Oxygen Management During Alcoholic Fermentation
Pontificia Universidad Católica de Chile
Oxygen additions are a common practice in winemaking, as oxygen has a positive effect in fermentative kinetics, biomass synthesis and improvement of color, structure and :flavor in treated wines. However, most oxygen additions are carried out heuristically through pump-over operations solely on a know-how basis, which is difficult to manage in terms of the exact quantity of oxygen transferred to the fermenting must. It is important to estímate the amount of oxygen added because very slight additions are not effective and; excessive additions could lead to organoleptic defects, in:fluencing the wine quality drastically. Therefore, oxygen management practices, including knowing the right amount of oxygen to add during winemaking ( depending the desired results ), the dynamics of oxygen transfer and consumption, and tools for regulating precisely the oxygen additions, are necessary for elaborating optimal quality wines. The aim of the present study is cover these aspects, providing a better understanding of the oxygenation during alcoholic fermentation at industrial winemaking scale, i. e. how oxygen is distributed inside large tanks and how much oxygen is dissolved and biologically consumed. Such studies were performed firstly at industrial scale and, later, brought to the laboratory scale. In these studies oxygen transfer was performed using two systems: direct oxygen bubbles sparging and through silicone membranes. The latter allows to transfer precise quantities of oxygen to the liquid phase and it could be used to study oxygen management and aroma compounds evolution during fermentation. Industrial scale studies included the evaluation of different pump-overs modes: closed, open pump over anda pump-over with Venturi (that incorporates air to the circulating must). Results showed that closed pump-overs incorporated negligible amounts of oxygen, while pump-overs with a Venturi injector incorporated the highest dissolved oxygen, i.e. almost twice more oxygen than open pump-overs. Oxygen distribution inside large tanks (40,000 L) was achieved by measuring with oxygen sensor probes at three different levels. Pump-overs resulted in a heterogeneous distribution of oxygen within the fermenting must, with almost 80% of the total oxygen added rapidly consumed at the top of the tank. Oxygen consumption during industrial winemaking was followed in 5,000 L tanks, measuring on the middle of the tank. When analyzing oxygen dissolution during the maceration and fermentation stages, lower oxygen levels were encountered in the presence of high free SOz concentrations and reduced yeasts activity, respectively. Furthermore, the oxygen dissolution kinetics during fermentation was analyzed by estimating the oxygen dissolution rate and a global consumption cons~t for different fermentation stages. Results showed that the COz has a negative impact on oxygen dissolution and, together with the elevated yeast biological uptake, are the main responsible for the low oxygen dissolution during the tumultuous fermentation. All-scale experiments were performed to better understand the industrial process and the oxygen dissolution. A model of oxygen kinetics was developed and tested at laboratory scale, using a pore diffuser, under similar conditions of those encountered in oenological fermentations. Results indicated that the fermentation phase, liquid composition, mixing process and carbon dioxide concentration must be considered when performing oxygen addition during oenological fermentations. Nevertheless, oxygen added through the sparger system is difficult to control, making its implementation at industrial scale still complicated. F or this reason, a bubble-free oxygenation system was developed and validated, which exhibited high reproducibility. The evaluation ofvarious parameters on the maximum oxygen transfer rate (OTRm) showed that, for fixed characteristics of the silicone tube and the partial pressure, dissolved COz and medium composition had negligible effects; and that the parameters with the biggest influence on the OTR.m were the liquid flow rate and the temperature. This information was used to build a mathematical model that allows to calculate the OTR in synthetic media and in real fermentation media. Furthermore, this bubble-free oxygenation system was used to manage precisely oxygen content ( quantity and time of oxygen addition) during alcoholic fermentation, and to study the kinetics of relevant higher alcohols and esters produced during alcoholic fermentation by coupling the bubble-free oxygenation system to an on-line GC system (data not shown). Altogether, the results of this study as well as the tools hereby developed will help developing better oxygen addition policies in wine fermentations on an industrial scale and this work makes a step toward innovative strategies for oxygen management during alcoholic fermentation through bubble-free oxygenation system.