Optimizing the yeast density of Saccharomyces cerevisiae and the fermentation time to improve the nutritional composition of red-fleshed dragon fruit (Hylocereus polyrhizus) peel

Nguyen Thi Hanh Chi^1,2, Huynh Thi My Tien^1,2, Chau Kim Ly^1,2, Chau Sóc Thai^1,2 and Tran Trung Tuan^1,2

¹ An Giang University, An Giang, Vietnam. No 18, Ung Van Khiem Street, Long Xuyen ward, An Giang province, Vietnam
tttuan@agu.edu.vn
² Vietnam National University Ho Chi Minh City, Vietnam.

Abstract

This study aimed to optimize yeast inoculation density and fermentation time to improve the nutritional composition of dragon fruit peel through biotechnological upgrading with Saccharomyces cerevisiae. Fresh dragon fruit peels were collected, cleaned, blanched and chopped into small pieces prior to fermentation. A pure culture of S. cerevisiae strain CT6b was prepared and inoculated into the peel at three different densities (10⁵, 10⁶ and 10⁷ cells/g), while untreated peel served as the control. Fermentation was carried out under ambient conditions for 0, 24, 48, and 72 h, following a completely randomized design with four treatments and four replications. Samples were analyzed for pH, dry matter (DM), organic matter (OM), crude protein (CP) and crude fiber (CF) at each incubation time. The results demonstrated that both yeast density and incubation duration significantly influenced the chemical composition of the peel. pH decreased progressively during fermentation, with the fastest decline observed at the highest yeast density, reaching 4.17 after 72 h at 10⁷ cells/g. Dry matter and organic matter contents decreased steadily over time, reflecting the utilization of carbohydrates and other fermentable substrates by yeast metabolism. In contrast, crude protein content increased substantially in yeast-supplemented treatments, rising from 8.13% at 0 h to 10.56% after 72 h at the highest inoculation density. This improvement was attributed to microbial protein synthesis and the relative concentration effect caused by substrate degradation. Crude fiber content declined markedly with fermentation, from 25.43% in the control to 18.74% in the 10⁷ cells/g treatment at 72 h, suggesting partial degradation of structural polysaccharides such as hemicellulose and pectin. In conclusion, fermentation with S. cerevisiae effectively enhanced the nutritional profile of dragon fruit peel by increasing crude protein while reducing fiber and indigestible organic matter. The optimal condition was identified as 10⁷ cells/g yeast inoculation for 72 h of incubation, which produced the most significant improvements. These findings highlight the potential of yeast fermentation as a sustainable strategy to convert fruit processing by-products into value-added feed resources, thereby improving digestibility and suitability for pig nutrition.

Key words: dragon fruit peel, fermentation optimization, nutritional composition, agro-industrial by-products, animal feed valorization, sustainable livestock nutrition

Introduction

Pitaya (Hylocereus spp.), commonly referred to as dragon fruit, has gained significant global attention owing to its distinctive appearance, palatable flavor and high content of bioactive compounds. Among its cultivars, the red-fleshed species Hylocereus polyrhizus is particularly valued because of its abundance of antioxidants, vitamins and dietary fiber, which are beneficial to human health and have wide applications in the food industry (Attar et al 2022). Nevertheless, industrial processing of dragon fruit generates substantial quantities of peel by-products, which constitute approximately 30–45% of the fruit’s total weight. These peels are often discarded, creating environmental challenges while wasting a biomass rich in nutrients and functional components (Anandari et al 2024; Taharuddin et al 2023; Eveline and Audina, 2018). Dragon fruit peel is notable for its high dietary fiber content, both insoluble and soluble, as well as bioactive compounds and essential minerals. Biswas et al (2022) reported that peel powder contains approximately 6% protein, 6% fat, 4% ash and nearly 60% dietary fiber, primarily insoluble fiber. Similarly, Ramil et al (2021) documented fiber levels exceeding 60% along with substantial amounts of potassium, calcium, magnesium and iron. Reterta and Trinidad (2018) observed that fermentation of dragon fruit peel produces short-chain fatty acids such as acetate, propionate and butyrate, which are beneficial for gut health. Despite these advantages, the high fiber and low protein content restrict its direct use in pig diets, require biotechnological strategies to enhance its nutritional value.

