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Article

Enhancing Steamed Bread Quality Through Co-Fermentation of Sourdough with Kazachstania humilis and Lactobacillus plantarum

by
Zicheng Wang
,
Ao Fu
,
Xin Wang
and
Guohua Zhang
*
School of Life Science, Shanxi University, Taiyuan 030006, China
*
Author to whom correspondence should be addressed.
Fermentation 2025, 11(6), 298; https://doi.org/10.3390/fermentation11060298
Submission received: 18 April 2025 / Revised: 15 May 2025 / Accepted: 21 May 2025 / Published: 23 May 2025
(This article belongs to the Section Fermentation for Food and Beverages)
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Abstract

Sourdough fermentation, a time-honored biotechnology known for enhancing the texture, flavor, and nutritional quality of steamed bread, has yet to be fully leveraged for optimizing microbial synergy, particularly between Kazachstania humilis (KH) and Lactobacillus plantarum (LP). In this study, we systematically evaluated the impact of fermentation dynamics on sourdough properties and steamed bread quality using single-strain (KH or LP) and co-fermentation (LP+KH) strategies. Our findings demonstrated that LP+KH co-fermentation significantly accelerated sourdough acidification, achieving the lowest pH (3.8) and highest total titratable acidity (TTA, 14.2 mL) among all groups. This synergy also enhanced dough gas retention, resulting in an 11.89% and 7.25% increase in specific volume compared to LP and KH monocultures, respectively. Steamed bread produced from the co-fermented dough exhibited markedly improved textural qualities, including reduced hardness, gumminess, and chewiness, along with increased cohesiveness. Moreover, the water content in bread from the LP+KH group remained significantly higher, contributing to better freshness retention over time. In conclusion, LP and KH co-fermentation offers a promising approach for elevating the quality and shelf-life of steamed bread, revealing untapped potential in microbial synergy during sourdough fermentation.

1. Introduction

Wheat is one of the most important crops in global agriculture, which is pivotal in East Asia, where Chinese steamed bread dominates wheat flour consumption, accounting for 40–50% [1]). Within the domain of cereal fermentation, the utilization of sourdough in the fermentation of steamed bread has been regarded as an ancient biotechnological practice, thus leading to enhanced texture [2], flavor [3] and nutritional value of the dough [4]. Sourdough is formed by the interaction of wheat flour, water, and yeast or lactic acid bacteria (LAB), which work together on the sourdough to produce a series of biochemical reactions that affect the rheological properties of sourdough and make the texture and flavor of steamed bread significantly different [5].
Sourdough provides a conducive habitat for microbes, which can support the growth of more than 50 species of LAB and 20 species of yeasts [6,7]. LAB produces a wide variety of metabolites, including exopolysaccharides and various organic acids, that not only give the sourdough its unique flavor but also have a positive effect on the rheological properties and gluten network structure of the sourdough [8]. It has been shown that Lactobacillus plantarum is a typical lactic acid bacterium in sourdough [9] which mainly uses saccharides in the sourdough to ferment, producing metabolites such as CO2 and alcohol. Kazachstania humilis, the second largest proportion of yeast isolated from many sourdoughs and naturally leavened doughs, contributes to the puffiness and softness of sourdough through CO2 and adds flavor to steamed bread with ethanol after fermentation [10,11]. The growth conditions of LAB and yeast in sourdough are similar; both can use glucose and maltose for fermentation, and yeast fermentation can provide nutrients, such as amino acids and vitamins, for LAB [12]. The metabolites of lactic acid bacteria can be used as the metabolic energy of yeast, which is conducive to the further proliferation of yeast [6]. Compared to steamed bread made from yeast only, fermented sourdough not only improves the flavor of steamed bread but also improves the nutritional value of steamed bread. Liu et al. compared the effects of single-strain yeast fermentation with the co-fermentation of Lactobacillus plantarum and Saccharomyces cerevisiae on the digestive and quality properties of steamed bread, highlighting the beneficial role of mixed fermentation involving plant-derived Lactobacillus plantarum in enhancing the nutritional value and sensory qualities [13]. Xu et al. found that Kazachstania humilis significantly increased the complex profile of volatiles and improved the specific volume of wheat sourdough bread in mixed fermentation [14].
In the history of research and production of steamed bread, a mixed fermentation system composed of LAB, yeast, and other bacteria existing in the natural environment was fermented, which was called a sourdough starter [15]. However, with the development of food science and technology, high-activity single yeasts have been used for the rapid fermentation of steamed bread. This fermentation method can significantly accelerate the fermentation rate of sourdough and improve production efficiency. However, single-yeast fermentation may result in a relatively simple flavor of steamed bread that lacks the rich taste and aroma produced by traditional fermentation methods [16]. To compensate for the deficiency of single fermentation, researchers have begun to explore combined fermentation technology, especially yeast combined with LAB, through the synergistic effect of the two to improve the flavor and texture of steamed bread. Composite fermentation technology not only improves the nutritional value of steamed bread but also gives it a unique flavor and taste [17]. However, the combined fermentation of Lactobacillus plantarum and Kazachstania humilis has not been thoroughly studied in terms of the number of viable bacteria, tastes, or flavors of the product.
The aim of this study was to compare the effects of single and combined fermentation of Lactobacillus plantarum and Kazachstania humilis on sourdough formation and steamed bread quality, including specific volume, texture, aging rate, and water content of steamed bread. Sensory evaluation and volatility analysis of steamed bread were performed. Research on the combined fermentation of LAB and yeast is helpful for promoting sustainable development and innovation in the pasta industry. Through continuous research and development of new pasta products and production processes, the pasta market can be broadened, and the added value and economic benefits of the pasta industry can be improved.

