Evaluation of the effect of Saccharomyces cerevisiae on fermentation characteristics and volatile compounds of sourdough (2024)

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J Food Sci Technol
v.55(6); 2018 Jun
PMC5976591

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J Food Sci Technol. 2018 Jun; 55(6): 2079–2086.

Published online 2018 Mar 19. doi:10.1007/s13197-018-3122-1

PMCID: PMC5976591

PMID: 29892108

Guohua Zhang,¹ Yurong Sun,¹ Faizan Ahmed Sadiq,^2,³ Hafiz Arbab Sakandar,⁴ and Guoqing He^2,³

Author information Article notes Copyright and License information PMC Disclaimer

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Supplementary Materials

Abstract

The objective of this study was to unveil insights into the effects of Saccharomyces cerevisiae on the development of volatile compounds and metabolites during the dough fermentation in making Chinese steamed bread. Changes in gluten structure under the influence of baker’s yeast were studied using scanning electron micrographs (SEM). A unique aroma profile was found comprising some previously reported aromatic compounds and some unreported aromatic aldehydes ((E)-2-Decenal and 2-Undecenal) and ketones (2-Heptanone and 2-Nonanone) in the baker’s yeast fermentation. Among metabolites, the most preferred sugar for this yeast (glucose) showed a significant decrease in contents during the initial few hours of the fermentation and at last an increase was observed. However, most of the amino acids increased either slightly or decreased by the fermentation time. SEM of fermented dough showed that the yeast had a very little effect on starch stability. This study provided some fermentation features of the bakers’ yeast which could be used for the tailored production of steamed bread.

Electronic supplementary material

The online version of this article (10.1007/s13197-018-3122-1) contains supplementary material, which is available to authorized users.

Keywords: Saccharomyces cerevisiae, Metabolites, Scanning electron micrograph, Volatile compounds

Introduction

Chinese steamed bread (CSB) is a wheat-based fermented product of Chinese origin representing around 40% of the total wheat consumption of China. It is now becoming popular in other Asian as well as American and some European countries (Zhu 2014). The microbial ecosystem of sourdough with a great microbial stability confers a plethora of benefits to steamed bread beyond simply endowing it with a unique flavor. CSB can be fermented by either a pure culture of yeast especially Saccharomyces cerevisiae or a combination of lactic acid bacteria (LAB) and mold/yeast as in traditional starter cultures, like sourdough, Jiaotou and Jiaozi (Luangsakul et al. 2009; Liu et al. 2012; Zhang and He 2013).

Materials and methods

Wheat flour

Flour with low gluten content was used for making CSB, which was purchased from a local supermarket. The protein, moisture, wet gluten and ash contents of the wheat flour were 7.56, 12.57, 18.5 and 0.05%, respectively. Instant dry yeast (Angel brand, made in China) was purchased from a local supermarket.

Fermentation of steamed bread

The steamed bread was made by the modification of the procedure described by Kim et al. (2009). Firstly, wheat flour and instant yeast (0.5% of wheat flour) were remixed and stirred for 10min, the dough was fermented for 1h at 28 ± 2°C and 75% relative humidity in a controlled fermentation cabinet (LX-X24, Shandong, China). Secondly, the dough was rolled into round shape by hands and fermented at 28 ± 2°C and 75% relative humidity for 30min in a cabinet. Then,the proofed dough was steamed for 30min in a steamer (Joyong, Hangzhou, China).

pH and total titratable acidity (TTA)

The pH and TTA values of each sample were determined by standard method (AACC 2000). All experiments were carried out in triplicate.

Metabolite analysis

Concentration of glucose in fermented dough samples was determined by Glucose Assay Kit (Sigma-Aldrich, China).

The compositions of amino acids during Saccharomyces cerevisiae-ZJU315 fermentation were determined using amino acid analyzer (L-8900 Hitachi-hitech, Japan), following the method described by Mariod et al. (2010). Samples containing 0.10g of dough were acid hydrolyzed with 4.0mL of 6mol/L HCl in a vacuum-sealed hydrolysis vials at 110°C for 22h. Ninhydrin was added to the HCl as an internal standard. The tubes were cooled after hydrolysis, opened and placed in a desiccator under vacuum until dry. The residue was dissolved in a suitable volume of a sample dilution with 0.02mol/L HCl, filtered through a Millipore membrane (0.45μm pore size) and analyzed for amino acids.