Microbial fermentation offers a promising method for improving the nutritional profile and digestibility of fibrous fruit by-products for pig feeding. Fermentation has long been employed to increase crude protein content, degrade complex carbohydrates and improve palatability in feed resources. Among microorganisms, Saccharomyces cerevisiae is particularly relevant due to its generally recognized as safe status, its widespread use in animal nutrition and its ability to synthesize microbial protein while partially degrading structural carbohydrates (Dhiman et al 2025; Marlida et al 2023; Płacheta et al 2022; Kamal et al 2024). However, the efficiency of fermentation depends on process parameters such as inoculation density and incubation time, which directly influence microbial growth dynamics, substrate utilization and nutrient transformation. Optimizing these parameters is essential for maximizing protein enrichment, reducing fiber and improving the overall feeding value of the substrate (Araújo et al 2024; Yafetto et al 2022; Bajić et al 2022; Sun et al 2022; Aruna et al 2017). The use of fermented dragon fruit peel as pig feed presents dual benefits: reducing agro-industrial waste and providing alternative, locally available feed resources to help offset the rising costs of conventional feed ingredients. In the context of sustainable pig production, such strategies represent an eco-friendly approach to converting underutilized by-products into value-added feed ingredients (Shah A. M. et al 2025).

Therefore, the present study aimed to optimize the inoculation density of S. cerevisiae and the fermentation duration to improve the nutritional composition of H. polyrhizus peel for potential use in pig feed. Changes in pH, dry matter, organic matter, crude protein and crude fiber were systematically evaluated to identify fermentation conditions that enhance protein content and reduce fiber fractions, thereby improving the digestibility and suitability of dragon fruit peel as a component of pig diets.

Materials and methods

Preparation of dragon fruit peel

Red-fleshed dragon fruit (Hylocereus polyrhizus) peels were collected from vegetable and food processing factories in An Giang province, Vietnam. The peels were washed thoroughly with clean water to remove dirt and debris, then blanched in hot water at 75–100 °C for 3–15 minutes to reduce microbial contamination. After cooling, the peels were cut into small pieces (2–3 cm in diameter) to facilitate uniform fermentation.

Preparation of yeast suspension

A pure culture of Saccharomyces cerevisiae strain CT6b was used for inoculation. Five colonies were transferred into 500 mL Erlenmeyer flasks containing yeast extract peptone dextrose broth (YPDB) and incubated on a rotary shaker at 200 rpm for 48 h. Subsequently, the culture was scaled up by transferring 100 mL of the suspension into five additional flasks containing 500 mL of YPDB and incubated under the same conditions for 48 h, until the yeast density reached approximately 1 × 10⁸ cells/mL.

Inoculation of dragon fruit peel

The prepared yeast suspension was diluted to achieve three inoculation densities: 1 × 10⁵, 1 × 10⁶ and 1 × 10⁷ cells/g of peel. For 1 × 10⁷ cells/g, a 1:10 dilution was prepared by mixing 100 mL of yeast suspension (10⁸ cells/mL) with 900 g of peel. For 1 × 10⁶ cells/g, a 1:100 dilution was prepared by mixing 10 mL of yeast suspension with 990 g of peel. For 1 × 10⁵ cells/g, a 1:1000 dilution was prepared by mixing 1 mL of yeast suspension with 999 g of peel. The treatments were designated as follows: DFP0 (control, no yeast addition), DFP1 (10⁵ cells/g), DFP2 (10⁶ cells/g) and DFP3 (10⁷ cells/g).

Fermentation process

Approximately 1 kg of prepared peel was placed into thick polyethylene bags. The contents were mixed thoroughly, left open for 5–6 h to allow initial fermentation activity and then sealed. The bags were incubated at room temperature under ambient conditions (cool areas during hot weather, warm areas during cold weather).

Experimental design

The study was conducted using a completely randomized block design with four treatments (DFP0–DFP3) and four replications per treatment. Samples were collected at 0, 24, 48 and 72 h of fermentation for analysis.

Analytical procedures

pH measurement: pH was determined using a calibrated pH meter. The electrode was rinsed with distilled water before each measurement and immersed into homogenized peel samples until stable readings were obtained.

Yeast enumeration: Yeast density was determined by serial dilution and plating. Ten grams of fermented peel were homogenized in 90 mL of sterile saline solution, followed by serial dilution to appropriate concentrations. Samples were plated on yeast extract peptone dextrose agar (YPDA) and incubated for colony enumeration. Yeast cells were further identified microscopically (400×) based on morphology, budding characteristics and pigmentation.

