Controlling Heat Instability in Wine
Publisher’s note: This article is the first in a four part series for winemakers by Nick Smith with the Enology Laboratory at the University of Minnesota. Future stories in the series include “Treatment of Wine to Reduce Hazing” and “Methods for Testing Heat Stability.”
Snow globes make a nice holiday present and collector item. Your wine however, should be purchased for drinking, not as a winter display. Several factors can contribute to cloudy and hazy wine. Those factors include pectin, protein instability, and microbial activity. Protein instability remains one of the least understood hazing phenomenons. I will focus this series of articles on heat stability. First, the basic science behind heat instability will be addressed. After covering the science, analysis and interpretation will be discussed.
Protein and Heat Stability: The Science
Question: What would make an otherwise perfectly clear wine turn into a goopy, cloudy mess? Answer: Heat induced aggregation of wine proteins and protein denaturation. Consider that wine contains a complex array of dissolved materials. Those include sugars, organic acids, polyphenols, and proteins. Only a few of the actual proteins involved in heat instability have been identified. To complicate matters, some of those proteins can actually prevent hazing in other situations. Most, if not all research on protein stability in wine has been aimed at v. vinifera. Hopefully for the Midwest, hybrid focused research will provide greater clarity on protein issues in the future.
Proteins are long strings of amino acids held together by peptide bonds. The long strings of amino acids arrange themselves in a complex series of shapes and forms. Think about a piece of yarn. It can be bound up into a ball, strung back and forth, and twisted along its axis. A protein is essentially the same, a twisted, strung back and forth, and wound up string of amino acids. The overall structure of the protein helps determine its function. If the shape of the protein is modified even the smallest amount, its function can be halted. This reshaping and or loss of protein function is referred to as denaturation.
Twenty different amino acids can be linked together to build a protein. The amino acids individually have properties that not only aid in the formation of the protein structure, but also affect how it interacts with neighboring proteins. Two of the functional differences among the amino acids are charge and polarity. The arrangement and amount of polar amino acids affects the proteins solubility in water or other solvents. Positive or negative charges can help hold a protein structure together as well as attract or repel neighboring proteins. Hydrogen bonding and disulfide bridges between adjacent amino acids in the same protein further contribute to the shape and function of a protein.
Two important interactions of proteins with wine and must influence solubility: ionic interactions and hydration. As mentioned previously, proteins interact with water and neighboring proteins. It is these interactions that allow proteins to be soluble and suspended in solution. The amount and type of basic (positive) and acidic (negative) amino acids affect the overall charge of that protein. When a protein has a net neutral charge, it is at its isoelectric point. At low pH, there is a greater amount of hydrogen ions in solution. The hydrogen ions interact with the negative charges on the acidic amino acids and lead to proteins with an overall positive charge. If the pH of the solution is lower than the isoelectric point, the protein will have a net positive charge. Wine proteins find themselves in this condition, one where the protein’s isoelectric point is greater than the pH of wine. Long story short, heat susceptible proteins in wine tend to be positively charged.
Why does the positive charge of the protein matter? We have all played with magnets at some point in our lives. Same charges repel, opposite charges attract. With the proteins having an overall positive charge, they do not attract to each other and settle out of solution. Instead, they repel each other and keep themselves suspended in solution, slowing their precipitation. In most cases, this situation has no effect on wine appearance. That is, until a factor outside the environment of the wine bottle changes. In a short period of time crystal clear wine turns cloudy due to one of wines biggest enemies: heat.
Consider the egg. An uncooked egg white is relatively translucent with a yellow-white color. Add heat and the egg white turns opaquely white. The protein albumen makes up a large portion of the proteins in the egg white. When heated those proteins denature and take on a completely different visual appearance. While wine is not commonly fried or hard boiled, increases in temperature encourage the formation of hazes and coagulation of wine proteins. The addition of heat to wine can cause a structural change to the wine protein and results in coalescence and settling. Hazing occurs during cooling of the wine following a heat event. Heat causes the proteins to change into a form that is more soluble at high temperature. Proteins interaction with water is disrupted at high heat. When the wine cools, the proteins are no longer soluble and settle out of solution (Ribéreau-Gayon and others). Single high heat events are not the only condition to be concerned about. Slightly elevated temperatures over an extended period of time can cause hazing.
Observant individuals will note that protein hazing happens mainly in white wines. Key differences between white wines verses red wines are tannin and phenolics. During fermentation the tannins from skin maceration act as a fining agent removing the heat sensitive proteins from solution. The risk of heat instability is not eliminated in low tannin red or rosé wines.
In the next article, we’ll discuss the careful use of bentonite as a fining agent to reduce wine hazing.
Further reading on protein function and shape:
Handbook of Enology Volume 2: The Chemistry of Wine Stabilization and Treatments. Ribéreau-Gayon, P., Glories, Y., Maujean, A., Dubourdieu, D. 2001.