2. Materials and Methods

2.1. Ethical Considerations

Sensory evaluation involving human panelists was conducted to assess the quality of steamed bread fermented with Kazachstania humilis and Lactobacillus plantarum. The study was approved by the Ethics Review Committee of Shanxi University (Approval no. SXULL-2020007). All participants signed an Informed Consent Form before participating. Data were collected anonymously, and where anonymity could not be guaranteed, written consent for publication was obtained. All procedures complied with ethical guidelines for research involving human subjects.

2.2. Raw Materials

The LAB species (Lactobacillus plantarum1 (LP1)) and the yeast species (Kazachstania humilis strain (KH2-1)) were used in this study and were isolated from the traditional sourdough preserved in the School of Life Sciences, Shanxi University, Taiyuan. MRS medium was provided by Beijing Aobo Star Biotechnology Co., Ltd., Beijing, China. YPD medium was provided by Qingdao Haibo Biotechnology Co. Ltd., Qingdao, China. Wudeli flour was provided by the Wudeli Flour Group Co., Ltd., Handan, China. Ordinary dry yeast was provided by Angel Yeast Co., Ltd., Yichang, China. Both MRS and YPD mediums provided were prepared using the below preparation. Both MRS and YPD media are non-selective and, thus, can support the growth of both bacteria and yeasts in the absence of selective agents. Therefore, in ensuring the purity of LP1 (Lactobacillus plantarum) and KH2-1 (yeast) cultures, appropriate antifungal and antibacterial agents (cycloheximide and chloramphenicol, respectively) were added to the media during solid culture preparation.

2.2.1. Preparation of MRS Medium and YPD Medium

MRS Medium
First, weigh each component according to the MRS medium formula in Table 1. Add it to 1000.0 mL of distilled water and stir evenly. Then, use a pH meter to adjust the pH of the solution to 5.7 ± 0.2. Dispense the prepared medium into suitable containers and sterilize it by high pressure at 121 °C for 15 min. After sterilization, the liquid medium can be used after cooling to room temperature and stored in a 4 °C refrigerator.
For solid culture medium, an additional 15.00 g of agar is required. MRS solid culture medium needs to be sterilized, and cycloheximide antibiotics are added to make the final concentration reach 100 mg/L in the plating process, ensuring reliable distinction between yeast and bacterial populations during the process. When cooled to 50–60 °C, gently shake it and pour it into a sterile plate. After cooling and solidification, it is ready for use.
YPD Medium
First, weighing each component according to the YPD medium formula in Table 2, add it to 1000.0 mL of distilled water and stir evenly. Autoclave at 121 °C for 15 min. After sterilization, the liquid medium can be used after cooling to room temperature and stored in a 4 °C refrigerator.
If preparing solid culture medium, add an additional 20.00 g agar powder and continue to stir and heat until the agar is completely dissolved. Then, divide the solution into suitable containers. After sterilization, add chloramphenicol antibiotic agents to the YPD solid culture medium to make the final concentration reach 100 mg/L in the plating process, ensuring reliable distinction between yeast and bacterial populations during the process. Gently shake it when it cools to 50–60 °C, pour it into a sterile plate, and cool it to solidify for use.

2.3. Statistical Analysis

All the data were analyzed using Microsoft Excel, and SPSS data were analyzed using IBM SPSS Statistics (Version 29.0, 2024). Means and standard deviations were calculated for each group at all time points. One-way ANOVA was conducted to compare the effects of time and treatment (single versus mixed culture) on microbial growth. Post-hoc comparisons were performed using Tukey’s test where appropriate. A p-value of <0.05 was considered statistically significant.

2.3.1. Flowchart Process

The overall experimental procedure is illustrated in the flow chart in Figure 1 below, which consists of three main stages: (A) microbial cultivation, discussed in Section 2.3.2, (B) physical and chemical determination of sourdough, discussed in Section 2.3.3, and (C) quality analysis of steamed bread, discussed in in Section 2.3.4.

