| Kelp is nutritious and inexpensive.China’s production ranks first in the world,it has great potential for food processing.However,the strong fishy odor of kelp discourages many Chinese and foreign consumers,limiting the development of kelp consumption and deep processing of the product.At present,the fishy odor of kelp is mainly removed by physical methods such as activated carbon adsorption and spice masking,but these methods are ineffective.The microbial method is mild and effective,and it is the mainstream deodorization method at present.However,the conditions need to be strictly controlled,otherwise,it is easy to be infected and the sensory of kelp will be affected.Meanwhile,the mechanism of microbial fermentation of kelp deodorization is not clear.Therefore,in this paper,we investigated the conditions of green protection and enzymatic digestion to enhance the effect of fermentation;used various flavor analysis methods to characterize the fishy-odor compounds of kelp;studied and optimized microbial fermentation conditions to analyze the changes of fishy compounds;investigated the mechanism of microbial deodorization from the perspective of enzyme catalysis.This paper is of academic value in controlling the deodorization effect of kelp and revealing the mechanism of microbial deodorization of kelp,as well as providing a scientific basis for the development of kelp market and product processing.Firstly,the kelp green protection was investigated using Zn Cl2 as a green protection agent,and the effects of addition concentration,temperature and time on kelp green protection were analyzed at p H 7.5,and the response surface optimization was carried out.The best results were obtained with a red-green color difference|Δa|of 6.28 when the Zn ion concentration was 250mg/L,the temperature was 77°C and the time was 32 min.Subsequently,the kelp was enzymatical y digested using cel ulase and papain（1:1,w/w）,and the enzymatic conditions were studied using the total sugar and reducing sugar contents as indicators.The enzymatic digestion time was 2 h,p H was 6.0,the temperature was 55°C,and the total sugar content was15.22%and reducing sugar content was 7.84%at 1.5%addition,and the free amino acid content was higher than the blank control at this time,and the electronic tongue taste showed better.Secondly,the volatile flavor compounds of kelp were measured by GC-MS,GC-O-MS,and odor activity values,and the key fishy-odor compounds were analyzed.The results showed that aldehydes,alcohols and ketones accounted for more than 3/4 of the flavor substances in kelp,and the total content reached 1313μg/kg.Unsaturated carbonyl compounds such as 1-octen-3-one,（E,Z）-2,6-nonadienal,（E,E）-2,4-decadienal,（E）-2-nonenal,2,4-nonadienal,and 1-octen-3-ol were the most important contributors to the fishy odor of kelp.Most of them have odors such as grass,grease and earth.And these unsaturated aldehydes and ketones tend to have a lower olfactory threshold than saturated ones,causing stronger undesirable odors.Subsequently,the fermentation of kelp with Saccharomyces cerevisiae,Lactobacillus plantarum and Pediococcus pentosaceus was used to deodorize the kelp,and the odor difference between before and after fermentation was determined using GC-IMS to study the effect of different strains on the deodorization of kelp and to optimize the conditions.The results showed that the content of unsaturated carbonyl compounds was proportional to the odor value;the fishy-odor value of kelp fermented by S.cerevisiae was 8.32 at a fermentation temperature of 32°C,a fermentation time of 3.5 h and an inoculum level of 3.5%,which predicted an 88%decrease in the content of unsaturated carbonyl compounds;the fishy-odor values of L.plantarum and P.pentosaceus were 9.55 and 9.35,respectively,under the optimal fermentation conditions.The GC-IMS showed a significant deodorization effect of yeast fermentation over the lactic acid bacteria,which was also consistent with the sensory evaluation.In the S.cerevisiae fermentation samples:the content and type of aldehydes and ketones decreased significantly,while the corresponding alcohols increased;the total OAV decreased from 8583.9in the blank samples to 516.3,a decrease of 94%;and a significant decrease of 1-octen-3-one（olfactory threshold 0.01μg/kg）was accompanied by the production of 3-octanone（olfactory threshold 23μg/kg）,which was not present in the lactic acid bacteria fermentation samples.This suggests the presence of enzymes in yeast that catalyze the reduction of unsaturated aldehydes and ketones to alcohols,which are enoate reductase and alcohol dehydrogenase,respectively.These two enzymes converted the original low threshold of strong undesirable volatile compounds into the high threshold and mild odor volatile compounds,which was the reason for the decrease of the fishy odor of kelp.Finally,the study of the pathway of catalytic reduction of unsaturated carbonyl compounds in S.cerevisiae revealed a preference and competition between enoate reductase and alcohol dehydrogenase in different substrates.The enoate reductase from S.cerevisiae was cloned and heterologous expression in Bacillus subtilis.The enzymatic properties were investigated to explore the optimal catalytic reaction conditions to act on the deodorization of kelp.The results showed that the concentration of enoate reductase was 0.78 mg/m L,the enzyme activity was2.08 U/mg,and the optimum temperature and p H were 35°C and 7.5,respectively.The efficiency of alkenone reductase in catalyzing carbon-carbon double bonds decreased as the number of carbon atoms and alkene bonds increased.And the position and cis-trans structure of carbon-carbon double bonds also affected the catalytic efficiency.After the complex action of enone reductase with S.cerevisiae,most of the aldehydes and ketones in kelp were extremely low,and the effect was better than that of yeast alone. |