Chemical composition: Samples were dried at 105 °C for 8 h to constant weight to determine dry matter (DM). Organic matter (OM), crude protein (CP) and crude fiber (CF) were analyzed according to AOAC (1990). CP was determined using the Kjeldahl method (N × 6.25).

The data were analyzed by using the ANOVA Linear Model (GLM) of Minitab 16.

Results and discussion

The changes of pH

The pH data presented in Table 1 were obtained from the same experimental batch that was previously reported by Chi N.T. et al. (2025) in the article entitled ‘Microbial Safety, Quality and Feed Potential of Fermented Red-fleshed Dragon Fruit Peel with Saccharomyces cerevisiae’, published in Advances in Animal and Veterinary Sciences, Article No. 2607–2614, Volume 13, Issue 12. In the present study, these pH data are transparently reused to support a different research objective, specifically the optimization of Saccharomyces cerevisiae inoculation density and fermentation time.

The pH of dragon fruit peel was significantly influenced by both the density of Saccharomyces cerevisiae inoculation and the incubation time (Table 1). At the beginning of the experiment (0 h), the pH values were relatively similar among treatments, ranging from 4.75 to 4.78. However, the treatment supplemented with the highest yeast density (DFP3, 10⁷ cells/g) already exhibited a slightly but significantly lower pH compared with the other treatments (p<0.05). As fermentation progressed, a gradual decline in pH was observed in all treatments, with the rate of reduction being more pronounced at higher yeast densities. After 24 h of incubation, the pH decreased from 4.68 in the control (DFP0) to 4.57 in DFP3, with intermediate values in DFP1 (4.64) and DFP2 (4.62). This trend continued at 48 h, where the pH values ranged from 4.53 in the control to 4.28 in DFP3, showing a clear yeast density-dependent effect. By 72 h, the lowest pH was recorded in DFP3 (4.17), followed by DFP2 (4.23) and DFP1 (4.29), while the control maintained the highest value (4.40). Overall, the results demonstrate that yeast supplementation accelerated the acidification of dragon fruit peel during fermentation, with the strongest effect observed at the highest inoculation density (10⁷ cells/g). This suggests that higher yeast populations enhanced metabolic activity, leading to increased organic acid production and a more rapid decline in pH.

The progressive decline in pH observed during the fermentation of dragon fruit peel reflects the metabolic activity of Saccharomyces cerevisiae. In this study, both inoculation density and incubation duration significantly influenced acidification dynamics. The lowest pH value (4.17) was recorded after 72 h at the highest inoculation level (10⁷ cells/g, DFP3), compared with 4.40 in the uninoculated control. This indicates a density dependent acceleration of fermentation, where larger yeast populations metabolized fermentable sugars more rapidly, producing ethanol and organic acids that contributed to substrate acidification.

Comparable acidification patterns have been widely reported in the bioconversion of fruit and vegetable residues. Weil et al (2025) demonstrated that microbial fermentation of plant-based substrates typically reduces pH from initial values of 5.0–6.5 to 3.5–4.5, depending on substrate characteristics and microbial load. This reduction was mainly attributed to the accumulation of lactic, acetic and other organic acids, which preserve the substrate, enhance digestibility and increase nutrient bioavailability. The findings also emphasized that both microbial density and fermentation duration are key factors governing acidification kinetics, consistent with the density-dependent effect observed in the present study. The results align with the established metabolic characteristics of S. cerevisiae. According to Parapouli et al (2020), this yeast species thrives in sugar-rich substrates, whereby sugars are initially converted to ethanol and organic acids even under aerobic conditions. The accumulation of these metabolites reduces pH, thereby creating selective pressure that inhibits undesirable microbial growth and stabilizes the fermentation environment. Moreover, Xu et al (2019) reported that inoculation with S. cerevisiae during silage fermentation influenced microbial community dynamics. Although lactic acid bacteria such as L. buchneri are traditionally considered the main drivers of acidification, the presence of S. cerevisiae enhanced the availability of fermentable substrates, indirectly stimulating lactic acid bacteria activity and improving silage stability. The study also highlighted the importance of inoculation density and microbial interactions in determining fermentation end-products and pH decline. Furthermore, Kuchen et al (2024) in beverage fermentation, observed that an initial juice pH of approximately 4.47 was optimal for yeast performance and inhibitory to spoilage organisms. Their optimization trials confirmed that pre-adjusting juice pH to around 4.6 improved fermentation outcomes. These findings are consistent with the current study, in which initial pH values near 4.7 provided a favorable environment for yeast activity and subsequent acidification.