2.3.2. Microbial Cultivation

Microbial Starters and Growth Curve
LP1 cells were cultivated in MRS medium at 30 °C for 24 h. KH was cultivated in the YPD medium at 30 °C for 24 h. The dosage of the LAB solution and yeast solutions was 2%.
Sourdough Fermentation
At the 24 h of fermentation, 20 mL of the LP1 culture medium of MRS broth and 20 mL of the KH culture medium of YPD broth were collected. Afterwards, centrifugation at 4000 rpm for 10 min was performed to obtain the sediment. Then, 10 mL of deionized water was added to each of the two sediments to prepare a suspension. LP1 sourdough was prepared by mixing 100 g of wheat flour, 45 mL of deionized water, and 5 mL of LP1 suspension. KH sourdough was prepared by mixing 100 g of wheat flour (strong flour, Wudeli Flour Group, Hebei, China), 45 mL of deionized water, and 5 mL of KH suspension. LP1- KH sourdough was prepared by mixing 100 g of wheat flour (strong flour, Wudeli Flour Group, Hebei, China), 40 mL of deionized water, 5 mL LP1 suspension, and 5 mL KH suspension. Each mixture was manually kneaded and rolled into sourdough to achieve an initial cell density of approximately 6–7 Log CFU/g for LP1 and 5–6 Log CFU/g for KH2-1. Thereafter, sourdoughs were placed in a biochemical incubator (HRSP-H series biochemical incubator, Haier, Qingdao, China) at 85% humidity and 30 °C temperature for 24 h, as shown in Figure 2 below.

2.3.3. Physical and Chemical Determination of Sourdough

Change of Viable Cell Count
Steamed bread was prepared according to the method described by [18]. First, 10 g of sourdough at different fermentation times was weighed, 90 mL of sterile saline (0.9% NaCl) was added, and it was beaten and homogenized for 2 min. Next, 1 mL of the sample homogenate was diluted to a 10-fold gradient, resulting in dilutions of 10−4, 10−5, and 10−6. LAB growth was confirmed using the plate count method in MRS agar medium, whereas yeast growth was confirmed in YPD agar medium. The agar medium was then incubated at 30 °C for 48 h.
Determination of pH, Total Titratable Acidity, Lactic Acid, and Acetic Acid
The pH and total titratable acidity (TTA) were determined according to the methods described by [19]. First, 10 g of sourdough was mixed with 90 mL of water, magnetically stirred for 15 min, and titrated against 0.1 N NaOH until a final pH of 8.2 was obtained. TTA was measured after the steamed bread-making process, and the results were expressed as the double volume (mL) of 0.1 N NaOH required to titrate 10 g of sourdough.
Lactic and acetic acids were quantified according to the reports of Lefebvre et al, 2002, with some modifications [20]. Briefly, 10 g of sourdough was diluted in 90 mL of ultra-pure water, and the mixture was homogenized with a homogenizer (flap-type sterile homogenizer, Tuohe Electromechanical Technology, Shanghai, China). The mixture was extracted in a water bath at 60 °C for 15 min. Then, the extracts were centrifuged at 4000 rpm for 15 min to obtain the supernatant, followed by syringe filtration using a 0.22 μm filter. The concentrations of organic acids in the sourdough were measured using a high-performance liquid chromatography (HPLC) system equipped with a Hi-Plex H column (300 × 6.5 mm; Agilent, Santa Clara, CA, USA) with phosphoric acid (0.01%) as mobile phase A and pure methanol as mobile phase B. The column temperature was maintained at 40 °C, the sample size was set at 10 µL, and a flow rate of 0.8 mL/min was used.

2.3.4. Quality Analysis of Steamed Bread

Steamed Bread Performance
The preparation of steamed bread was modified according to the method described by [13], and the formulations are presented in Table 1. Specifically, the sourdoughs constituted 50% of the total dough weight, with the remaining 50% comprising a mixture of flour and deionized water in a 2:1 ratio. The sourdough was stirred in a flour mixer for 10 min and then kneaded manually until it became smooth. After that, the dough was cut into 50 g portions, and each portion was kneaded by hand until the dough was moistened and formed. The portions were left in a fermenting box with 85% humidity and a temperature of 30 °C for 1 h to increase, and then they were steamed in a pot for 25 min. Steam bread fermented using ordinary dry yeast, named OFB, was regarded as the control group, prepared using a 2:1 proportion of flour and deionized water.
Determination of Specific Volume
The specific volume of steamed bread was measured using the millet replacement method described by [21]. The results were calculated using the following formula:
λ = V m
where λ is the specific volume of steamed bread, V is the volume, and m is the mass of steamed bread.
Determination of Water Content
According to the Chinese food national standards [22], the steam bread was stored at room temperature and exposed to air for 0, 24, 48, and 72 h, and 1 g of samples was taken from the central part, placed in a weighing bottle of constant mass, and placed in a drying oven at 101–105 °C until a constant mass was obtained. Moisture content was calculated based on the difference in mass before and after. Results: The average values of three repeated measurements were recorded.
Determination of Texture
The method of detecting texture described by [23] was modified. The steamed bread was cooled to room temperature for 1 h, and a texture analyzer (TMS-PRO, Food Technology Corporation, Sterling, VA, USA) was used to determine the texture of freshly steamed bread. Briefly, steamed bread was cut into a cuboid measuring 2 cm × 2 cm × 2 cm. The measurement conditions were set as follows: the sensor range was 25 N; the detection speed was 60 mm/min; the initial force was 0.05 N, and the shape variable was 50%. The test was repeated three times for each group of samples, and the average value was calculated.
Sensory Evaluation Analysis
A sensory evaluation panel comprising six professionals assessed eight quality attributes: specific volume, appearance, internal structure, color and luster, elasticity, crispness, aroma, and flavor. The evaluators (n = 6, aged 20–40 years, evenly distributed by sex) rated these attributes according to the scoring criteria outlined in Table 3.

