Evolution And Regulation Of Volatile Compounds In Chinese Steamed Bread During Production

Chinese steamed bread（CSB）is very important in the diet structure of the Chinese people.Nowadays,yeast is used in the industrial production of CSB.However,CSB could not satisfy the need of consumers as a result of its bland aroma.In contrast,CSB fermented with sourdough is popular among people because of its unique texture and abundant aroma.Aroma is an important factor affecting food consumption.However,current researches in CSB aroma are limited in the level of volatile compounds（VOC）.The key aroma compounds of CSB and the effect of production and storage on VOC of CSB are still unclear.In this paper,the key aroma compounds of CSB fermented with both yeast and sourdough were systematically studied by modern flavor chemistry,and the evolution of VOC during CSB production and storage was also investigated.This paper could provide theoretical guidance and technical support for improving the aroma of CSB fermented with yeast.The main contents are as follows:（1）The aroma profile and key aroma compounds of yeast-CSB（Chinese steamed bread）and sourdough-CSB were different.VOC in CSB were qualitative and quantitative analyzed by gas chromatography-olfactory mass spectrometry（GC-O-MS）combined with headspace-solid phase microextraction（HS-SPME）.Besides,the key aroma compounds were characterized using descriptive sensory analysis,aroma extract dilution analysis（AEDA）,calculation of odor activity value（OAV）,partial least squares regression（PLSR）correlation analysis and aroma recombination.Results showed that,2-pentylfuran（wheat-like）,1-octen-3-ol（mushroom）,ethyl lactate（floral）,benzaldehyde（almond）,nonanal（citrus）and hexanal（green）with the largest flavour dilution factor（FD）were characterized as key aroma compounds in sourdough-CSB.While in yeast-CSB,2-pentylfuran,1-octen-3-ol,2-methyl-1-propanol（alcoholic）,hexanal and nonanal with high FD value were considered as key aroma compounds.It was indicated that the main differences between the two CSB were attributed to the concentration of 2-pentylfuran and 1-octen-3-ol with high FD factors.In addition,ethyl lactate found in sourdough-CSB contributed to the floral odor,while2-methyl-1-propanol found in yeast-CSB contributed to the strong alcoholic odor.（2）Dough kneading could promote the formation of VOC such as 2-pentylfuran and short chain aldehydes in CSB.However,the kneading time of the best CSB aroma was not consistent with that of the best CSB texture.Results of HS-SPME-GC-MS and PLSR analysis showed that the raw-flour odor of CSB was positively correlated with long-chain aldehydes（octanal,nonal and decal）,while wheat-like aroma was positively correlated with2-pentylfuran and short-chain aldehydes.It was indicated that raw-flour odor decreased and wheat-like aroma increased with the extension of dough kneading time,which was beneficial to improve the CSB aroma.The contents of long-chain aldehydes such as octanal,nonanal,decanal and（E）-2-nonenal decreased by 60.00%、82.08%、71.74%和64.29%,respectively,with the kneading time increasing from 3 to 11 min.Meanwhile,the prolongation of kneading time could promote the formation of 2-pentylfuran and short-chain aldehydes.However,with the kneading time increased from 9 to 11 min,the contents of many VOC such as2-pentylfuran,octanal,（E）-2-octanal and benzaldehyde significantly（P<0.05）decreased.Combining the results of sensory evaluation and analysis of texture,specific volume and other parameters,it was indicated that the optimal aroma formation time（9 min）was later than the optimal texture formation time（7 min）.