The changes of DM

The dry matter (DM) content of dragon fruit peel was significantly affected by both yeast inoculation density and incubation time (Table 2). At the start of fermentation (0 h), DM content ranged from 7.25% to 7.67%. The control (DFP0) and the lowest yeast density (DFP1, 10⁵ cells/g) maintained the highest DM values (7.67% and 7.63%, respectively), whereas higher yeast densities (DFP2 and DFP3) showed slightly but significantly lower values (7.25% and 7.35%, p<0.05). As fermentation progressed, DM content declined steadily across all treatments. After 24 h, DM decreased from 7.51% in the control to 6.87% in DFP3, with intermediate values in DFP1 (7.29%) and DFP2 (7.13%). A more pronounced reduction was observed at 48 h, where DM values were 7.05% in the control, 6.70% in DFP1, 6.32% in DFP2 and 5.97% in DFP3. By 72 h, the lowest DM was recorded in DFP3 (5.78%), followed by DFP2 (6.05%) and DFP1 (6.35%), while the control retained the highest value at 6.80%.

The general trend demonstrates that yeast inoculation accelerated the reduction of dry matter content in dragon fruit peel during fermentation, and this effect became more pronounced with increasing yeast density and incubation time. The decline in DM can be attributed to the utilization of soluble carbohydrates and other organic substrates by S. cerevisiae for growth and metabolic activity, leading to the release of carbon dioxide and other fermentation by-products. This suggests that yeast fermentation not only alters the nutritional profile but also reduces the total solid content of the substrate, with the strongest effect observed at the highest inoculation density (10⁷ cells/g).

The reduction in dry matter (DM) content of dragon fruit peel during fermentation reflects the metabolic activity of Saccharomyces cerevisiae. The decline was more pronounced at higher inoculation densities, particularly at 10⁷ cells/g, indicating accelerated substrate utilization. This observation is consistent with the known biochemical function of yeast, in which fermentable carbohydrates are converted into ethanol, carbon dioxide and other metabolites. As described by Maicas (2020), the central pathway involves glycolytic conversion of glucose to pyruvate, followed by its reduction to ethanol with the simultaneous release of CO₂. These transformations account for the measurable loss of solids, as soluble carbohydrates are converted into volatile or soluble products. Yeasts are well adapted to sugar-rich fruit surfaces, which supports vigorous fermentation when substrates such as dragon fruit peel are available. Previous publications have reported a reduction in DM associated with fermentation of S. cerevisiae. Salafia et al (2022) documented up to ~85% DM loss during fermentation of pineapple waste cell wall sugars by S. cerevisiae ATCC 4126, highlighting that substrate composition and sugar availability strongly influence fermentation kinetics and the extent of DM reduction. Similarly, Kasprowicz-Potocka et al (2016) observed that yeast fermentation of blue lupin seeds decreased DM by up to 10% due to nitrogen-free extract depletion. Importantly, this reduction coincided with improvements in nutrient quality, as crude protein increased and digestible protein fractions were enhanced. Wrobel et al (2023) reported that microbial conversion of carbohydrates into metabolic end products including ethanol, CO₂ and organic acids, reduced DM content while simultaneously contributing to substrate stabilization. In agreement, Yuan et al (2023) found that higher yeast densities increased sugar depletion rates, leading to greater DM reductions. The mechanisms underlying DM loss are supported by Parapouli et al (2020), described how S. cerevisiae metabolizes sugars through glycolysis and alcoholic fermentation, yielding ethanol and CO₂. While ethanol remains within the substrate, CO₂ escapes, contributing directly to DM reduction. Additionally, part of the substrate carbon is assimilated into microbial biomass, altering the overall DM balance. Xu et al (2019) also reported measurable reductions in DM after inoculation with S. cerevisiae, attributing this to enhanced microbial activity and greater substrate utilization. Their findings underscore the trade-off inherent in yeast fermentation: although DM yield decreases, nutritional quality improves through protein enrichment and enhanced digestibility.