3. Results

Several studies have been conducted to explore the interactions between lactic acid bacteria and yeasts during sourdough fermentation. For example [24], emphasized the importance of using well-defined sourdough starter cultures in industrial applications, and research done by [25] highlighted the synergistic roles of Lactobacillus plantarum and Kazachstania humilis in promoting dough development and enhancing fermentation stability. Their findings are consistent with our results, where the presence of K. humilis significantly promoted the growth of L. plantarum in mixed fermentations. Similar microbial dynamics and mutualistic effects were observed, confirming what the researchers [26] highlighted: that controlled co-fermentation can improve both microbial viability and product quality in sourdough production.

3.1. Growth Curve and Change of Viable Cell Count During Fermentation of Sourdough

As can be seen from Figure 3, in the 0–2 h and 0–6 h periods, LP1 and KH were in the delayed period, and their growth rates were slow. Then, LP1 entered the logarithmic growth period from 2–14 h, KH grew rapidly from 6–12 h, and entered the logarithmic growth period with rapid growth, and then LP1 and KH strains entered the stable period at 14 h and 12 h, respectively. The fermentation broth of lactic acid bacteria and yeast in the logarithmic stage at 12 h with a high concentration of bacteria was selected to prepare sourdough. Selective media were used for the accurate enumeration of LP and KH. Cycloheximide (100 mg/L) was added to MRS medium to inhibit yeast growth, and chloramphenicol (100 mg/L) was added to YPD medium to inhibit bacterial growth. This ensured that OD600 measurements reflected the growth of the target microorganisms in the monoculture.
We measured the changes in the number of colonies during the mixed fermentation of LP and KH and compared them with their separate fermentations, and the results are shown in Figure 4a,b. As shown in Figure 4a, the variation trend of lactic acid bacteria colony number in LP+KH mixed fermented sourdough was similar to that in the LP-alone fermented sourdough. At the early stage of fermentation 0–6 h, LP increased rapidly, the number of colonies continued to increase, and the number of colonies increased by one order of magnitude, and there was no significant difference between the two groups. Between 6 and 20 h, the LP grew slowly. At 20–24 h, the number of colonies in both groups reached a maximum at 20 days, and the number of LP colonies in the LP+KH mixed fermentation group was nearly twice that of the LP fermentation group alone. This indicates that the presence of KH promotes the growth of LP. As shown in Figure 4b, at 0–10 h, the number of colonies on LP+KH mixed fermented sourdough was always greater than that on KH-alone fermented dough, and after 10 h, the number of colonies on KH-alone fermented dough was greater than that on LP+KH-mixed fermented sourdough. Selective media were also used for viable cell counting. MRS agar containing cycloheximide (100 mg/L) was used for enumerating LP, and YPD agar with chloramphenicol (100 mg/L) was used for enumerating KH. This allowed separate and reliable counting of each microorganism in both single and mixed cultures. However, this may be due to the gradual accumulation of lactic acid produced by LP and the gradual decrease in pH as fermentation progresses. The growth of KH was inhibited to some extent; however, there was no significant difference in the number of colonies between the two groups. The number of colonies in the two groups also reached a maximum at the 20 h, and the number of colonies began to decline after the 20 h. There may be two main factors that make the living environment of KH and LP unfavorable. First, as fermentation progressed, the pH of the dough dropped significantly, reaching an overly acidic environment that was no longer suitable for their growth. Second, owing to the surge in the number of microbes in the dough, there was increased competition for resources, which may have led to a large die-off of microbial populations.

3.2. Changes of pH and TTA During the Fermentation of Sourdough

During sourdough fermentation, microbial metabolism can metabolize carbohydrates in sourdough, producing lactic acid, propionic acid, and other acids that lower the pH of the dough. As shown in Figure 5, the pH values of the three experimental groups (LP, KH, LP+KH group) before fermentation were in a similar range of around 6.1 and did not change significantly. As fermentation progressed, the pH values of the three groups showed a downward trend, while the TTA values showed an upward trend. At the end of fermentation, the pH of group KH decreased slowly, from 6.2 at the beginning to 4.2 at the end of fermentation, which was relatively small, but lasted for a long time. The pH and TTA values of the LP+KH group were not significantly different from those of the LP+KH group, but the pH and TTA values were significantly lower than those of the KH group. These results show that LP1 plays an important role in the production of organic acids during sourdough fermentation. The organic acid content of the LP and KH combined fermentation was higher than that of KH alone fermentation. Our findings are in agreement with [27], who reported that the co-fermentation process of LAB and yeast produces more acidic substances than yeast alone. After 16 h of fermentation, the pH and TTA change rate of the three kinds of dough slowed down, and sourdough fermented for 20 h was selected as the starter to make steam bread.

