Orange-Peel Waste Enzymatic Saccharification: Scaling-Up Under Diverse pH-Control Strategies

Waste from the fruit juice industry presents high sugar and phenolic contents, high humidity and biological activities and cumbersome disposal or low-added valorization. Orange-peel waste (OPW) represents 35–55% w/w of processed fruit, with oranges being the main citric crop. OPW saccharification leads to sugar-rich hydrolysates that can be further processed via fermentative and catalytic routes. In this work, OPW enzymatic hydrolysis was studied via batch and fed-batch processing using either a 50 mM citrate buffer or a 9 g/L NaCl solution with pH control by adding CaCO3 to ensure high enzyme activity across the enzymatic process. Preliminary runs showed that particle size of 3.4 mm diameter and a 300 r.p.m. stirring speed, a six-blade Rushton turbine and wall baffles were adequate to reach high sugar yields in batch. Further scale-up in batch at medium solid loading (12.5% w/w) and fed-batch operation at high-solid loading (20% w/w) led to high yields and glucose and fermentable sugars (up to 74 and 136 g/L, respectively, when using the saline solution and CaCO3 as pH-controlling agent, in only 50 h; notably shorter and higher than when using the citrate buffer). Fractal kinetic models have been shown to accurately represent the compositional change across all batch and fed-batch conditions, highlighting NaCl reaction medium and alkali-driven pH control as the most appropriate approach to achieve high yields at low process times, a promising result for further developments at demonstration and industrial scales using automatic pH control.