Effects of salting and drying on quality attributes of snoek (Thyrsites atun)

[摘要] ENGLISH ABSTRACT: Snoek (Thyrsites atun) is an important commercial fish species in South Africa, particularly in the Western Cape province. Snoek is mainly sold as fresh fish with excess fish being processed into dried salted fish. Although fishing is regulated by the various government agencies, the processing chain of snoek is largely unmonitored and unstandardized which has resulted in variable snoek quality becoming available to the consumer.Therefore, the aim of this study was to assess the quality (proximate, biochemical composition and microbiological safety) of salted and dried snoek products both experimentally and through market sampling. This was achieved by investigating two lines of research: 1) Establishing the quality of locally sold salted and dried snoek from informal vendors and 2) Investigating the effects of different levels of salting (%) and drying (relative humidity and temperature) conditions on snoek meat quality. Product quality was assessed through the analysis of proximate composition, fatty acid composition, physico-chemical attributes, water activity, moisture loss, salt content and microbial safety and oxidative stability.Moisture, protein, lipid, and ash contents of snoek varied substantially between vendors, thereby providing products with variable quality to consumers. The water activity and TBARS (thiobarbituric acid reactive substances) of snoek from all vendors were higher than the reported threshold for good quality dried fish, indicating high levels of lipid oxidation. The level of TBARS and water activity varied between 9.58-19.83 mg MDA.Kg-1, and 0.75-0.85, respectively. This is undesirable in dried fish products as high water activity predisposes the products to spoilage. These findings indicate that the salted and dried snoek retailed in the Western Cape province require further drying to reduce the water activity in order to ultimately prolong the shelf life of the product. However, low histamine, high salt, low microbial counts and the absence of pathogens (Salmonella spp. and Escherichia coli) were observed which suggests that the products were wholesome at time of sale.An assessment of the effects of salting and drying on water activity, moisture loss, and salt content of Cape snoek was carried out. The treatments included 0% salt (unsalted) and salting at 20% salt concentration and salt saturation; the treated snoek portions were dried at 30°C/40% RH (relative humidity), 40°C/40% RH and 40°C/50% RH. During the salting process higher moisture loss resulted in high weight loss and low water activity in snoek portions salted to saturation compared to 20% (w/w). Once oven drying commenced the unsalted snoek portions had significantly higher moisture loss, higher weight loss but higher water activity than the salted groups. Lower salt content (12.8%), higher product yield and higher water activity were observed in snoek treated with 20% salt compared to those treated with saturated salt (23.6% salt). Low water activity and a salt content above 6% inhibits microbial growth and prevents spoilage of salted dried fish; therefore, the salt content of the salted snoek (12.8% for 20% salted).after the drying process is adequate for preventing microbial spoilage.A study examining the drying kinetics of snoek determined the moisture ratio, drying rate and effective moisture diffusivity of salted (20% w/w) and unsalted snoek dried at different drying environments (30°C/40% RH; 40°C/40% RH; 40°C/50% RH). A number of mathematical models (Lewis/Newton, Page, Henderson, Pabis and asymptotic logarithmic) were tested to predict the drying kinetics of snoek where the most effective predictive model was the Page model (r2 = 0.9999; mean square error = 6.6 x 10-6). The Page model showed that salting at 20% and drying at 40°C/40% RH was the best treatment with the highest effective moisture diffusivity (1.54 x 10-5) among the salt treatments, which is an indication of high drying rate. Therefore, the Page model can be used to optimise the drying process and design of dryers for snoek processing. The drying conditions, various treatments and their interactions (p ≤ 0.05) significantly influenced the drying rate and moisture content of snoek portions. Low effective moisture diffusivity and drying rate were observed in salted portions, emphasising the effect of salting on drying of snoek.This study has provided baseline information on the quality of locally sold dried snoek and established that salting at 20% and drying at 40°C/40% RH is adequate for the production of consistent quality salted dried snoek. Further research is needed to study the effects of salting and drying on the nutritional quality and shelf life of snoek.

[发布机构] Stellenbosch University