Study On Semi-solid Low-salt Fermentation Of Cowpea By Lactobacillus Plantarum MG-1

Cowpea has high moisture content,crisp and tender tissue,and it is difficult to store fresh cowpea after harvest.Cowpea is processed in various ways to prolong its shelf life,such as sun-curing,dehydration,natural fermentation and inoculationed fermentation.However,the sun-curing process is susceptible to weather and mildew easily due to improper operation;the limited scale of dehydration process technology makes it difficult to guarantee product quality and safety;the amount of salt used in natural fermentation is large,and the content of nitrite exceeds the standard.Therefore,inoculation of lactic acid bacteria fermentation has become a research hotspot in recent years.Inoculated fermentation has many advantages,such as shorten the fermentation cycle,conducive to cowpea nutrition,color,fragrance,texture maintenance.At present,most of the fermented cowpea is fermented by putting cowpea in the jar.This fermentation method is not easy to carry,easy to cause secondary pollution,preservative addition,texture softening and other problems,which bring inconvenience to transportation,storage,marketing and eating.The low-salt soft packaging fermented cowpea overcomes the above shortcomings of high salt,secondary pollution,additive use,serious softening of texture after sterilization and so on.。Vacuum packaging of Lactobacillus plantarum purebred semi-solid fermentation cowpea is currently less studied.It is of great practical significance to study the semi-solid fermentation technology and sterilization method of cowpea to solve this problems.In order to improve the nutritional value of cowpea,the process optimization（water content,inoculation volume,salt concentration,fermentation temperature,fermentation time）and sterilization methods（microwave,pasteurization,pasteurization steam and high temperature sterilization）of semi-solid fermentation of cowpea by Lactobacillus plantarum MG-1 were used to study the changes of total acid content,nitrite content,Vc content and sensory evaluation during fermentation.To prolong the storage period and make the fermented cowpea taste,color,better taste,low salt,low nitrite,so as to solve the above quality and safety problems.In this study,the total acid content,nitrite content,VC content and sensory evaluation of fermented cowpea were weighted,and the comprehensive weighting index X=0.2×total acid content+0.2×VC content+0.3×（1/nitrite content）+0.3×sensory evaluation index.At the same time,the key parameters of cowpea fermentation process were determined by comparing natural fermentation.On the basis of single factor,the response surface optimization experiment was carried out.Based on the optimum parameters of response surface optimization,different sterilization methods and conditions were explored with the evaluation indexes of total acid content,nitrite content,VC content,sensory evaluation,color difference,hardness and total number of colonies.The main results are as follows:1.Single factor test:The results of single factor test with salt concentration,inoculation amount,water content,fermentation temperature and fermentation time showed that salt concentration was 6%,inoculation amount was 5%,water content was 85%,temperature was 31℃,fermentation time was 6days;The inoculated ferric nitrite content was（1.360±0.0027）mg/kg,which was 12.43%lower than that of natural fermentation,which was much lower than the nitrite limit（NaNO₂）of GB2714-2003 hygienic standard of salt pickles≤20 mg/kg.the total acid content was（0.60±0.0243）g/kg higher than natural fermentation by 30.43%;the Vc content was（10.22±0.2222）mg/100g higher than that of natural fermentation by 88.21%;the sensory evaluation score of 86 was 14.67%higher than that of natural fermentation;The comprehensive weighted scores of 5%inoculation,85%water content and 6%salt concentration were 1.000 and higher than those of other levels in the same group.The comprehensive weighted scores of 6 days fermentation time were 0.849higher than those of other fermentation time.2.Response surface optimization experiment was carried out on the basis of single factor experiment.Five factors affecting cowpea fermentation,including salt concentration,inoculation amount,water content,fermentation temperature and fermentation time,were tested by response interview.The optimum technological parameters of cowpea fermentation were determined.The fermentation temperature was 31.09℃,inoculation amount was5.10%,salt concentration was 5.59%,water content was 85.56%,and fermentation time was6.60 days.The weighted comprehensive fermentation index was 0.9766.In order to facilitate operation,the conditions were adjusted to 6%salt concentration,5%inoculation volume,85%moisture content,31℃fermentation temperature and 7d fermentation time.Under these conditions,the nitrite content（1.224±0.0002）mg/kg decreased by 18.40%and 10%respectively compared with the pre-optimization group and the natural fermentation group,compared with the pre-optimized and natural fermentation groups,which was far lower than the sanitary standard of Chinese pickles（GB2714-2003）.Nitrate limit（NaNO₂）≤20 mg/kg;the total acid content（0.92±0.0123）g/kg increased by 53.33%and 31.43%respectively compared with the pre-optimization group and the natural fermentation group,and the Vc content（14.35±0.2670）mg/100g respectively.increased by 40.41%and 88.57%respectively compared with the pre-optimization and natural fermentation groups;the sensory evaluation of 88 points increased by 2.32%and 15.79%respectively compared with the optimized and control groups;3.On the basis of the results of response interview,cowpea was treated with different sterilization methods and conditions.The quality changes of cowpea were studied after being placed at 37℃for 7 days.The results showed that microwave 800w-180s sterilization had the highest comprehensive weighted score of 1.0,nitrite（1.188±0.0065）mg/kg,total acid（0.92±0.0123）g/kg,Vc（0.55±0.0339）mg/100g,color difference（3.45±0.20）,hardness 6.7±0.10/10⁵kg/cm²,sensory quality 85,pathogenic bacteria（Salmonella,Shigella,Staphylococcus aureus）not detected,coliform group<3.0MPN/g,It conforms to the hygienic standard GB 2714-2003 for pickled vegetables,and is a qualified product.