Search

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 utilisand Pleurotus ostreatus, either individually or sequentially, into an enriched substrate with increased digestibility for use as ruminant feed. After C. utilisfermentation, 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. utilisand P. ostreatusachieved 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. ostreatusalone due to pectin and hemicellulose degradation. Considering the time required for fermentation, the C. utilistreatment was the most efficient treatment to convert apple pomace into a more nutritive substrate for ruminant feed.

Lignocellulolytic enzymes from conventional and non-conventional yeasts are not commonly studied, and they have never been described for Candida utilisspecies. After solid-substrate cultivation of C. utilis(CCT 3469) on apple pomace, degradation of cellulose, pectin and lignin fragments was observed. Production of the main lignocellulolytic enzymes by C. utiliswas investigated and high activity for pectinase (239 U ml−1) as well as a significant manganese-dependent peroxidase (19.1 U ml−1) activity was found. Low cellulase (3.0 U ml−1) and xylanase (1.2 U ml−1) activities were also observed suggesting that C. utilismay have lignocellulose degradation ability.