solubility index increased nearly linearly. Bulk density increased, and expansion ratio decreased consistently with higher AP contents. For textural attributes, primary and secondary hardness values declined in a quadratic manner, while chewiness and cohesiveness also exhibited significant curvature (R² > 0.95). Cutting strength in both parallel and vertical directions decreased steadily, accompanied by reductions in texturization degree and integrity index. In contrast, antioxidant activities (DPPH, ABTS) increased with AP addition, but showed plateau effects at higher levels, as indicated by negative quadratic coefficients. Overall, the regression models explained a high proportion of variance (R² values ranging from 0.71 to 0.97), confirming that AP incorporation exerted both linear and non-linear influences across physicochemical, structural, and functional attributes. These findings highlight a clear trade-off between enhanced functional properties and deteriorated textural attributes, underlining the importance of balancing AP levels when designing senior-friendly or plant-based meat analog products.

4. Conclusions

This study investigated the incorporation of apple pomace (AP), an agro-industrial by-product rich in fiber and polyphenols, into high-moisture meat analogs. Across a range of analyses (ANOVA, MANOVA, and quadratic regression), AP addition significantly influenced multiple physicochemical, structural, and functional traits. Increasing AP content consistently reduced hardness, cohesiveness, springiness, chewiness, and integrity index, reflecting the disruption of the protein-starch matrix by dietary fibers. In parallel, antioxidant activities (DPPH and ABTS) increased markedly due to polyphenol release during extrusion. These results highlight a clear trade-off: enhanced nutritional functionality at the expense of structural integrity. Moderate inclusion levels (below approximately 10–15%) appear optimal for balancing antioxidant benefits with acceptable texture and processability. From an applied perspective, this balance is particularly relevant for the development of texture-modified food systems where softer textures and enhanced bioactivity are desirable. At the same time, valorizing AP aligns with sustainability goals by reducing agro-industrial waste, promoting circular bioeconomy practices, and offering food companies practical avenues to improve ESG performance. Future research should extend this work through sensory evaluation, shelf-life testing, and assessment of interactions with other plant protein sources. Moreover, future research should be expanded to include product optimization strategies such as fiber modification, enzyme treatment, or hybrid protein systems. Such studies would not only validate the techno-functional role of AP but also expand its application in broader sustainable food systems. Overall, this work makes a strong contribution to the field of sustainable food engineering.