Dough microstructures

The fermented dough samples at the initial fermentation stage and end of fermentation stage were fixed in30g/L glutaraldehyde in 0.1M phosphate buffer, pH 7.0, for 4h at 25°C. Samples were washed with phosphate buffer for15min three times. Then grains were post fixed in 10g/L osmium tetroxide in phosphate buffer for 1h at 25°C. After washing with phosphate buffer, samples were dehydrated in ethanol: 15, 30, 50 and 70% ethanol for 10min each, 85 and 95% for 15min each, and 99.5% for 1h. After dehydrating, samples were critical-point dried and coated with gold. The preparations were observed using a scanning electron microscope (XL30ESEM; Philips Amsterdam, Netherlands).

Volatile compounds analysis

SDE method

Volatile compounds in the steamed bread, fermented by S. cerevisiae, as well the control bread were determined by the SDE method using dichloromethaneas theextractionsolvent as previously described by Zhang et al. (2016). Anhydrous sodium sulphate (Na₂SO₄) was mixed with the solvent-phase extract which was then concentrated to 1mL at 40°C using a Vigreux column (30cm × 2cm). The samples were then stored in a freezer at − 80°C for GC-O-MS (Gas Chromatography–Olfactometry-Mass Spectrometer) analysis.

SPME method

Volatile compounds in the samples were determined using the SPME method as previously described by Zhang et al. (2016). Steamed bread crumbs (1.5g) were placed in 15mL vials and spiked with 1.5mg of internal standard d4-pyrazine and sealed with an aluminum crimp cap provided with a needle-pierceable polytetrafluoroethylene/silicone septum. SPME was performed with 75µm CAR/PDMS (Carboxen/Polydimetylsiloxane) fiber mounted in a manual SPME holder assembly (Supelco, Bellefonte, USA). The sample vial was placed in a 75°C water bath for 30min to equilibrate and then the septum was pierced with the SPME needle. The fiber was exposed to the headspace of the sample for 30min and after the extraction, the fiber was retracted into the needle and immediately transferred to the injection port of a gas chromatograph and desorbed in theGC injection portfor 10min at 250°C.

Gas chromatograph-mass spectrometer (GC-MS) analysis

The analysis of volatiles was performed on a GC 7890A (Agilent, Palo Alto, CA) coupled to a Triple Quad 7000B (Agilent, Palo Alto, CA), and equipped with a Sniffer 9000 Olfactometer (Gerstel, KG, Germany). Separations in GC were performed on DB-WAX (30 m × 0.320 mmi.d., 0.25µm film thickness; J & W Scientific, USA). The carrier gas used was ultra-high purity helium and the column had a flow rate of 1.0ml/min.

The oven temperature was programmed from 40°C for 3min, then increases at 5°C/min to 200°C, and then increasing at 10°C/min to 230°C, whereby it was held for 5min. The temperatures of the injector and the GC/MS interface were 250 and 280°C.

Electron-impact mass spectra were generated at 70eV, with m/z scan range from 35 to 550 amu, with the ion source temperature of 230°C. Compounds were identified according to NIST 2.0 mass spectra libraries installed in the GC–MS equipment.

Compounds identification

The identification of volatile compounds was based on the comparison of the mass spectrum and retention index with reference compounds. The RI values and odour descriptions on DB-Wax column, with those of linear retention indices (RIs), having the same/similar odor quality and RI, previously have been reported in www.odour.org.uk. n-Alkanes (C₇–C₂₂) were analyzed under the same conditions to calculate LRIs: LRI = 100N + 100 n (t_Ra−t_RN)/(t_R(N+n)−t_RN), which was described by Dool and Kratz (1963).

Statistical analysis

Statistical analyses were performed by SPSS.17.0.

