Properties and Changes during Fermentation of Salted Shrimp Paste as Affected by Quality of Row Material and Selected Halophilic Bacteria

Abstract

Eleven commercial Kapi, obtained from various places of Thailand, were characterized. The samples had varying compositions, but protein constituted as the major component (29.44-53.27%, dry weight basis). Salt content and Aw of all samples were in the range of 22.77-35.47% and 0.695-0.774, respectively. Various formal nitrogen contents (11.96-22.87 mg N/g sample) and degree of hydrolysis (DH) (12.68-20.76%) suggested the varying cleavage of peptides among the samples. The samples had different colors with different total carotenoid contents (0.54-1.97 mg/g sample). Amino acid composition and volatile compounds were also varied. Glu/Gln and pyrazine derivatives were dominant in all samples. Moreover, all samples also possesses antioxidative activities. Thus, Kapi could be a good source of protein and served as the nutritious condiment. When small shrimp Acetes vulgaris and Macrobrachium lanchesteri, used as alternative sources for Kapi production, were characterized, both shrimp mainly contained serine proteases, which were strongly inhibited by PMSF and SBTI. Maximal proteolytic activity of A. vulgaris and M. lanchesteri was found at pH 7, 60°C and pH 8, 60°C, respectively. Activity of both proteases decreased with increasing NaCl concentration (0-30%). During Kapi production, protease, trypsin and chymotrypsin activities were detectable throughout all processes. Activities were decreased when salting was implemented but continuously increased with increasing fermentation time. Those changes coincided with increasing TCA-soluble peptide content and disappearance of myosin heavy chain and actin. Those proteases more likely involved in proteolysis of shrimp during Kapi production. Post-mortem storage time of both shrimp prior to salting affected the quality of resulting Kapi. Both shrimp underwent deterioration when stored at room temperature (28-30°C) as indicated by the increases in pH, total volatile base (TVB),trimethylamine (TMA) contents, thiobarbituric acid reactive substances (TBARS) and total viable count (TVC). Resulting Kapi became darker and had higher intensity of volatiles and antioxidative activities when shrimp used as raw material were stored for a longer time. However, odor- and overall-likeness scores of Kapi were decreased when unfresh shrimp (more than 6 h and 12 h of post-mortem storage time for A. vulgaris and M. lanchesteri, respectively) were used. Quality of Kapi produced from A. vulgaris (KA) and M. lanchesteri (KM) were comparatively examined. Both samples had different chemical compositions, physical and sensory properties. KM had the higher protein content but lower fat content, compared with KA (p<0.05). However, KA showed the higher DH, peroxide value (PV), TBARS value as well as browning intensity (A420), compared with KM. This indicated the higher protein degradation, Maillard reaction as well as lipid oxidation occurred in KA. The higher antioxidative activities were also found in KA sample (p < 0.05). Overall, A. vulgaris showed higher potential as an alternative raw material for Kapi production, in comparison with M. lanchesteri. Chemical and microbiological changes of shrimp A. vulgaris during Kapi production including salting, drying and fermentation were monitored. The pH decreased within the first 10 days of fermentation and continuously increased as fermentation progressed. Color of Kapi turned to be browner or darker as fermentation time increased (p < 0.05). Several changes of Kapi components took place throughout all processes but mainly occurred during fermentation period. Those included degradation of proteins and adenosine triphosphate (ATP), lipid oxidation as well as the formation of volatiles. Those changes played a role in the characteristic of resulting Kapi, especially taste or flavor. A. vulgaris containing 4.21% lipid (dry weight basis), which mainly comprised triacylglycerol (TAG) (28.03%) and sterol (ST) (24.03%). Both TAG and ST gradually decreased during Kapi production, while free fatty acid (FFA) increased and became dominant in the final product (49.29%). TAG and FFA fractions mainly consisted of saturated and monounsaturated fatty acids. Polyunsaturated fatty acids (PUFAs), especially eicosapentaenoic acid (EPA) (C20:5(n-3)) and docosahexaenoic acid (DHA) (C22:6(n-3)), were accumulated in phosphatidylethanolamine (PE) and phosphatidylcholine (PC) fractions. During Kapi production, lipolysis and lipid oxidation occurred throughout all processes as evidenced by increases in PV, TBARS value and FFA content. Those changes might partially contribute to the Kapi characteristics, especially flavor. Bacillus spp. K-C3, isolated from commercial Kapi, was characterized for its growth and production of extracellular enzymes including protease, lipase and chitinase. The maximal growth and enzyme secretion of this strain were obtained at pH 6-8, 25-35°C in the presence of 10% NaCl. Kapi inoculated with Bacillus spp. K-C3 at the level of 102-106 CFU/g dry weight sample were characterized throughout the processes, compared with naturally fermented Kapi. Based on polymerase chain reaction-denaturing gradient gel electrophoresis (PCR-DGGE) result, Bacillus spp. K- C3 bands were detected in all steps of Kapi production, indicating that this strain could survive throughout the entire process. All inoculated samples exhibited the higher proteolytic, lipolytic and chitinolytic activities, corresponding well with the higher TCA-soluble peptide, DH and FFA content of the final products, compared to the control. The greater rate of fermentation in the inoculated samples directly affected characteristics of Kapi including the brown color development, lipid oxidations, antioxidative activities as well as formation of volatiles. Therefore, inoculation of shrimp A. vulgaris with Bacillus spp. K-C3 seemed to be the potential means to accelerate the fermentation and yielded Kapi with the preferable characteristics.