Studies On Changes Of Flavor And Texture In Sauerkraut During Natural Fermentation

Sauerkraut is one of traditional fermented vegetables in northeastern China. The indigenous Chinese fermented vegetable products formed by microorganisms from Chinese cabbage during natural fermentation. With rich nutrition, unique flavour and good texture, sauerkraut has attracted consumers’ preference deeply. Unique flavor and texture are important factors which are used to evaluate the quality of sauerkraut. However, there is little known about the changes of flavor and texture of sauerkraut during natural fermentation and it badly hinders the development of natural fermented crafts. Based on Chinese cabbage changes of flavor and texture of sauerkraut during natural fermentation systematically studied. It can provide theoretical basis for traditional crafts improvement and optimal parameters. The main contents and conclusions are as follow:Main physical and chemical indexes and microbial indexes of sauerkraut were determined during natural fermentation. The results showed that pH and soluble protein content of sauerkraut firstly decreased and then changed not obviously during natural fermentation; Nitrite content increased in the initial stage of fermentation then decreased. In the end stage of fermentation it fluctuated in a certain range; The nitrite content reached the peaking content (16mg/kg) on the eighteenth days. In the end of fermentation, the nitrite content of Chinese sauerkraut decreased to1.6mg/kg and reached a steady state. When they reached maturity, nitrite content of Chinese sauerkraut reached a steady state and ranged from0.57mg/kg to2.11mg/kg; The content was far below national hygiene standards(20mg/kg). Aerobic plant content, lactic acid bacteria content and yeasts content increased firstly and then decreased during natural fermentation; No mold was detected and afer18days no coliforms was detected. In the end of fermentation lactic acid bacteria content kept from105-106. It provided favorable conditions for flavor formation and improving stability.Nonvolatile flavor compounds of sauerkraut during natural fermentation were determined. Kinds of free amino acids in sauerkraut were determined by amino acid analyzer. External standard method was used for quantitative. The results showed that naturally fermented sauerkraut were rich in17kinds of free amino acids. All of the free amino acids had different degrees of increase except His. Total free amino acids reached the highest levels in the middle stage, and its content was676.7mg/L. Taste activity value of Glu and Asp were greater than1, so they contributed great to flavor formation of sauerkraut and they were the main source of umami. Taste activity value of others were less than1, so they contributed insignificantly to flavor formation of sauerkraut. The ratio of essential amino acids and nonessential amino acids were0.85and0.83respectively. It showed that sauerkraut were high nutritive value when they reached maturity. Soluble sugar content changed with titratable acid content growth and decline. Soluble sugar content continued to decrease and titratable acid content continued to increase during fermentation. The ratio of soluble sugar to titratable acid was lower in the middle and end stage of fermentation. The fermentation flavor was very rich and intense.Volatile flavor compounds of sauerkraut during natural fermentation were analyzed by headspace solid-phase micro-extraction (HS-SPME) and gas chromatography-mass spectrometry (GC-MS). The headspace solid-phase micro-extration conditions were optimized by single factor test and orthogonal experiment. The optimum conditions were using50/30um DVB/CAR/PDMS fiber to extract samples adding0.32g/mL sodium chloride salt at40℃for40min. The results showed that17,45,57and55volatile flavor compounds were detected in four different fermentation stages respectively. They belonged to10varieties, including esters, isothiocyanates, alcohols, aldehydes, ketones, nitriles, hydrocarbons, acids, heterocycles and sulfides. Dimethyl disulfide, dimethyl trisulfide and (E, Z)-2,6-nonadienal are the main volatile flavor compounds of sauerkraut.2-Ethyl furan and acetaldehyde also contributed to flavor formation of sauerkraut.Changes of texture propertise of sauerkraut were measured during natural fermentation by using instrumental texture profile analysis (TPA). The correlation coefficients between the textural parameters and texture change regularities of sauerkraut were analyzed. The results showed that TPA parameters including hardness, fracturability, resilience, cohesiveness and chewiness had significant correlations. They decreased with time prolonged during natural fermentation. The the optimum product texture of Chinese sauerkraut could be achieved when fermented for40-60days.