Results and discussion

Changes in pH and TTA values

The changes in pH and TTA values (g/100g of the sample) during the fermentation are presented in Fig.1. A gradual decrease in pH with a corresponding increase in TTA was noticed during the fermentation process. The initial pH and TTA values of the sourdough sample at the beginning of fermentation was 5.43 ± 0.05 and 7.46 ± 0.11, respectively, which reached to the final values of 4.8 ± 0.03 and 8.11 ± 0.06, respectively after 3.5h of fermentation. It is known that low pH as a result of high acid contents during the fermentation enhances the capacity of dough to retain gases which are produced as a result of LAB or yeast fermentation (Rocha and Malcata 2012).

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Fig.1

Changes of pH and TTA during S. cerevisiae fermentation

Changes in metabolites during fermentation

Metabolite changes in terms of glucose and amino acid contents were studied during the dough fermentation of yeasted and non-yeasted samples. Changes in the glucose content during the fermentation process are shown in Fig.2. It is known that the trace amounts (0.3–0.5%) of free sugars in dough (glucose, sucrose, maltose and fructose) are important sources of carbon for the metabolism of yeast fermentation. These sugars are utilized by yeast producing CO₂ and ethanol. In our trial, we only focused on glucose as it is considered as the most important and preferred sugar for S. cerevisiae during the fermentation (Hopkins and Roberts 1936). The results show that during the initial stage (30min) of fermentation, the control dough (without yeast) contained 1.02 ± 0.08mg/g glucose contents, which increased to 1.56 ± 0.06mg/g after 220min of fermentation. However, in the fermented dough, the glucose contents decreased rapidly during the initial few hours of the fermentation and at last an increase in glucose content was observed. A sharp decrease in glucose content just after the fermentation process shows a huge consumption of glucose as a result of yeast metabolic activity. It is well established that yeast preferentially utilizes glucose and assimilates other sugars like fructose or maltose (Hopkins and Roberts 1936; Verstrepen et al. 2004). Transcriptomic analysis of three different strains of S. cerevisiae in bread dough fermentation revealed that glucose genes are expressed just after the onset of fermentation (Aslankoohi et al. 2013). The increase in glucose content in control bread and in yeasted dough at the end of fermentation may be attributed to the hydrolysis of starch or sucrose by the activity of indigenous amylases (a-amylase) or inverses in wheat flour that act on damaged starch granules (Jayaram et al. 2013).

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Fig.2

Changes of glucose contents during S. cerevisiae fermentation

Amino acids are essential component of dough and serve as important flavor precursors. Variation in the concentrations of different amino acids, which include total amino acids, aromatic amino acids, essential amino acids, branched-chain amino acids, sulphuric amino acids and glutamic acid in control and fermented dough at three different stages is shown in Fig.3. A very slight increase in amino acid contents (essential, branched and aromatic) was observed in yeasted dough sample with a slight decrease in sulphuric and glutamic acid groups. It is known that only a little increase in amino acid contents could be observed during the yeast fermentation of dough as compared to a mixed culture of bacteria and yeast (Spicher and Nierle 1988). The amino acid metabolism of the yeast S. cerevisiae has recently been confirmed by transcriptomic studies of different strains of this species during the dough fermentation. It was revealed that different strains (regardless of their genetic background) expressed genes involved in amino acid metabolism in the middle fermentation phase (Aslankoohi et al. 2013).

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Fig.3

Changes of amino acids during S. cerevisiae fermentation (1: beginning of fermentation stage; 2: middle of fermentation stage; 3: end of fermentation stage)

Microstructural changes during S. cerevisiae fermentation

Gluten network plays an important role in retaining the gases produced by yeast and bacteria during the fermentation process. In dough, normally starch granules are embedded firmly in the protein matrix. However, starch granules adapt the reticular pattern or become swollen (Tester 1997) once gluten network is damaged.

SEMs (scale bar 100 and 50μm) of yeasted dough samples at the initial and end stages of fermentation are shown in Fig.4. Before the fermentation, it can be seen that the smooth and uniform starch granules are entrapped in an extensible protein or gluten network. These observations are in line with the previously described observation for a dough microstructure (Kim et al. 2003).

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Fig.4

SEM observation of dough during S. cerevisiae fermentation. A Beginning of fermentation stage, B end of fermentation stage

After fermentation only a slight change in starch granules can be observed. However, some starch granules are deformed or ditches are formed over its surface. This may be attributed to the action of endogenous α-amylase in flour or the activity of microorganisms in sourdough on damaged starch granules, which resulted in the alteration of the dough consistency as it is well known that S. cerevisiae does not produce starch degrading enzymes, amylases (Ostergaard et al. 2000). It can be concluded that yeast may help in developing the dough structure by less affecting the starch granules and thus helps in improving the gas retention capacity of dough.

