Effect of storage temperature on the physicochemical, nutritional and microbiological quality of pasteurised soursop (Annona muricata L.) Juice

Abstract

This work was aimed at investigating the effect of storage temperature on the physicochemical, nutritional, and microbial quality of soursop juice. Physicochemical quality was determined by measuring changes in pH, titratable acidity, total soluble solids and colour (L*a*b* values). Ascorbic acid levels, total phenolic content and total antioxidant capacity were analysed to determine the effect of storage temperature on nutritional quality. The changes in aerobic mesophilic and psychrophilic bacteria, lactic acid bacteria, Enterobacteriaceae, as well as yeast and moulds were enumerated to determine the effect of storage temperature on microbial quality. Soursop juice was pasteurised at 85°C for 5 min and stored at varying temperatures of 4, 10 and 25°C. Storage was carried out for 12 weeks at both 4 and 10°C, while at 25°C the juice was stored for 3.5 weeks when visible signs of spoilage was detected. Storing soursop juice at 4ºC did not affect quality, however, at 25°C, decreases in pH and total soluble solids with an increase in titratable acidity was observed. In addition, a faster rate of ascorbic acid degradation was observed at 25°C. The main group of microorganisms that were responsible for the spoilage of soursop juice stored at 25°C were lactic acid bacteria, and yeasts and moulds. The results of this work show that pasteurised soursop juice can be stored at refrigeration temperatures without changes in quality.   Key words: Soursop juice, pasteurization, storage temperature, lactic acid bacteria, ascorbic acid.