3.3. Lactic Acid and Acetic Acid Content of Sourdough

As illustrated in Figure 6, lactic and acetic acid concentrations in the three sourdough groups (LP, KH, and LP+KH) were quantified after 20 h of fermentation. The LP group exhibited the highest lactic acid content (approximately 7.5 mg/g), whereas the KH group showed significantly reduced lactic acid production (approximately 4.5 mg/g) compared to the other sourdoughs. Notably, the lactic acid content in the LP+KH co-fermentation group remained comparable to that in the LP monoculture, with only a marginal reduction that lacked statistical significance (p > 0.05). The acetic acid content in the LP+KH group was significantly lower than that in the LP group. Our study revealed that acetic acid contributes minimally to overall acidity compared to lactic acid; lactic acid accumulation strongly correlates with pH decline and TTA elevation, whereas LP dominates lactic acid biosynthesis, exerting no significant synergistic effect on acid yield when supplemented with KH. Consistent with previous reports [28], metagenomics analyses have confirmed that LAB species dominate sourdough, and the lactic acid produced by their metabolism is the main driver of the decrease in pH and the increase in TTA of sourdough. Other studies have reported that during fermentation, lactic acid not only imparts a unique flavor to sourdough bread and enhances its texture by strengthening the gluten network, thereby softer the internal structure and positively influencing sourdough quality; acetic acid also significantly affects the flavor profile, exhibits strong antibacterial properties, and further improves overall dough quality [29].

3.4. Appearance, Section and Specific Volume of Steamed Bread

The appearance and section of steamed bread exhibited pronounced variations across the fermentation groups (Figure 7). The steamed bread in the OFB group exhibited a smooth surface and uniform coloration, devoid of noticeable cracks or indentations. The sectional structure was dense, and the distribution of pores was indicative of adequate fermentation. In contrast, steamed bread in the LP group was slightly darker and had lower surface smoothness than that in the OFB group. The sectional structure was somewhat loose, and the distribution of the pores was uneven. The KH group presented irregular surface blisters and localized collapse, and the sectional structure was relatively compact with a sparse distribution of pores, which may be ascribed to the limited capacity for CO2 retention. Conversely, the steamed bread in the LP+KH group demonstrated an appearance and surface smoothness that was between those of the LP and KH groups. The sectional structure was more uniform, the distribution of pores was even greater, and the pore size was moderate. This may be due to the activation of transglutaminase (TGase) under acidic conditions, which promotes the cross-linking of gluten proteins, enhances network stability, and improves CO2 retention capacity [30]. Other studies have reported that, in the process of yeast and LAB species composite fermentation, starch is more fully degraded to produce small molecule glucose, and at the same time, LAB species are added to increase the acid production of dough, while low pH environment promotes the growth of yeast species, which is beneficial for yeast species to produce CO2 [31]. Our study revealed that the synergistic effect of LP and KH facilitated dough fermentation and augmented pore formation, thereby rendering steamed buns more voluminous in appearance.
The specific volume of the steamed bread obtained by synergistic fermentation of LP+KH sourdough was significantly increased that of the LP and KH group steam bread by 11.89% and 7.25%, but there was no significant difference (p > 0.05) compared to OFB group (Figure 5). These observations align with the appearance and section data, reflecting the improved CO2 entrapment efficiency. Our findings are in agreement with Liu et al., 2023, who reported that the specific volume of fermented steam bread in L. plantarum and yeast co-fermentation increased compared with that in monocultures [13].

3.5. Changes of Water Content in Steamed Bread

As shown in Figure 8, the water content of the newly made steam bread core was approximately 38%. With the extension of storage time, the water content of the four groups of steamed bread cores showed a gradual decline. During the whole storage period, compared with steam breads of OFB group and LP group, the water content of LP and KH co-fermented sourdough steam breads was significantly higher than that of the other two groups (p < 0.05). During the storage period of 1 to 3 days, there was no significant difference in water content between LP and KH groups and KH group (p > 0.05), but on the fourth day, the water content of KH group was 28.55%, which was significantly lower than 36.49% in LP and KH groups. During storage of steamed bread, water loss leads to many changes, including starch recovery, skin hardening, and flavor loss. The water retention of steam breads increased after LP and KH were added, mainly because the lactic acid and acetic acid produced by the fermentation of LP and KH would reduce the pH value of the dough and create an acidic environment. An acidic environment is conducive to protein denaturation and crosslinking, enhancing the stability of the gluten network, and improving the water retention of steam breads [32]. The CO2 produced by KH fermentation forms bubbles in the dough and expands the dough, while the exopolysaccharide produced by LP can improve the gluten network structure, making the space between the steam breads more delicate and uniform, and helping to lock in [33].

