Nisin is the only FDA-approved bacteriocin with a GRAS (Generally Regarded as Safe) status, used as a biopreservative/shelf-life extender in foods and many other products including pharmaceutical, veterinary and health care products. It is produced commercially by fermentation of a milk-based medium, using strains of Lactococcus lactis subsp. lactis. However, being a dairy-based product requires its labeling since milk is an allergen. Also, since its high production cost is mainly dominated by its medium costs, an emerging trend has been observed with a growing number of studies aimed at finding alternative, non-dairy and preferably non-allergen-based substrates. In order to cut costs, and to also be an environmentally friendly approach, several of these substrates have been sourced from agriculture or aquaculture-based waste streams. The review included in this dissertation highlights the advantages of value-addition to waste and the promising potentials of some of these low-value nutritive sources for the production of high-value nisin. An in-depth study of soy whey (SW) as a non-dairy, nisin fermentation feedstock was performed using Lactococcus lactis. Results indicated that SW was able to produce equivalent biomass and nisin yields (2.18g/L and 619mg/L), as compared to the commercial medium, de Man-Rogosa-Sharpe (MRS) broth (2.17g/L and 672mg/L, respectively), without the need for external nutrient supplementation. This indicated the nutritive qualities of this co-product stream in being able to support the growth of a fastidious bacterial culture.
The success of SW as a bacterial growth medium motivated the second study which focused on growing the oleaginous algal strain Chlorella vulgaris under mixotrophic and heterotrophic culture conditions. Microalgae are cultivated in large scale for several commercially important products among which algal lipids have gained increasing interest due to their use as biodiesel feedstock as well as a source of essential fatty acids with neutraceutical value. Photoautotrophic algal cultivation suffers from low growth rates while heterotrophic/mixotrophic modes have high media costs. In order to boost algal lipid production with lower media costs, two co-product streams, SW and thin stillage (TS) were tested as growth substrates. TS is a high-organic strength co-product generated from the dry-grind corn-ethanol industry. Traditional use of TS involves an energy-intensive concentration process to form "syrup" which finally ends up as animal feed in Distillers' Dried Grains with Solubles (DDGS). The cost-effectiveness and success of the corn-ethanol industry is highly dependent on the value of its co-products. Therefore, in order to be an economically viable and an environmentally sustainable process, researching new avenues for value-addition to TS is very important. As per the results from the current study, biomass yields (dry basis, db) from TS, SW and a synthetic control medium-MBM (modified basal medium) after 4 days of incubation were 9.8, 6.3 and 8.0 g/L with oil contents at 43, 11, and 27 % (w/w) respectively. Polyunsaturated fatty acids (PUFAs) or essential FAs, were found to be highest in Chlorella-TSoil (56%), followed by 38% in Chlorella-SWoil and 31% in Chlorella-MBM oil. Therefore, mixotrophic cultivation of C. vulgaris in TS and SW produced high yields of both algal biomass and lipids at low cost, thus adding value to co-product streams and improving economic viability of algal cultivation.
The high algal oil yields from TS motivated the third study involving the use of oleaginous fungal strain, Mucor circinelloides for its potential in adsorbing/assimilating the oil and nutrients present in TS, for the production of lipid and protein-rich fungal biomass. Fungal batch cultivation for 2 days using a 6-L airlift reactor led to a 92% increase in oil yield from TS, relative to the original oil content, with concomitant reduction in suspended solids and chemical oxygen demand (COD) in TS by 95% and 89% respectively. M. circinelloides, when grown on TS gave a biomass yield of 20 g/L (dry basis), with a lipid content of 46 % (dwb). The polyunsaturated fatty acids were 52% of the total lipids. Overall, fungal cultivation on TS produced a high-protein animal feed and high-value fungal oil, thus improving corn-ethanol process economics.
The fourth study focused on employing the "hurdle concept" and developing more efficient antimicrobial systems by combining plant essential oils (EOs) with food-grade organic acids. The seven most active oils among the sixteen tested, were combined with five different organic acids and the antimicrobial interactions (synergistic/additive/antagonistic) were examined against food-related pathogens. Malic and citric acid were the most inhibitory acids which showed synergism with mountain savory, redistilled oregano (RO), and cinnamon oils against S.aureus 25923 and with cassia, RO, and cinnamon oils against E.coli 25922. The only synergistic EO-acid combination against C.albicans 10231 was lemon myrtle-citric acid.