# Development and Analysis of an Intensified Batch-Fed Wine Fermentation Process
## 1 Introduction  
Wine production is the oldest bioprocess in existence, and has been a major driver in the advancement of biology, chemistry, and process science: pasteurization, for example, arose from Pasteur's efforts to prolong wine shelf life [1]. Despite this, wine fermentations are still quite primitive from a bioprocess engineering point of view. White wine fermentations are carried out by expressing juice from white grapes and adding an initial inoculum of cultivated yeast (_Saccharomyces cerevisiae_). At the most, industrial winemakers might measure and correct initial nutrient levels and control fermentation temperature via the use of a tank jacket or external heat exchanger. White wine fermentations are typically carried out at low (14-20 \({}^{\circ}\)C) temperatures [2]. These low temperatures result in slow process cycle times for a single batch of white wine, on the order of 1-3 weeks. Wine presents another challenge in its seasonality: wineries only operate during the \"harvest\" or \"crush\" period, September-November in the Northern Hemisphere, February-April in the Southern. Slow fermentations with only \(\sim\)1/3 yearly uptime means that wineries are capital intensive versus breweries or distilleries with similar yearly throughputs. Wine juice is a nitrogen limited media where _Saccharomyces_ rapidly dominates [3], even when not inoculated. Yeast derives nitrogen from ammonia and alpha-amino acids, typically referred to as Yeast Assimilable Nitrogen (YAN) [4; 5], with uptake following Monod style saturation kinetics. YAN is typically present in grape juice in the 100-300 mg/L level(though lower and higher levels do occur naturally) and it is often corrected up to 350 mg/L by the addition of ammonia or amino acids by the winemaker prior to fermentation, or occasionally at the beginning of the exponential phase of yeast growth. Yeast consumes hexoses (in grape juice, glucose and fructose) in a non-growth associated manner and produce ethanol in proportion to hexose consumption. Metabolic activity is a strong function of temperature [6]; biomass deactivation is a function both of ethanol concentration and temperature. While Monod-style models have historically been utilized to understand wine fermentation kinetics, the application of genome scale metabolic models to explore the production of dilute sensory metabolites for fermentation optimization has also been performed [7]. The basic nature of industrial wine fermentations, coupled with the large global demand for wine, the capital-intensive nature of wineries, and recent advancements in fermentation engineering, make wineries a ripe target for fermentation process intensification. Process intensification is the science of increasing the productivity of existing processes and has been applied widely to industrial bioprocesses [8], such as pharmaceuticals [9] or biodiesel production [10; 11]. Typical fermentation process intensification approaches include the engineering of optimized organisms, the application of fermentation engineering to improve process conditions, and the switch from totally batch operation to either fed-batch or continuous operation. Increased first-principle understanding of wine fermentations has led to multiple attempts to accelerate wine fermentations. Increasing fermentation temperature with thermotolerant yeast [12] has led to faster fermentations at the cost of a significant negative sensory impacts. Attempts to convert wine fermentations to continuous processes [13] have similarly met with mixed success; while feasible at the benchtop scale, continuous fermentations have sterility requirements well outside the scope of current winery processing capabilities and are difficult and expensive to implement. For an intensified process to be widely applicable, it must readily slot into the processes and equipment of existing wineries while offering minimal sensory impact.