Volatile aromatic compounds

A sum of 27 volatile aromatic compounds was extracted using two different methods of volatile extraction (SDE and SPME) followed by their identification using the GC coupled with an Olfactometric detector. Results of the identification are shown in Table1. A total of 11 aldehydes, 7 alcohols, 4 ketones, 1 acid, 1 ester and 3 others compounds were detected. In a previous study, over 40 flavor compounds (20 alcohols, 7 esters, 3 alkanes, 1 sulphur, 6 lactones, 6 aldehydes and 3 alkenes) were detected in French sourdoughs fermented with S. cerevisiae using the vacuum desorption method (Frasse et al. 1993). Similarly, in another study, a total of 7 different types of volatile compounds (acetaldehyde, ethyl acetate, acetone, ethanol, hexanal, isobutanol and propanol) were found in bread doughs fermented with S. cerevisisae using SPME (Torner et al. 1992). Compared to these two reports, more aldehydes were detected in the current study. Among, alcohols, propanol was not found in our study, however, 1-Pentanol, 1-Hexanol and 1-Octanol were recovered that remained undetected in the mentioned study (Torner et al. 1992). A variation in different result is probably due to the usage of different wheat flour varieties as well as different strains of leavening agent. In this context, it is also important to mention that the ability of forming volatile aromatic compounds significantly varies among the species as well among the strains of the same species (Hansen and Schieberle 2005).

Table1

GC-MS identification of volatiles of S. cerevisiae steamed bread obtained with SDE and SPME methods

Compounds	RI	SDE		SPME		Odor
Compounds	RI	Control	S. cerevisiae	Control	S. cerevisiae	Odor
Aldehydes (11)
Hexanal	1081	–	19.22	5.78	–	Green
Heptanal	1183	37.13	15.22	–	1.90	Planty green
(E)-2-Heptenal	1318	–	–	1.06	1.88	Fatty
Nonanal	1392	–	2.23	–	3.98	Fattt
(E)-2-Octenal	1425	30.50	9.02	–	–	Nutty
Furfural	1460	–	–	–	2.82	–
Benzaldehyde	1515	–	–	13.27	5.71	Almond
(E)-2-Nonenal	1532	11.53	6.88	–	–	Cucumber
(E)-2-Decenal	1642	–	2.08	–	–	Fatty
2-Undecenal	1750	–	1.41	–	–	Bitter
(E,E)-2,4-Decadienal	1807	19.84	6.24	–	–	Fried potato
Total		100.00	62.30	20.11	16.29
Alcohols (7)
Ethanol	925	–	–	–	12.78	Alcohol
3-Methyl-1-Butanol	1198	–	–	–	27.24	Sweet,green
2-Methyl-1-Butanol
1-Pentanol	1252	–	15.29	7.01	2.60	Alcoholic
1-Hexanol	1361	–	–	19.71	7.37	–
1-Octanol	1551	–	–	1.42	1.02	–
Phenylethyl alcohol	1903	–	–	–	16.66	–
Total		0	15.29	28.14	67.67
Ketones (4)
2-Heptanone	1180	–	6.04	–	–	Cheese
2-Octanone	1283	–	8.34	–	–	Fruity
3-Hydroxy-2-Butanone	1292	–	–	–	1.21
2-Nonanone	1387	–	2.16	–	–	–
Total		0	16.54	0	1.21
Acids (1)
Acetic acid	1449	–	–	–	9.46	Acid
Total			0	0	9.46
Ester (1)
Ethyl acetate	880	–	–	–	1.10	Fruity
Total			0	0	1.10
Others (3)
2-Pentylfuran	1231	–	5.87	43.95	1.60	Fruity
Naphthalene	1740	–	–	7.80	1.25	Camphoric
2,3-butanedione	945	–	–	–	1.42	Sweet
Total		0	5.87	51.75	4.27

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More aldehydes were detected using SDE method, while more alcohols were detected using SPME method (Table1). Among aldehydes, (E)-2-Octenal, Heptanal, (E)-2-Nonenal and (E,E)-2,4-Decadienal were detected in the control bread as well as in the bread fermented with the yeast S. cerevisiae. The presence of aromatic compounds in the control bread may be due to the endogenous odorants of wheat flour which are produced as a result of natural enzymes and microflora of flour (Hansen and Schieberle 2005). Also, the variety and type of wheat flour affect the aroma generation in sourdough bread to a great extent (Starr et al. 2015).