3.6. Texture of Steamed Bread

The cohesiveness, springiness, and resilience of steamed bread were positively correlated with bread quality, while hardness, gumminess, and chewiness values were negatively correlated with the quality of steamed bread [34]. As can be seen from Table 4, the hardness, cohesiveness, gumminess, and chewiness of the cooperative fermentation of LP and KH sourdough steamed buns were significantly improved compared with the separate fermentation of OFB-steamed buns, LP, and KH sourdough steamed buns. The hardness, gumminess, and chewiness of steamed buns in the LP+KH group decreased by 23.24%, 18.55%, and 23.22% compared with the OFB group, respectively, and steamed buns in the LP+KH group decreased by 34.74%, 33.19%, and 40.27% compared with the LP group, respectively. Steamed buns in the LP+KH group were reduced by 32.07%, 28.13%, and 31.28% compared to those in the KH group, respectively. In terms of cohesiveness of steamed buns, steamed buns in the LP+KH group increased by 10.44%, 7.25%, and 10.44% compared with those in the OFB, LP, and KH groups, respectively. This indicates that the combination of fermented products can reduce the protease activity of KH, further weaken the decomposition of gluten protein, promote the cross-linking of starch and gluten protein, and then give the steamed bread a closer gluten network structure to maintain good dough shape [35] It can be seen from Figure 8 that the specific volume of steamed bread in the LP+KH group was the largest, which can explain why the hardness and chew ability were lower and better, respectively [36]. The results showed that LP+KH sourdough significantly improved the quality of steamed bread. Through cooperative fermentation of different strains in a reasonable ratio, the competition and complementary effects between them can provide benefits for the formation of an ideal quality of steamed bread [8]. Our findings are consistent with those of [37] who used the co-fermentation system of LAB and yeast to treat wheat bran, which could improve the texture characteristics of wheat bran and give it a larger specific volume and softer texture.

3.7. Sensory Evaluation Results

A review panel evaluated the specific volume, appearance, internal structure, color and luster, masticatory effect, elasticity, crispness, aroma, and flavor of steamed bread (Figure 9) produced using four different starter agents (ordinary dry yeast, LP1 sourdough, KH sourdough, and LP1+KH sourdough). As can be seen from Figure 9, the surface of the LP1+KH steamed bread was smooth and flat without collapse, the internal pores were tight and uniform, the aroma was rich, the taste was delicate, the color was uniform, the highest, and the sensory score was the highest, up to 88.3 points, which shows that the sensory attributes of the LP1+KH group were significantly improved. In contrast, fermentation using either LP1 or KH alone was not as effective as co-fermentation [38], also reported similar results: the co-fermentation of S. cerevisiae and L. plantarum can enhance the sensory quality of bread, recommending its use as a substitute for traditional naturally fermented dough processes. The LP1+KH group also received the highest score for aroma, which may be due to the synthesis of certain alcohols, acids, and esters such as lactic acid, acetic acid, propionic acid, ethanol, n-propanol, isopropanol, and ethyl acetate [13]. In addition, the degradation of proteins produces certain amino acids that can be used as precursors for the synthesis of flavor substances, thus enhancing the flavor of steamed bread [39,40].

4. Limitations and Future Perspectives

This study demonstrated promising co-fermentation effects of Lactobacillus plantarum and Kazachstania humilis in sourdough. However, limitations exist. Despite the use of selective antibiotics, the cycloheximide and chloramphenicol used to distinguish between bacterial and yeast growth, interactions with other sourdough microorganisms were not considered, which could impede full results without any other components present. Secondly, the experiments were conducted under controlled laboratory conditions, which may not fully reflect industrial-scale processes. The LP+KH co-culture shows potential as a functional starter culture for industrial sourdough production, offering enhanced microbial growth and fermentation performance. Therefore, future researchers should validate this system at pilot and industrial scales, assess flavor compound development, and evaluate consistency in real dough environments. Such efforts will support the broader application of defined mixed cultures in commercial baking.

5. Conclusions

The present study, through the study of the interactions between LP and KH during sourdough fermentation and their subsequent impact on steamed bread quality, aimed to compare the effects of single and combined fermentation on the fermentation of sourdough and the quality attributes of the final steamed bread product. This comprehensive analysis revealed that pH and TTA were significantly reduced during sourdough fermentation, with the LP+KH group showing the lowest pH and highest TTA, suggesting a more pronounced fermentation activity. The co-fermentation process resulted in a marked increase in specific volume, up to 32% compared to single yeast fermentation, indicating improved sourdough aeration and CO2 retention. The steamed bread from the LP+KH group exhibited superior textural properties, with reduced hardness, gumminess, and chewiness and increased cohesiveness, indicating a softer and more palatable crumb. Additionally, LP+KH fermented bread demonstrated better water retention, which is essential for maintaining freshness over time. The sensory evaluation panel rated the LP+KH group the highest in terms of appearance, internal structure, flavor, elasticity, crispness, and aroma, achieving an overall sensory score of up to 88.3 points. These findings underscore the potential of microbial co-fermentation to enrich the nutritional value and sensory attributes of steamed bread, which could be beneficial for consumer acceptance and market expansion and could be a promising strategy for developing high-quality fermented food products by enhancing sourdough characteristics.

Author Contributions

Z.W. and A.F. conceptualized and developed the methods for conducting this study. X.W. provided technical assistance. Z.W., A.F. and X.W. obtained the data, and Z.W. and A.F. analyzed the data. X.W. managed data collection. G.Z. supervised the entire work. Z.W. wrote the first draft of the manuscript. All authors have read and agreed to the published version of the manuscript.

