foods Review Fermentation of Agri-Food Waste: A Promising Route for the Production of Aroma Compounds Jasmine Hadj Saadoun , Gaia Bertani, Alessia Levante , Fabio Vezzosi, Annalisa Ricci, Valentina Bernini * and Camilla Lazzi   Citation: Hadj Saadoun, J.; Bertani, G.; Levante, A.; Vezzosi, F.; Ricci, A.; Bernini, V.; Lazzi, C. Fermentation of Agri-Food Waste: A Promising Route for the Production of Aroma Compounds. Foods 2021, 10, 707. https://doi.org/10.3390/foods 10040707 Academic Editors: Graziana Difonzo, Silvia Grassi and Maria Paciulli Received: 26 February 2021 Accepted: 22 March 2021 Published: 26 March 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 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/). Department of Food and Drug, University of Parma, Parco Area delle Scienze 49/A, 43124 Parma, Italy; jasmine.hadjsaadoun@unipr.it (J.H.S.); gaia.bertani@unipr.it (G.B.); alessia.levante@unipr.it (A.L.); fabio.vezzosi.fv@gmail.com(F.V.); annalisa.ricci@unipr.it (A.R.); camilla.lazzi@unipr.it (C.L.) * Correspondence: valentina.bernini@unipr.it; Tel.: +39-0521906527 Abstract: Food waste and byproducts are generated along the entire food processing and storage chain. The large amount of waste deriving from the whole process represents not only a great economic loss but also an important ethical and environmental issue in terms of failure to recycle potentially reusable materials. New, clear strategies are needed to limit the amount of waste produced and, at the same time, promote its enhancement for further conversion and application to different industrial fields. This review gives an overview of the biological approaches used so far to exploit agri- food wastes and byproducts. The application of solid-state fermentation by different microorganisms (fungi, yeasts, bacteria) to produce several value-added products was analyzed, focusing on the exploitation of lactic acid bacteria as workhorses for the production of flavoring compounds. Keywords: waste; valorization; fermentation; lactic acid bacteria; flavor 1. Are We Sure They Are Really Waste? In recent decades, a new challenge concerning the reduction of food waste (FW) and food loss has been raised for the world population. Indeed, food production is increasing annually worldwide, and the Food and Agriculture Organization of the United Nations (FAO) has estimated that one-third of the products intended for human consumption (corresponding approximately to 1.3 billion tonnes/year) is wasted or lost every year [1]. This event, in developing countries, mainly originates from the first steps of the food supply chain, due to technical and management limits in the post-harvest, processing, and distribution steps. On the other hand, in medium- and high-income countries, food is lost or discarded in the final steps, by retailers and consumers, due to the high productivity and quality standards required by the market [2]. As a result, many resources are lost in terms of land, water, and nutrients, and therefore the production, processing, manufacturing, and transport steps become a waste of energy. Considering the growth of the global population projected to 2050, to ensure an appropriate food supply, food production must increase in the next decades [3]. This will cause a rise in FW, ultimately posing a serious problem in terms of waste management and disposal. Thus, the modern challenge is to minimize FW, optimize production processes, and move from a linear consumption and production model to a new one organized according to a circular economic strategy. In this view, FW is considered as a byproduct and resource, and several studies about waste and byproduct valorization have been conducted in recent decades [4–6]. FW is defined as any part of food that is discarded, regardless of its potential content of compounds retaining a high value [7]. Depending on its origin or production, FW can be characterized by a variable chemical composition of carbohydrates, proteins, lipids, and other components that could be used in different industries and fields [8], such Foods 2021, 10, 707. https://doi.org/10.3390/foods10040707 https://www.mdpi.com/journal/foods