The change of CP

The crude protein (CP) content of dragon fruit peel was markedly influenced by both yeast density and incubation time (Table 3). At the initial stage (0 h), CP values were similar across all treatments, ranging from 8.13% to 8.15%, with no significant differences (p>0.05). This indicates that the baseline protein content of the substrate was uniform prior to fermentation. After 24 h of incubation, CP content began to increase, particularly in treatments supplemented with higher yeast densities. The lowest value was observed in the control (8.31%), while the highest was recorded in DFP3 (8.67%), with DFP1 (8.48%) and DFP2 (8.56%) showing intermediate increases. By 48 h, the differences among treatments became more pronounced, with CP ranging from 8.71% in the control to 10.43% in DFP3. Notably, DFP2 (10.04%) and DFP1 (9.75%) also exhibited substantial increases compared with the control. At 72 h, CP content reached its peak, with DFP3 maintaining the highest value (10.56%), followed by DFP2 (10.24%) and DFP1 (9.98%), while the control increased only slightly to 8.90%.

Overall, the results demonstrate that yeast supplementation significantly enhanced the CP content of dragon fruit peel during fermentation and the effect was positively correlated with inoculation density and incubation time. This increase in crude protein can be attributed to microbial protein synthesis and yeast cell proliferation, which contribute additional nitrogenous compounds to the substrate. Furthermore, yeast activity may also concentrate protein levels by utilizing carbohydrates and reducing dry matter content, thereby proportionally increasing CP concentration. These findings highlight the potential of Saccharomyces cerevisiae fermentation to improve the protein value of fruit by-products, with the strongest effect observed at the highest inoculation density (10⁷ cells/g).

The crude protein (CP) content of dragon fruit peel increased progressively with both yeast inoculation density and fermentation duration. As shown in Table 3, CP rose from 8.90% in the uninoculated control to 10.56% after 72 h at the highest inoculum level (10⁷ cells/g, DFP3). This enrichment can be explained by two main mechanisms: (i) microbial protein synthesis associated with the proliferation of Saccharomyces cerevisiae and (ii) a relative concentration effect resulting from carbohydrate degradation and dry matter loss, which proportionally increased the protein fraction. These findings confirm that yeast fermentation is a viable strategy to upgrade low protein fruit by-products, aligning with the broader principle of single cell protein production.

Comparable results have been reported for other agro-industrial residues. Tropea et al (2022) demonstrated a substantial protein increase, from 8.52% to 40.19%, using a multi-waste substrate combining plant residues with nitrogen rich fish waste and cellulolytic enzyme supplementation. Their study highlights the role of substrate composition and enzymatic hydrolysis in enhancing microbial growth and protein yield, while the dragon fruit peel system emphasizes inoculum density as a determining factor under nitrogen-limited conditions. Similarly, Suriyapha et al (2021) showed that citric waste enriched with yeast waste achieved 535 g/kg DM CP compared with only 110 g/kg DM in the unfermented substrate. Other studies underscore the influence of substrate type. Dunuweera et al (2021) found that CP enrichment varied across fruit residues, reaching 48.32% in pineapple based medium but only 9.64% in pomegranate. This variation reflects differences in fermentable sugars and nutrient availability, factors that also explain the relatively modest enrichment observed in dragon fruit peel. Aruna et al (2017) reported that yam peel fermentation with S. cerevisiae increased CP from 6.60% to 11.08% without nitrogen supplementation, but supplementation with ammonium sulphate further elevated CP to 15.54% and improved essential amino acid content. Similarly, Dygas and Berłowska (2023) demonstrated that sugar beet processing waste supported high single-cell protein yields due to its favorable nutrient profile. Maxwell et al (2019) also reported that potato peel fermentation under solid-state conditions with nitrogen supplementation achieved CP increases from 12.5% to 21.86%, representing a 74.88% improvement.

The change of CF

The crude fiber (CF) content of dragon fruit peel was significantly reduced by yeast inoculation and incubation time (Table 4). At the beginning of fermentation (0 h), CF levels were relatively high across treatments, ranging from 25.17% to 25.43%, except for DFP3 (22.88%), which was already significantly lower (p<0.05). A gradual decline in CF was observed as fermentation progressed and the effect was more pronounced with increasing yeast density. After 24 h, CF content decreased slightly in the control (25.28%) compared with the initial value, whereas yeast-supplemented treatments exhibited greater reductions, particularly DFP3 (22.48%). By 48 h, the differences became more marked: the control remained at 24.83%, while DFP1, DFP2 and DFP3 declined to 24.25%, 23.58% and 20.87%, respectively. At 72 h, CF reached the lowest level in DFP3 (18.74%), followed by DFP2 (21.80%) and DFP1 (22.97%), whereas the control maintained the highest content (24.13%).

