Microbiological Quality and Shelf Life of Pickled African Walnut (Tetracarpidium conophorum) Preserved with Lactic and Citric Acids

Abstract

The short shelf life of locally processed ready-to-eat African walnut is a challenge limiting its utilisation. Pickling is an ancient traditional practice of preserving certain types of food. Therefore, the addition of organic acids to African walnut pickle is aimed at elongating shelf life of the product. In this study, raw African walnut was prepared, then pickled using brine solution (5 % NaCl) that separately contain 1 %, 3 %, 5 % citric and 1 %, 3 % and 5 % lactic acid. The samples were stored for 6 Weeks at room temperature (28±2ºC). Total bacterial and fungal counts of the product were monitored weekly using standard methods. Microbiological challenge test study of the pickles using Bacillus cereus and Staphylococcus aureus isolated from fresh African walnut was also performed. Pickle without organic acid was the control sample. Using molecular identification methods, Bacillus cereus strain EV-1 (KY689737), Ochrobactrum ciceri strain EV-2 (KY689738) and Bacillus amyloliquefaciens strain EV-3 (KY689739) were isolated from the pickles. Fungi isolated were Penicillium sp. and Fusarium sp. Total bacterial and fungal count of the pickles range between 3.52-7.34 log10cfu/g and 3.48-5.73 log10cfu/g, respectively was higher than that of the control sample. Importantly, lactic acid demonstrated more antibacterial effect than citric acid in preserving African walnut pickles. The challenge test result revealed that B. cereus was more dominant than S. aureus. Based on Microbiological Guidelines for Ready-to-Eat Food that B. cereus and S. aureus must be less than 5 log10cfu/g and 4 log10cfu/g, respectively, the developed pickle is best consumed within 2 weeks.