Biological approach for effective utilization of worthless onions—vinegar production and composting

Abstract

About 10% of all the onions harvested are disposed of or cheaply treated in a market as worthless onions every year in Japan because they failed to meet the quality standards required for marketing to the customer. Therefore, in order to allow both value additions and the effective reuses of worthless onions, a biological approach combining vinegar fermentation and composting was investigated. Onions were pressed in a mechanical juicer and separated to give onion juice (60 wt.%) and onion residues (40 wt.%). The juice was converted to onion vinegar by a two-step fermentation system. Nutritional analysis revealed that the onion vinegar contained the various minerals, amino acids and organic acids, which is expected to be a new valuable product from onions. The onion residues were then utilized as a source of composting, and found to be successfully composted within a week. Based on these investigations, it became possible to establish the scheme so that the worthless onions can be totally recycled as onion vinegar and fertilizer (compost).