Thermal stability of lactic acid bacteria metabolites and its application in preservation of tomato pastes

Aims: Tomato (Lycopersicum esculentum) is one of the highly nutritious and perishable food ingredients. This work is intended to apply bio-preservation in extending the shelf life of tomato paste. Methodology and results: Isolation of lactic acid bacteria (LAB) and spoilage tomato microorganisms was done from healthy and spoilt tomato samples respectively using De Mann Rogosa and Sharpe and nutrient agar and potato dextrose agar respectively. Antimicrobial metabolites of LAB (lactic acid, diacetyl and hydrogen peroxide) were determined and effect of treatment temperatures (60, 80 and 100 °C for 30 min) on its efficacy was also determined. The results obtained from this study revealed that Lactobacillus brevis and L. plantarum were isolated from healthy tomato samples while Bacillus megaterium, B. laterosorum, Pseudomonas syringe, Aspergillus fumigates, Fusarium solani, F. acuminatum and F. funjikuroi were isolated from the spoilt tomato samples. LAB isolated produced antimicrobial compounds with L. plantarum I recording the highest lactic acid (2.7 g/L), diacetyl (2.2 g/L) while 2.4 g/L of hydrogen peroxide was observed. Metabolite extract from L. brevis I (ML7) exhibited the largest broad spectrum activity with inhibition zones ranging from 10 mm to 18 mm against spoilage microorganisms except F. funjikuroi. At 60 °C and 80 °C metabolites produced by LAB species were inhibitory to some of the tomato spoilage microorganisms and at 100 °C no inhibition was detected. The shelf life of tomato paste treated with metabolite extracts from L. plantarum I, L. plantarum II, L. brevis I and L. brevis II (LAB3) and kept in refrigerator showed the longest shelf life of 20 days. Conclusion, significance and impact study: Metabolite extracts from selected LAB were more effective than methyl propylparaben in preserving tomato paste stored at 4 °C and 25 °C against spoilage bacteria and the application of biopreservative should be encouraged in food processing industries.