Chemical characteristics changes associated with extracted protein and salt addition and effect of different additives in the curing process on sensory acceptance of liquid fermented fish from tilapia frame

Liquid fermented fish (Nam pla ra) is a traditional local condiment and popular consumption in Thailand. According to previous research, tilapia frame could be used as a raw material for the production, but the amount of protein received is still below the standard. In addition, there is no information of additives in curing process on consumer preferences. In this research, response surface methodology was used to study the influence of extracted protein from soybean meal (5–15%) and salt (15–20%) contents on chemical characteristics of Nam pla ra after 4 months of fermentation. The amount of roasted rice and roasted rice bran addition in curing process on sensory preferences were also studied. The results showed the linear effect of extracted protein and salt contents on NaCl content (P < 0.05). When extracted protein increased, NaCl content of sample decreased, while increased salt concentration, NaCl content increased. Extracted protein showed linear relationship with autolytic degradation products and formaldehyde nitrogen of sample (P < 0.05). Increase in extracted protein concentration caused an increment to both autolytic degradation products and formaldehyde nitrogen. In addition, salt content showed linear relationship with protein, total nitrogen, ammonia nitrogen and amino nitrogen (P < 0.05). When increased salt concentration, protein and all kind of nitrogen tested in this study decreased. The statistical models for protein and NaCl contents (as an index of product quality) and our experimental validation, indicated that the models were appropriate to predict values. For acceptability test, there was no significant different in all attributes scores of cured samples with 10% roasted rice, roasted rice bran and mixture of both. In addition, their showed a significant different in all attributes scores lower than the commercial sample. In conclusion, the highest protein content was found in a treatment prepared by added salt content 15% by weight of digested material. Addition of proteins extracted from soybean meal did not lead to increase in protein content of finished product. However, chemical properties of all treatments could meet Thai community product standard.