The Effects of Fermentation and Roasting of Hass Avocado Seeds on Colour and Antioxidant Activities
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Nowadays, the food industry has paid more attention to using food by-products for conversion to value-added products. Nearly 1000 tons of avocado wastes including avocado peels, avocado seeds, wastewater, and the pulp are generated in the production of avocado oil in New Zealand each year (Anuja Nadkarn, 2017). These wastes are ideal raw materials for food use as they may have high nutritional value and be beneficial for health. Avocado by-products are utilized as food ingredients may contribute to solving the serious environmental problems caused by the disposal of processing wastes. In this study, we wanted to investigate the influence of fermentation and roasting on colour, total phenolic content (TPC) and antioxidant capacity of New Zealand ‘Hass’ cultivar avocado seeds. Avocado seeds were subjected to natural fermentation and inoculated fermentation with either Lactobacillus plantarum or kefir. These fermented samples were evaluated in terms of visual plate count values of lactic acid bacteria (LAB), acetic acid bacteria (AAB) and yeasts. The number of microorganisms increased significantly (p<0.001) over 7 days of fermentation in all samples. The colony forming units (cfu) of LAB, AAB, and yeast significantly (p<0.001) increased in the first five days. A statistically significant difference was observed between the day5 and day7 counts of LAB and yeast, but this increase was no more than 1 log cfu after 5 days indicating that growth had reached its stationary phase. Although TPC and antioxidant capacity significantly (P<0.05) decreased after fermentation of avocado seeds, fermentation with Lactobacillus plantarum led to significantly (P<0.05) higher TPC and antioxidant activities compared to kefir and naturally fermented samples. The dried fermented avocado seeds in all three fermentation conditions (L. plantarum, kefir and natural fermentation) were further subjected to roasting. Roasting time and temperature were found to significantly (p < 0.05) affect the colour (L*, a* and b* value), TPC, and antioxidant capacities. L*and b* values of roasted avocado seeds significantly (P <0.001) declined with the increase of roasting temperature and time. Roasting also significantly (P<0.05) increased TPC with longer roasting time. In terms of antioxidant activity, only the value of FRAP assay significantly (P <0.001) rise with increasing roasting temperature and time. In addition, roasted samples fermented by L. plantarum represented significantly (P<0.001) higher TPC and antioxidant capacity (using both CUPRAC and FRAP assays) compared to kefir and naturally fermented samples. This study demonstrated that fermentation of avocado seeds in combination with roasting could yield a powder with high TPC and good antioxidant activity, which can be potentially be incorporated in food as ingredients to enhance nutrition and confer value-added benefits.