Funding

The study was supported by the Shanxi higher education institution students’ innovation and entrepreneurship training project in 2024, project number: 20240076.

Institutional Review Board Statement

Not applicable for studies not involving humans or animals.

Informed Consent Statement

Not applicable.

Data Availability Statement

The original contributions presented in this study are included in the article. Further inquiries can be directed to the corresponding author.

Conflicts of Interest

The authors declare no conflicts of interest.

Abbreviations

The following abbreviations are used in this manuscript:
KHKazachstania humilis
LPLactobacillus plantarum
TTAtotal titratable acidity

References

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Fermentation 11 00298 g001
Figure 1. Experimental flow chart.
Figure 1. Experimental flow chart.
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Figure 2. Sourdough fermentation process.
Figure 2. Sourdough fermentation process.
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Figure 3. Growth curves of Lactobacillus plantarum (LP) and Kazachstania humilis (KH) in monoculture. OD600 values were measured at 2 h intervals up to 24 h in MRS medium (for LP, with cycloheximide to inhibit yeast) and YPD medium (for KH, with chloramphenicol to inhibit bacteria). Values are presented as means ± SD.
Figure 3. Growth curves of Lactobacillus plantarum (LP) and Kazachstania humilis (KH) in monoculture. OD600 values were measured at 2 h intervals up to 24 h in MRS medium (for LP, with cycloheximide to inhibit yeast) and YPD medium (for KH, with chloramphenicol to inhibit bacteria). Values are presented as means ± SD.
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Figure 4. Changes in viable cell counts of LP (a) and KH (b) during sourdough fermentation. Viable cells were counted using selective agar plates: MRS + cycloheximide for LP and YPD + chloramphenicol for KH. Sourdough samples were taken at 2, 4, 6, 8, 10, 12, 16, 20, and 24 h. Values are presented as means ± SD.
Figure 4. Changes in viable cell counts of LP (a) and KH (b) during sourdough fermentation. Viable cells were counted using selective agar plates: MRS + cycloheximide for LP and YPD + chloramphenicol for KH. Sourdough samples were taken at 2, 4, 6, 8, 10, 12, 16, 20, and 24 h. Values are presented as means ± SD.
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Figure 5. pH (a) and TTA (b) changes during sourdough fermentation. The pH and TTA of different sourdoughs, including LP (Lactobacillus plantarum), LP+KH (Lactobacillus plantarum and Kazachstania humilis), and KH (Kazachstania humilis), were measured after 2, 4, 6, 8, 10, 16, 20, and 24 h. Values are presented as means + SD (standard deviation).
Figure 5. pH (a) and TTA (b) changes during sourdough fermentation. The pH and TTA of different sourdoughs, including LP (Lactobacillus plantarum), LP+KH (Lactobacillus plantarum and Kazachstania humilis), and KH (Kazachstania humilis), were measured after 2, 4, 6, 8, 10, 16, 20, and 24 h. Values are presented as means + SD (standard deviation).
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Figure 6. Lactic acid (a) and acetic acid (b) concentrations in the sourdough groups. The lactic acid and acetic acid contents in the three sourdoughs, including LP, LP + KH, and KH, were measured at the 20th hour. Values are presented as means + SD (standard deviation). Lowercase letters (a, b, c) indicate significant differences (p < 0.05) among samples. Different letters indicate significant differences.
Figure 6. Lactic acid (a) and acetic acid (b) concentrations in the sourdough groups. The lactic acid and acetic acid contents in the three sourdoughs, including LP, LP + KH, and KH, were measured at the 20th hour. Values are presented as means + SD (standard deviation). Lowercase letters (a, b, c) indicate significant differences (p < 0.05) among samples. Different letters indicate significant differences.
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Figure 7. (a) Appearance and internal structure; (b) specific volume of steamed bread fermented by different microbial groups (ordinary dry yeast, LP sourdough, LP + KH sourdough, and KH sourdough). Values are presented as means + SD (standard deviation). Lowercase letters (a, b, c) indicate significant differences (p < 0.05) among samples. Different letters indicate significant differences.
Figure 7. (a) Appearance and internal structure; (b) specific volume of steamed bread fermented by different microbial groups (ordinary dry yeast, LP sourdough, LP + KH sourdough, and KH sourdough). Values are presented as means + SD (standard deviation). Lowercase letters (a, b, c) indicate significant differences (p < 0.05) among samples. Different letters indicate significant differences.