These results indicate that fermentation with Saccharomyces cerevisiae effectively degraded the fibrous fraction of dragon fruit peel and the extent of fiber reduction was dependent on yeast density and incubation duration. The decline in CF may be explained by the enzymatic activity of yeast and associated microbial populations, which are capable of partially breaking down structural polysaccharides such as pectin, cellulose and hemicellulose. Additionally, the utilization of soluble fiber components as energy sources during fermentation likely contributed to the observed reduction. The most substantial decrease occurred under the highest inoculation density (10⁷ cells/g), suggesting that increased microbial activity enhanced fiber degradation and improved the digestibility potential of the substrate.

The crude fiber (CF) content of dragon fruit peel decreased progressively with increasing yeast inoculation density and fermentation time, with the most pronounced reduction observed at 72 h in DFP3 (10⁷ cells/g), where CF declined from 25.43% at 0 h to 18.74%. This pattern indicates that Saccharomyces cerevisiae contributed to the partial degradation and solubilization of structural carbohydrate fractions, particularly hemicellulose and pectin, as well as some amorphous cellulose regions. The density-dependent effect suggests that higher yeast populations enhanced metabolic activity, accelerating carbohydrate breakdown and reducing measured fiber levels while proportionally concentrating protein due to dry matter losses.

These results align with previous reports on the capacity of yeast to alter fiber-rich substrates. Cano et al (2024) demonstrated that pectinase-producing S. cerevisiae strains significantly modified pectin-rich fractions of crude fiber, lowering fiber content and improving fermentability of fruit residues. Similarly, Thu et al (2025) observed consistent reductions in CF during the fermentation of jackfruit by-products, attributing the changes to yeast-mediated breakdown of non-cellulosic fractions, primarily hemicellulose and pectin. Both studies highlight the importance of substrate composition and microbial enzyme activity in determining the extent of fiber reduction. Hoyer et al (2013), used baker’s yeast, Saccharomyces cerevisiae for fiber degradation, investigated the influence of fiber degradation on simultaneous saccharification and fermentation of high-solids spruce slurry to ethanol. Results demonstrated that extensive fiber degradation by cellulolytic enzymes not only reduced the recalcitrant polysaccharide fraction but also enhanced the release and availability of fermentable sugars.

The change of OM

The organic matter (OM) content of dragon fruit peel was influenced by both yeast inoculation density and incubation time (Table 5). At 0 h, OM content was similar across treatments (86.59–86.64%) and showed no significant differences (p>0.05), indicating a uniform baseline composition of the substrate before fermentation. After 24 h of incubation, slight but significant reductions in OM were observed in yeast-inoculated treatments compared with the control. The control (DFP0) retained the highest OM value (86.30%), while the lowest was recorded in DFP3 (86.01%). A clearer trend emerged at 48 h, where OM decreased progressively with higher yeast densities: 85.86% in the control, 85.18% in DFP1, 84.79% in DFP2 and 84.16% in DFP3. By 72 h, OM continued to decline, with the highest value maintained by the control (85.55%) and the lowest by DFP3 (83.28%), while DFP1 (84.58%) and DFP2 (84.04%) showed intermediate reductions.

These findings indicate that yeast fermentation led to a gradual decline in OM content of dragon fruit peel and the effect was amplified by higher inoculation densities and longer incubation times. The reduction in OM can be attributed to the utilization of fermentable organic substrates, including soluble carbohydrates and portions of hemicellulose and pectin, as energy sources for Saccharomyces cerevisiae. The metabolic conversion of these organic fractions into carbon dioxide, ethanol and other fermentation by-products effectively reduced the proportion of organic matter remaining in the substrate. This suggests that fermentation not only modifies nutrient composition but also reduces indigestible organic fractions, with the greatest effect occurring under the highest yeast density (10⁷ cells/g).

