Citation: Sigüenza-Andrés, T.; Pando, V.; Gómez, M.; Rodríguez-Nogales, J.M. Optimization of a Simultaneous Enzymatic Hydrolysis to Obtain a High-Glucose Slurry from Bread Waste. Foods 2022, 11, 1793. https:// doi.org/10.3390/foods11121793 Academic Editors: Maria Martuscelli and Dino Mastrocola Received: 4 May 2022 Accepted: 16 June 2022 Published: 17 June 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). foods Article Optimization of a Simultaneous Enzymatic Hydrolysis to Obtain a High-Glucose Slurry from Bread Waste Teresa Sigüenza-Andrés 1, * , Valentín Pando 2 , Manuel Gómez 1 and José M. Rodríguez-Nogales 1 1 Food Technology Area, College of Agricultural Engineering, University of Valladolid, 34004 Palencia, Spain; mgpallares@uva.es (M.G.); josemanuel.rodriguez@uva.es (J.M.R.-N.) 2 Department of Statistics and Operational Investigation, College of Agricultural Engineering, University of Valladolid, 34004 Palencia, Spain; vpando@uva.es * Correspondence: teresa.siguenza@uva.es Abstract: Bread and bakery products are among the most discarded food products in the world. This work aims to investigate the potential use of wasted bread to obtain a high-glucose slurry. Simulta- neous hydrolysis of wasted bread using α-amylase and glucoamylase was carried out performing liquefaction and saccharification at the same time. This process was compared with a traditional se- quential hydrolysis. Temperature and pH conditions were optimized using a response surface design determining viscosity, reducing sugars and glucose concentration during the enzymatic processes. The optimal conditions of pH and temperature in the saccharification stage and the simultaneous hydrolysis were pretty similar. Results show that the slurry produced with simultaneous process had a similar glucose yield at 2 h, and at 4 h a yield higher than that obtained by the sequential method of 4 h and could reduce time and energy. Keywords: discarded bread; liquefaction; saccharification reducing sugars; glucose; α-amylase; glucoamylase 1. Introduction It is estimated that one-third of the food produced in the world is wasted. In retail shops, bread and bakery products are usually the most discarded, after fruits and veg- etables [1]. Melikoglu et al. [2] estimated that 10% of the world production of around 100 million bakery products are wasted annually. Moreover, the European Union recom- mends reducing food waste, but when not possible, the institution advises the redirection of this waste for human consumption before other uses [3]. Currently, despite these recommendations, discarded bread is mostly used for the production of animal feed, while studies focused on the use of bread waste in human consumption are scarce. Gélinas et al. [4] suggested the use of discarded bread for the production of sourdoughs More recently, wasted bread has been studied as a potential source to generate extruded products [5,6]. Another frequent use of wasted bread is its hydrolysis and fermentation as bread is an ideal substrate for enzymes and microorganisms. This amorphous matrix, consisting mainly of gelatinized starch, will be further modified during cooling as the starch retrogrades [7]. Besides, gelatinized starch can be attacked by amylases, owned or added, to generate sugars [8]. Immonen et al. [9] worked on the effects of bread slurry addition on bread quality and Rosa-Sibakov et al. [10] replaced sucrose with bread hydrolysate. The use of bread waste for the production of a growth medium for starters cultivation [11] or a new antifungal ingredient [12] have been also studied. Many studies took advantage of these changes and used bakery products as good feedstocks for ethanol production with high fermentation efficiencies through starch enzymatic hydrolysis [2,13,14]. For alcohol production, a first phase of starch hydrolysis is necessary to obtain a glucose syrup. This Foods 2022, 11, 1793. https://doi.org/10.3390/foods11121793 https://www.mdpi.com/journal/foods