Abstract
A carrier media is any food material that is used as a protective base during starter culture development. It encapsulates the bacterial cell for preservation purposes. Skim milk powder (SMP) is generally used as carrier media for starter culture preservation. However, current trends in functional foods have indicated a strong consumer demand for plant-based food products. In addition, dairy-free food products are a good alternative for consumers that have dairy allergies or those that are lactose intolerance. Freeze-drying is known as the best drying techniques that preserve cell viability of starter cultures during long term storage, however it is expensive and thus, not suitable for rural communities who do not have access to advanced drying techniques.
This study investigated the potential of coconut (CCN) and tiger nut (TGN) powders as an alternative protective carrier media to SMP. The oven drying technique was used as an alternative and cost effect drying method to freeze-drying, while the protective effect of CCN and TGN during drying and subsequent storage was investigated with three lactic acid bacteria (LAB) strains, Levilactobacillus brevis OK 3264 , Lactiplantibacillus plantarum UNFT 100 and Streptococcus thermophilus. The protective effects of the carrier media were investigated after drying and during storage at 4°C and room temperature (approx. 25°C) for 112 days. Cell viability, cell vitality, water activity, moisture content, acidification kinetics, glass transition temperature, cell morphology, and changes in color were monitored throughout storage to evaluate the culture stability. Thereafter, the fermentation potential of LAB strains from the best carrier media and drying technique was investigated in the controlled fermentation of ting, with spontaneously fermented ting serving as control. The fermentation potential in ting was monitored through changes in pH, total titratable acidity (TTA), total soluble solids (TSS), proximate composition, functional group analysis (FTIR), microbial quality, metabolite compounds (Gas Chromatography Mass Spectrometry) and consumer acceptability study.
Among the drying techniques, freeze-drying maintained better LAB cell viability during the drying process than oven-drying. The extent of cell viability loss during drying was influenced by the type of carrier media, with SMP providing the best protection, while TGN resulted in the highest loss in LAB cell viability. During long-term storage and across all carrier media, LAB strains stored at 4°C performed better than cultures stored at RT. Overall, the quality (cell viability, cell vitality, moisture content, aw, Tg, cell morphology) of LAB strains prepared with freeze-drying was maintained better during storage than that of oven-dried cells. In general, the quality of LAB strains preserved in CCN powder was comparable to that of SMP LAB strains at 4°C storage
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temperature. However, the extent of cell protection by the carrier media was dependent on the type of LAB strain.
Thereafter, the fermentation performance of two LAB strains (L. plant UNFT 100 and L. brevis OK 3264) that performed better when preserved in CCN and SMP was analysed during the fermentation of ting. Overall, the use of mixed LAB strains supported a more rapid fermentation than single strains. The LAB strains preserved in SMP and CCN resulted in ting with more rapid reduction in pH, higher TTA and lower final pH than the naturally fermented ting. Total soluble solids decreased with fermentation, reflecting the complex biochemical transformations occurring during the fermentation process, including the consumption of sugars by microorganisms. High LAB counts were observed in CCN and SMP ting when compared to control ting. Total aerobic counts were within the acceptable limit on all ting, while Enterobacteriaceae and yeasts and moulds were not detected. Furthermore, FTIR analysis confirmed the presence of different functional groups with ting prepared with SMP and CCN LAB strains showing increased intensities when compared to control ting. Ting produced with SMP and CCN mixed LAB strains had a higher protein content than control ting. With consumer acceptability, CCN ting was the most preferred by consumers as observed by to its highest overall score, followed by SMP ting. The PCA biplots showed that TTA exhibited a positive correlation with LAB counts, indicating that as the total LAB count increased, so did the TTA. Overall, the findings demonstrated that the use of CCN to preserve LAB strain can potentially contribute to the quality fermentation of ting that is acceptable to consumers.
Lastly, the metabolite profile of ting prepared with mixed CCN and SMP LAB strains was determined. Esters dominated the overall volatile compounds detected in all ting followed by acids. More diverse volatile compounds were detected in the control ting, while a higher concentration of acids was detected in the LAB inoculated fermentations than control, with the highest detected in CCN ting. Also, CCN ting had the highest concentration of volatiles compounds than SMP ting, including antioxidative (e.g. Oxalic acid and 1-(2,5-Bis-methoxymethoxy-phenyl), anti-inflammatory (3,4,5-Trihydroxybenzoic acid ethyl ester (3TMS)), and antitumor and antibacterial agents such as Aziridine, 2-(phenylmethyl)-, indicating its potential increased health benefit. The PCA biplot highlighted specific compounds that contributed to the main differences among the ting samples, with SMP ting compounds clearly separated from CCN and control. The study demonstrated that the production of volatile compounds depends on the carrier media used and the type of fermentation since the LAB inoculated ting produced different volatile compounds compared to the control. Overall, the findings of this work present the potential of CCN as an alternative carrier media for LAB strains. For proper function of the starter cultures, the study suggests that CCN strains be stored at 4°C for 112 days where prepared with freeze-
drying and for 84 days when prepared with oven drying. Further optimization of the CNN carrier can be performed to improve its cell protection efficiency.