Chemical and Biochemical Transformations during the Industrial Process of Sherry Vinegar Aging VICTOR PALACIOS,* ,† MANUEL VALCA Ä RCEL, ‡ ILDEFONSO CARO, † AND LUIS PE Ä REZ † Department of Chemical Engineering, Food Technology and Environmental Technology, Faculty of Sciences, University of Cadiz, Campus Rı ´o San Pedro, Apdo. 40, Puerto Real, 11510 Ca ´diz, Spain, and Department of Research and Development, Pedro Domecq, Jerez de la Frontera, Spain The work described here concerns a study of the chemical and biochemical transformations in sherry vinegar during the different aging stages. The main factors that contribute to the nature and special characteristics of sherry vinegar are the raw sherry wine, the traditional process of acetic acid fermentation in butts (the solera system), and the physicochemical activity during the aging process in the solera system. A number of chemical and biochemical changes that occur during sherry vinegar aging are similar to those that take place in sherry wine during its biological activity process (where the wine types obtained are fino and manzanilla) or physicochemical activity process (to give oloroso wines). Significant increase in acetic acid levels was observed during the biological activity phase. In addition, the concentrations of tartaric, gluconic, succinic, and citric acids increased during the aging, as did levels of amino acids and acetoin. A color change was also produced during this stage. Glycerol was not consumed by acetic acid bacteria, and levels of higher alcohols decreased because of the synthesis of acetates. On the other hand, in the physicochemical phase the microbiological activity was lower. Concentrations of some cations increased because of evaporation of water through the wood. A color change was also produced in this stage. Concentrations of different amino acids decreased because of reaction with carbonyl compounds. A precipitation of potassium with tartaric acid was also observed. KEYWORDS: Sherry vinegar; aging stages; chemical changes; biochemical changes INTRODUCTION Wine vinegar is an enological product that achieves, depend- ing on the type produced, a distinctive personality that engenders a high degree of appreciation among consumers. Sherry vinegar is one of the most renowned products of this type in the world. The long enological tradition associated with the regions around Jerez de la Frontera (Spain) has led to the development of a very special vinegar of premium quality, which has subsequently become widely appreciated in overseas markets. Production of such vinegar has reached levels of nearly four million liters per year. The special characteristics of this vinegar, which have been described in many studies (1-3), along with its positive image, has led to the creation of denomination of origin “sherry vinegar”. However, despite the long-term development of this product there are only a few very short studies concerned with the nature of the phenomena linked to the elaboration process (4). Owing to market demands, production of sherry vinegar today is performed by the majority of companies in a specific and controlled way, although this was not always the case in the past, when it was considered as a subproduct resulting from problems with the sherry-making process. Indeed, we could say that the nature of sherry vinegar evolved in tandem with the history of sherry production. Initially, vinegar was simply the result of an alteration in the critical phase of the production process or an unsuccessful attempt to obtain a good sherry. For example, a batch of this product may for some reason have been left during long periods of aging, during which it underwent a complicated process of acetification conditioned by the high alcohol content and the complex nature of the aged sherry. Nowadays, a high proportion of the raw material used for the production of sherry vinegar consists of perfect young sherry wine because accidental incidents in which the sherry wine is left for prolonged periods very rarely occur during the elabora- tion process. However, such incidents have not been completely eliminated and some of the faulty matured wines enter this system. For this reason some proportion of the sherry wine, in the first phases of maturing, is destined for the production of vinegar. This situation significantly changes the production methods, because the acetification process is currently carried out using biological reactors and the product is later subjected to the traditional methods of aging (5). Therefore, the raw material * To whom correspondence should be adressed. Fax: 00 34-956-016411. E-mail: victor.palacios@uca.es. † University of Cadiz. ‡ Department of Research and Development, Pedro Domecq. J. Agric. Food Chem. 2002, 50, 4221-4225 4221 10.1021/jf020093z CCC: $22.00 © 2002 American Chemical Society Published on Web 06/14/2002