The potential of strawberry extract and nisin for shelf-life extension of pasteurised milk at 4 and 10°C

Abstract

Pasteurised milk is a mild heat-treated milk product that has a limited shelf life at refrigerated storage temperature. This research was aimed to extend the keeping quality of pasteurised milk by adding strawberry extract and/or nisin. In the first part of the study, two commercial pasteurised milk products were investigated for their physicochemical and microbiological qualities at the end of their shelf lives. Moreover, microbial colonies were also collected and identified. The results showed that the commercial pasteurised milk had pH values of 6.46 to 6.62, total titratable acidity of 0.16-0.20% lactic acid and total soluble solid of 11.50-13.33ºBrix. For the microbial qualities, the milk products contained 5.30-5.40 log cfu/mL of total bacteria, 5.40-5.60 log cfu/mL psychrotroph as well as 2.53-2.55 log cfu/mL lactic acid bacteria at the end of the milk shelf life. From the collected colonies, there were 35 isolates were identified. These identified colonies were belonged to genus Bacillus for 9 isolates, genus Exiguobacterium for 12 isolates, genus Aeromonas for 2 isolates, genus Pseudomonas for 2 isolates, genus Lacticaseibacillus for 4 isolates and 1 isolate for each genus of Kurthia, Stenotrophomonas, Massilia, Paenibacillus, Limosilactobacillus and Escherichia. In the second part of the study, strawberry was extracted with distilled water or ethanol for either 6 or 48 h maceration period. The extract had yields in the range of 3.57-3.85% with moisture contents of 6.60-6.95% and water activity of 0.60 to 0.67. When the antimicrobial activity was tested against Pseudomonas aeruginosa, an identified isolate from commercial pasteurised milk, it was found that the strawberry extract macerated with ethanol for 6 h could inhibit and eliminate the studied microorganisms at a concentration of 25% of the extract. At the same time, this extract at 100% concentration could produce a diameter inhibition zone of 16.77 ± 0.31 mm, which was closed to the inhibition zones of the control treatments of 300 µg furanone and 10 µg penicillin G for 17.37 ± 0.45 and 17.57 ± 0.42 cm, respectively. The last part of the study was focused to comprehend the capability of strawberry extract with and without nisin to inhibit P. aeruginosa in milk substrate. Four pasteurised milk samples were prepared by individually adding with 300 µg/ml strawberry extract, 300 µg/ml furanone, 500 IU/ml nisin and a combination of 300 µg/ml strawberry extract and 500 IU/ml nisin. The control pasteurised milk was the milk without any addition of antimicrobial agent. Physicochemical and microbiological assessments of the pasteurised milk stored at 4 and 10ºC revealed that the control pasteurised milk had a pH of 6.03 ± 0.01 and total acidity of 0.23 ± 0.01% lactic acid after 14 days at chilled storage and a pH of 5.90 ± 0.01 and total acidity of 0.29 ± 0.01% lactic acid on the 7th day storage at 10ºC. On the other hand, the treated pasteurised milk had pH values of 6.28-6.34 and total acidities of 0.15-0.17% lactic acid after 21 days at refrigerated temperature and pH values of 6.10-6.20 with total acidities of 0.24-0.28% lactic acid on the 14th day storage at abuse temperature. Moreover, the control pasteurised milk had a total microbial count of 5.00 and 7.00 log cfu/mL at the end of 21 days storage at 4 and 10ºC, respectively. In contrast, the treated pasteurised milk possessed total microbial counts of less than 3.00 and up to 5.80 log cfu/mL at 4 and 10ºC storage temperature, respectively. These findings indicated the potential of strawberry extract with and without nisin to extend the shelf life of pasteurised milk kept at refrigerated and abuse storage temperatures.