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Figure 8. Water content of the newly made steam bread core. Values are presented as means + SD (standard deviation). Lowercase letters (a, b, c) indicate significant differences (p < 0.05) among samples. Different letters indicate significant differences.
Figure 8. Water content of the newly made steam bread core. Values are presented as means + SD (standard deviation). Lowercase letters (a, b, c) indicate significant differences (p < 0.05) among samples. Different letters indicate significant differences.
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Figure 9. A review panel evaluation.
Figure 9. A review panel evaluation.
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Table 1. MRS medium formula.
Table 1. MRS medium formula.
IngredientContent (L−1)
Pepton10.00 g
Beef Powder10.00 g
Yeast Powder5.00 g
Glucose20.00 g
Diammonium Citrate2.00 g
Sodium Acetate3.00 g
Dipotassium Hydrogen Phosphate2.00 g
Magnesium sulfate0.20 g
Manganese sulfate0.04 g
Tween-801.0 mL
Agar (Solid Culture medium)15.00 g
Table 2. YPD medium formula.
Table 2. YPD medium formula.
IngredientContent (L−1)
Yeast extract powder10.00 g
peptone20.00 g
Glucose20.00 g
Agar (solid culture medium)20.00 g
Table 3. Sensory evaluation score table.
Table 3. Sensory evaluation score table.
ProjectScoresScoring Criteria
Specific volume15The specific volume is greater than 2.0 and scores a full 15 points; the specific volume is between 2.0 and 1.4, with 0.2 points deducted for every 0.01 decrease; the minimum score for a specific volume of 1.4 or less is 3 points
Appearance10The steamed bread is upright, full, and smooth (7–10); the steamed bread is slightly collapsed and creased (3–6); the steamed bread collapsed and atrophied seriously (0–2)
Internal structure10The stomata structure is fine, the distribution is uniform, the skin and flesh are not separated (7–10); there are more pores, but the size of pores is different (3–6); the epidermis was seriously separated, the stomata were unevenly distributed (0–2)
Color and luster10The color is bright yellow, bright, and evenly distributed (7–10); the color is slightly yellow (3–6); the color is dark (0–2)
Masticatory effect10The steamed bread is palatable, soft, and easy to swallow (7–10); the steamed bread is medium, soft, and firm (4–6); the steamed bread is hard to chew and difficult to swallow (0–3)
Elasticity10Rebound is good, pressing more than 1/2 can quickly recover (7–10); rebound is slightly weak, but pressing more than 1/4 can recover (4–6); rebound is poor or nonexistent (0–3)
Crispness10The steamed bread is pleasant to chew and does not stick to your teeth (7–10); the steamed bread is slightly sticky or crumbly when chewed (4–6); the seamed bread chewing sticky teeth or not refreshing debris feeling serious (0–3)
Aroma15The steamed bread has a strong wheat flavor and no bran odor (11–15); the steamed bread has a strong wheat flavor with a light bran odor (6–10); the wheat flavor of steamed bread is light, or the bran odor is serious (0–5)
Flavor10Slightly sour and sweet (7–10); more sour or less sweet (4–6); too acidic or bitter (0–3)
Table 4. Texture characteristics of wheat bran.
Table 4. Texture characteristics of wheat bran.
Hardness (g)CohesivenessSpringiness (%)Gumminess (g)Chewiness (g)Resilience
OFB280.53 ± 14.14 b0.67 ± 0.01 b0.80 ± 0.03 bc187.43 ± 6.91 b150.67 ± 7.64 c0.27 ± 0.01 ab
LP329.73 ± 20.48 a0.69 ± 0.02 b0.87 ± 0.02 a228.50 ± 17.72 a193.67 ± 9.87 a0.22 ± 0.02 c
KH317.00 ± 20.73 ab0.67 ± 0.02 b0.79 ± 0.02 c212.43 ± 8.19 ab168.33 ± 7.02 b0.24 ± 0.01 bc
LP+KH215.33 ± 18.82 c0.74 ± 0.03 a0.85 ± 0.03 ab152.67 ± 17.11 c115.67 ± 5.51 d0.29 ±0.02 a
Note: Different superscript letters (a, b, c, d) within the same column indicate statistically significant differences between groups (p < 0.05), as determined by one-way ANOVA followed by Tukey’s post hoc test. Values sharing at least one letter are not significantly different.
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Wang, Z.; Fu, A.; Wang, X.; Zhang, G. Enhancing Steamed Bread Quality Through Co-Fermentation of Sourdough with Kazachstania humilis and Lactobacillus plantarum. Fermentation 2025, 11, 298. https://doi.org/10.3390/fermentation11060298

AMA Style

Wang Z, Fu A, Wang X, Zhang G. Enhancing Steamed Bread Quality Through Co-Fermentation of Sourdough with Kazachstania humilis and Lactobacillus plantarum. Fermentation. 2025; 11(6):298. https://doi.org/10.3390/fermentation11060298

Chicago/Turabian Style

Wang, Zicheng, Ao Fu, Xin Wang, and Guohua Zhang. 2025. "Enhancing Steamed Bread Quality Through Co-Fermentation of Sourdough with Kazachstania humilis and Lactobacillus plantarum" Fermentation 11, no. 6: 298. https://doi.org/10.3390/fermentation11060298

APA Style

Wang, Z., Fu, A., Wang, X., & Zhang, G. (2025). Enhancing Steamed Bread Quality Through Co-Fermentation of Sourdough with Kazachstania humilis and Lactobacillus plantarum. Fermentation, 11(6), 298. https://doi.org/10.3390/fermentation11060298

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