Characterisation of fermented coconut milk using probiotoc lactic acid bacteria

Abstract

Fermentation of milk enhances its nutritional values and biological activity through the improvement of nutrients bioavailability and production of bioactive compounds. Coconut milk is widely used in food products since it contains no cholesterol or lactose and only small quantities of saturated fatty acids. The main objective of this study was to develop fermented coconut milk with potent biological activity using probiotic lactic acid bacteria (LAB). The primary goal of this study was to investigate the probiotic properties of four strains of LAB: Lactiplantibacillus plantarum ngue16, L. plantarum ng10, Enterococcus durans w3 and Lactobacillus bervise w6. All four strains exhibited resistance (70.27-85.71%) to low pH conditions, displayed tolerance to 0.3% bile salt and demonstrated proteolytic but not haemolytic activities. The cell surface hydrophobicity of bacterial cell adhesion to xylene showed significant difference (p<0.05). Auto aggregation capacities were observed at all incubation period (1, 2 and 5 h). Additionally, all strains displayed co-aggregation activities against 6 pathogens following 2 h of incubation. Both LAB cells and cell free supernatants (CFS) exhibited antibacterial activity against selected Gram-positive and Gram-negative pathogenic bacteria. These strains exhibited resilience across pH (3-8) and temperature (60-100 °C) ranges but lost their activity when subjected to enzyme treatment (proteinase K and pepsin). Evaluation of DPPH and FRAP assays showed that all CFS can inhibit the formation of radicals compared to the negative control. Utilising the 1H-NMR technique, the bioactive metabolites in the LAB supernatant were identified: anserine, GABA, acetic acid, lactic acid, uracil, uridine, propylene glycol, isopropanol, serine, histidine and indol-3-lactate. Another objective was to optimise the fermentation conditions for coconut milk using response surface methodology (RSM), resulting in optimum conditions at 34.4 °C for 16.8 h using L. plantarum ngue16. The study then assessed the effect of fermentation and storage over 28 days on physicochemical, shelf life and textural characteristics of fermented coconut milk. The pH of fermented milk decreased from 4.26 to 3.92 after 28 days of cold storage. The LAB counts significantly increased (p<0.05) during fermentation and initial storage (1 to 14 days), reaching 6.42×108 CFU/mL, before decreasing significantly (p<0.05) to 1.66×108 CFU/mL at 28 days. Yeasts and mould were detected only on days 21 and 28, ranging from 1.74×102 to 1.22×104 CFU/mL, while coliforms and E. coli appeared from day 14 and onwards. Compared to un-fermented coconut milk, fermented coconut milk showed improved textural properties, decreased syneresis, increased water holding capacity and higher viscosity during storage. Sensory evaluation further revealed that fermented coconut milk was more acceptable than un-fermented milk. Finally, the study focused on the antioxidant and antibacterial activities of fermented coconut milk during storage. The fermented coconut milk exhibited potent antibacterial activity against target bacteria compared to un-fermented milk as well as antioxidant activity. The application of 1H-NMR revealed the presence of 40 metabolites detected in both fermented and un-fermented coconut milk with elevated levels of ethanol, valine, GABA, arginine, lactic acid, acetoin, alanine, phenylalanine, acetic acid, methionine, acetone, pyruvate, succinic acid, malic acid, tryptophan, uridine, uracil, and cytosine in fermented coconut milk. Overall, fermentation of coconut milk using L. plantarum ngue16 exhibited potential to extend shelf life, enhancing the flavour, texture, sensory proprieties and improved biological activities along with other beneficial nutrients.