（3）Increasing yeast concentration and prolonging fermentation time could not only promote the production of VOC derived from yeast metabolism,but also inhibit the production of VOC derived from lipid oxidation.Besides,prolonging steaming time could promote the production of 2-pentylfuran and benzaldehyde.VOC influenced by yeast concentrations,fermentation time and steaming time was analyzed using HS-SPME-GC-MS.Results showed that,the concentrations of 3-methyl-1-butanol in CSB fermented with 0.5%yeast increased by 25.6%and 39.6%when the fermentation time increased from 20 min to 40min and to 60 min,respectively.Remarkable 3-methyl-1-butanol accumulation of 64.0%and63.5%could be observed when the fermentation time increased from 20 min to 40 min in CSB with 1.0%and 1.5%yeast addition,respectively.Other VOC（such as2-methyl-1-propanol,phenethyl acetate and hexyl acetate）derived from yeast metabolism showed similar trends in contents with 3-methyl-1-butanol.Prolonged fermentation time could inhibit the formation of lipid oxidation compounds.In CSB fermented with 0.5%yeast,the contents of octanal,nonanal,decanal,（E）-2-nonenal and（E,E）-2,4-decadienal were reduced by 47.6%,41.9%,62.8%,57.4%and 35.5%,respectively,when the fermentation time increased from 20 to 60 min.Similar results were found in CSB fermented with higher yeast concentrations（1.0%and 1.5%）.Prolonged steaming time facilitated the production of benzaldehyde and 2-pentylfuran（P<0.05）and promoted the loss of alcohols and esters.The contents of 2-pentylfuran and benzaldehyde increased by 356.33%and 132.9%,respectively,with steaming time increasing from 5 to 30 min.Network analysis showed that 2-pentylfuran contributed to wheat-like aroma of CSB while 2-methyl-1-propanol and 3-methyl-1-butanol were positively correlated with alcoholic odor.（4）The rate of VOC loss in CSB stored at 25°C was slower than that stored at 4°C.The evolution of VOC in CSB during storage was investigated by HS-SPME-GC-MS and heat map analysis.Results showed that,the contents of 2 alcohols（1-octanol and 1-nonanol）,3aldehydes（hexanal,nonanal and（E）-2-octenal）,3 esters（1-Butanol,3-methyl-,acetate,pentanoic acid,ethyl ester and acetic acid,2-phenylethyl ester）,2 ketones（3-octen-2-one and geranylacetone）and 2-pentylfuran significantly decreased（P<0.05）in CSB after 1 d of storage at both 4°C and 25°C.With further increase of storage time,their contents decreased slowly or had no significant change（P>0.05）.After storage at 25°C for 2 d,the contents of1-octanol,nonanal,3-octen-2-one and 2-pentylfuran decreased by 28.46%,47.36%,39.14%and 64.15%,respectively,which were similar to the VOC loss in CSB stored at 4°C for 1 d.Furthermore,PCA analysis also showed that the VOC profile of CSB stored at 25°C for 1 d and 2 d was similar to that of CSB stored at 4°C for 1 d.Results of differential scanning calorimetry（DSC）,Fourier transform infrared spectroscopy（FTIR）and low field nuclear magnetic resonance（LF-NMR）showed that the retrogradation rate of CSB stored at 4°C was higher than that at 25°C.（5）Suitable addition of Na₂CO₃ in CSB production could neutralize excess acid and inhibit the production of lipid oxidation products.Results of HS-SPME-GC-MS showed that,the content of butanoic acid（rancid and sweaty）was sharply reduced by 95.52%with 0.4%Na₂CO₃addition in sourdough-CSB.With the addition of Na₂CO₃ in CSB increasing from 0to 0.5%,butanoic acid even disappeared,and the contents of lipid oxidation compounds such as 1-octen-3-ol,nonanal and（E）-2-octenal decreased by 68.12%,72.91%and 80.70%,respectively,while（E）-2-heptenal and（E）-2-nonenal disappeared completely.The changes of lipid oxidation compounds in yeast-CSB were similar to those in sourdough-CSB.In addition,Sodium dodecyl sulfate polyacrylamide gel electrophoresis（SDS-PAGE）and size-exclusion high-performance liquid chromatography（SE-HPLC）analysis showed that Na₂CO₃ could promote the hydrolysis of glutenin macropolymer（GMP）under alkaline conditions,and thus endowed CSB with soft texture.