Results of aldehydes, detected by both SDE and SPME, show that the bread fermented by S. cerevisiae produced only four aldehydes that could not be detected in the control bread, namely Nonanal, Furfural, (E)-2-Decenal, and 2-Undecenal. The rest of the seven aldehydes were found in both bread samples by either of the two methods. To the best of our knowledge, the two aldehydes, (E)-2-Decenal and 2-Undecenal and the two ketones (2-Heptanone and 2-Nonanone) have not previously been reported in Chinese steamed breads. The two aldehydes E,E)-2,4-Decadienal and (E)-2-Nonenal are key aroma components of bread crumb (Grosch and Schieberle 1997). It was noted that, in the absence of the yeast the control bread produced higher contents of (E,E)-2,4-Decadienal, (E)-2-Nonenal, (E)-2-Octenal and Heptanal, while in the presence of the yeast high contents of 3-methyl-1-Butanol, 2-methyl-1-Butanol, 2,3-Butanedione and Phenylethyl alcohol were obtained. These findings are in agreement with a previous report (Frasse et al. 1993). It could be explained from the fact that 2,4-(E,E)-Decadienal, (E)-2-Nonenal, (E)-2-Octenal and Heptanal are lipid oxidation products (Birch et al. 2013), whilst 3-methyl-1-Butanol, 2-methyl-1-Butanol, 2,3-Butanedione and Phenylethyl alcohols are fermentation products from the Ehrlich pathway (Pico et al. 2015). In the presence of low yeast content (control bread) there was an abundant availability of oxygen and thus lipoxygenases efficiently utilized oxygen to convert lipids into aldehydes, while in the presence of yeast, there was a deficiency of oxygen as it was rapidly consumed by yeast to produce the products via Ehrlich pathway. Thus, lipoxygenase enzymes remained unable to transform lipids into aldehydes and ketones (Pauline et al. 2008).

Yeast also converts amino acids into alcohols through transaminase, decarboxylase and reductase reactions in the Ehrlich pathway. For instance, phenylalanine, leucine, isoleucine and valine are converted into aromatic compounds (mainly 2-Phenylethanol and 3-methyl-1-Butanol) by the metabolic activities of yeasts which impart highly desirable flavors to bread crumb (Dickinson et al. 2003). Among all alcohols, 3-methyl-1-Butanol showed the highest proportion, which was surprisingly higher than ethanol. It shows a high amino acid metabolism of the yeast during the fermentation period. It is well known that the production of free amino acids via proteolysis during ferment at ion greatly influence the aroma profile.

Conclusion

This study provided information about the role of S. cerevisiae in the production of volatile aromatic compounds and metabolites during the fermentation of steamed bread. A decrease in the concentration of glucose over the course of fermentation was probably related with the metabolism of the yeast during the fermentation. Most of the amino acids increased either slightly or decreased during the fermentation time. A total of 27 aromatic compounds were identified. SEM of fermented dough showed that the yeast had a very little effect on starch stability. This study may be helpful in selecting desirable flavors and metabolites correlated with the specific S. cerevisiae strain used in this study which may further allow tailored formulation of CSB.

Electronic supplementary material

Below is the link to the electronic supplementary material.

Supplementary material 1 (DOCX 2645kb)^{(2.5M, docx)}

Acknowledgements

This research was supported by National Natural Science Foundation of China (Grant No. 31601461).

Footnotes

Electronic supplementary material

The online version of this article (10.1007/s13197-018-3122-1) contains supplementary material, which is available to authorized users.

Contributor Information

Guohua Zhang, Phone: +86-18035181052, Email: nc.ude.uxs@hggnahz.

Guoqing He, Phone: +86-13306526782, Email: nc.ude.ujz@ehqg.

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