Bioconversion of apple pomace into a nutritionally enriched substrate by Candida utilis and Pleurotus ostreatus

Apple production in the world has increased significantly over the last 10 years. A considerable fraction of these fruits, mainly those not approved for fresh consumption, is industrially processed to produce juices, flavours and concentrates. During this processing, a large amount of solid residues is produced, comprised mainly of peels, seeds, and pulp, which are collectively known as 'apple pomace'. This work aims to select biological treatments and conditions for the bioconversion of apple pomace by Candida utilis and Pleurotus ostreatus, either individually or sequentially, into an enriched substrate with increased digestibility for use as ruminant feed. After C. utilis fermentation, the protein level increased 100% and the mineral content 60%, accompanied by 8.2% of increase in the digestibility. The level of free sugars decreased 97% after substrate supplementation with ammonium sulphate (10 g l(-1)). After optimization, sequential fermentation with C. utilis and P. ostreatus achieved a high protein level with 500% of crude protein enrichment after 60 days of fermentation as well as a considerable increase in the mineral level. The level of free sugars increased after the fermentation with P. ostreatus alone due to pectin and hemicellulose degradation. Considering the time required for fermentation, the C. utilis treatment was the most efficient treatment to convert apple pomace into a more nutritive substrate for ruminant feed.