Kiwifruit waste valorisation: Chemical characterisation of by-products and enzymatic production of seed protein hydrolysates

Abstract

Despite the economic and nutritional significance of kiwifruit, large quantities are discarded during a post-harvest selection process, before industrial processing. This study explored the valorisation of undersized kiwifruit discard by first detailing the chemical composition of its peel, pulp, and seeds and identifying valuable components, such as protein, sugars and dietary fibre. In parallel, enzymatic-assisted extraction (EAE) represents an eco-friendly extraction technique and a green and efficient treatment to provide high-value-added products from underutilised agri-food waste. Subsequently, considering their nitrogen content (1.5% dw), kiwifruit seeds were selected as a matrix for protein recovery by EAE using a protease from Bacillus licheniformis. This method achieved a protein extraction yield of 50±3% within 2 h during the medium-scale trial, substantially greater than control extractions without the addition of the enzyme (< 10% at 2 h and < 45% at 4 h). The seed hydrolysate achieved a high degree of protein hydrolysis (73±2%), resulting in a high occurrence of free amino acids and short peptides, and a limited number of medium-chain peptides. Its total amino acid profile, particularly high in glutamic acid, cysteine, aspartic acid, leucine, and methionine, suggests its potential application as a plant biostimulant.