The progressive reduction in organic matter (OM) observed during fermentation of dragon fruit peel reflects the metabolic activity of Saccharomyces cerevisiae. Both yeast inoculation density and fermentation duration significantly influenced this decline, with the lowest OM recorded at 72 h under the highest inoculation density (10⁷ cells/g, DFP3). The magnitude of reduction (~2–3%) suggests that yeast preferentially utilized soluble carbohydrates, followed by partial hydrolysis of hemicellulose and pectin, converting these substrates into ethanol, carbon dioxide and organic acids. Carbon flux into ethanol, which remained in solution and CO₂, which escaped into the atmosphere, effectively reduced the measurable OM fraction of the substrate. Such metabolic processes align with the general biochemical characteristics of S. cerevisiae, as reviewed by Parapouli et al (2020), emphasized its capacity to initially ferment sugars into ethanol and organic acids before utilizing ethanol for further growth.

The selective reduction in OM during fermentation can be attributed not only to sugar metabolism but also to partial degradation of amorphous structural polysaccharides. Martins et al (2020) highlighted the role of pectin as a major carbon fraction in fruit residues, serving as a key substrate for microbial fermentation. This finding is consistent with the present study, as pectin and hemicellulose fractions in dragon fruit peel were likely important contributors to the observed OM decline. Mgeni et al (2024) also emphasized that fruit wastes, rich in pectin, hemicellulose and cellulose, undergo selective OM reduction during fermentation, thereby enhancing their potential for bioethanol production. Haske et al (2023) demonstrated that S. cerevisiae produced pectinase when cultivated on orange peels and maize cobs under solid-state fermentation. Their results showed enzymatic hydrolysis of pectin, leading to the release of galacturonic acid and reducing sugars, which decreased the complex carbohydrate fraction of the substrate. Similarly, peak pectinase activity was observed at ~72 h before declining due to nutrient depletion, a trend consistent with the OM reduction pattern in the present study.

Yeast cell walls are rich in β-glucans and mannoproteins and undergo structural changes during autolysis (Avramia I. and Amariei S., 2021; Coradello G. and Tirelli N 2021; Wang J et al, 2018). Autolysis has been shown to release structural polysaccharides into the surrounding medium, increasing the availability of β-glucans and mannoproteins (Ciccone M et al 2024; Martínez J M  et al  2016). Recent in vitro study showed that fungal β-glucans significantly shortened the lag phase and increased the growth rate of several lactic acid bacteria strains, demonstrating a direct prebiotic effect (Bukša et al 2025). In pig nutrition study, dietary supplementation with yeast cell wall preparations (containing β-glucans and mannoproteins) has been shown to improve intestinal morphology, enhance nutrient digestibility and modify gut microbiota composition. Although this study did not report a specific increase in Lactobacillus spp., the overall improvement in gut microbial environment is consistent with the hypothesis that yeast cell wall polysaccharides can support beneficial lactic acid producing bacterial communities (Lee et al 2021). Although the present study did not directly quantify β-glucan release or lactic acid bacteria, the observed reductions in pH and improvements in nutrient composition are consistent with a mechanism in which yeast autolysis and the release of cell-wall polysaccharides support lactic acid bacteria activity during fermentation. Because β-glucan and mannoproteins were not directly measured in this study, future work should include its quantification to confirm its contribution to the observed fermentation improvements.

Conclusion

Fermentation of red-fleshed dragon fruit (Hylocereus polyrhizus) peel with Saccharomyces cerevisiae significantly altered its nutritional composition and the extent of change depended on both yeast inoculation density and incubation time. Increasing yeast density accelerated acidification, as reflected by a greater reduction in pH, particularly at 10⁷ cells/g. Dry matter and organic matter contents declined progressively during fermentation, indicating the utilization of carbohydrates and other organic substrates by yeast metabolism. In contrast, crude protein content increased substantially in yeast-inoculated treatments, with the greatest enhancement observed at the highest yeast density, likely due to microbial protein synthesis and concentration effects from substrate degradation. Crude fiber content decreased markedly with increasing yeast density and incubation time, suggesting partial breakdown of structural polysaccharides such as hemicellulose and pectin.

Acknowledgements

This research was funded by Vietnam National University Ho Chi Minh City (VNU-HCM) under grant number C2024-16-18.

The authors thank to An Giang University (AGU), Vietnam National University Ho Chi Minh City (VNU-HCM, the Experimental Laboratory

I'd like to extend my sincere thanks to my colleagues in the Department of Animal Science and Veterinary Medicine and the students of class DH22CN for their continuous support and collaboration on this project.

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