Microorganisms in deteriorating Nigerian melon (Colocynthis citrullus L.) soup (egusi): Heat susceptibility profile and nutrient degradation potential

Abstract

Egusi is a popular melon (Colocynthis citrullus L.) soup in Nigeria that deteriorates within 24 hours. The study aimed to identify microorganisms associated with spoilage of egusi soup, determine their heat susceptibility profile and nutrient degradation capabilities. Nutrient and Potato Dextrose agar were used to isolate microorganisms from egusi soup obtained from 5 restaurants and kept on the shelf for three consecutive days. The isolates were identified by standard procedure involving microscopy and biochemical tests. Egusi spoilage capabilities of the isolates were tested using losses of nutrients (lipid, carbohydrate, protein) and changes in turbidity and pH as indicators after 24 h. The isolates were also tested for survival at varying temperatures (30-80⁰C). The bacteria isolated were, Bacillus sp., Enterococcus sp., Streptococcus sp., Staphylococcus sp., Escherichia coli and Klebsiella sp. while the fungi were Aspergillus niger, Penicillium sp., Microspora sp. and Aphanoascus sp. Changes in pH and turbidity of egusi were minimal (pH, 6.0-6.2; turbidity reduction, 1.0-1.2%) without interspecies differences (P>0.05) while nutrient losses were 5.5-43.2% with inter-species differences for protein and lipid (P<0.05). Most isolates tolerated a temperature range of 30-50⁰C with only Bacillus sp surviving at 80⁰C. Fungal isolates were less tolerant of heat (≤40⁰C). In conclusion, the isolates depleted egusi nutrients and tended to be thermophilic, thereby indicating the need for additional measures to complement 50-60⁰C treatment for preservation. The findings present an insight into the strategy to be adopted to ensure microbial and shelf stability of egusi