Did you know that heat can destabilize wine?!
We’ve already taken a look at how cold stability works in your downstream processing when preparing for the bottling season. The other half of the stability equation is how to manage your wine on the hot side of things. While undesirable, mostly for the impact on the long-term flavor profile of your wine, there are instances in the trade where your product may be subject to heat spikes. Whether it’s a shipping truck stopping in Phoenix during the summer, or customers who leave a bottle in the trunk while out on a day of tastings, it is a reality that can occur when the mercury begins to rise.
The Physical Process of Heat Instability in Wines
Despite flavor quality being the main concern if your wine is exposed to excessive heat, it’s the cosmetic aspect of heat stability that the typical winery will deal with when going through bottle preparation. The issue revolves around wines which have been exposed to excessive heat developing a noticeable haze as a result of heat labile proteins becoming denatured and no longer remaining stable in solution; instead forming a sol in which the proteins do not dissolve in solution yet remain suspended in the wine.
If you conceptually look at protein solubility in wine, the key points to understand are pH, alcohol concentration, and temperature. Plotting general solubility over pH, what you see are the individual protein molecules forming U-shaped curves over the range of pH with the bottom of the curve being the least soluble state of the protein, which is its isoelectric point, or the point where it has no net charge.
The vertical axis is general solubility. The higher the proteins are on this axis, the more likely they are to remain in solution. The lower on the axis, the more likely they are to come out of solution. There is no definitive break point for insolubility, but a range where proteins start to become insoluble. Once they drop into that range, they start denaturing and have the potential to come out of solution. Once they drop below that range, they will certainly denature and come out of solution. This graphical representation occurs at a fixed temperature and a fixed concentration of alcohol.
As the pH of the solution changes, the charge of the protein changes over the range of pH. Generally, proteins will remain stable in regards to pH over the pH range common to wine, but larger pH shifts can cause the proteins to denature as the shift causes a breakdown in how proteins fold and arrange themselves. Small pH shifts however, such as ones from an acidity adjustment, can cause insolubility if the isoelectric point falls into the insolubility range. Conversely, an unstable protein can go back into solution if the pH shift moves it back into the soluble range higher up on the U-curve away from an insoluble isoelectric point.
If you alter the alcohol concentration you change the overall solubility properties. Lower amounts of alcohol increase solubility and the U-curves would move up from their current positions, while higher alcohol volumes cause a decrease in solubility and would move the U-curves down on the vertical axis.
The property of most interest for winemakers in this instance though is temperature. Higher temperatures actually cause an increase in solubility, while lower temperatures decrease general solubility. The issue comes at the extremes in either case. Proteins can come out of solution and haze at low temperatures if solubility decreases enough. Wines will commonly freeze though before showing a chill haze, as one may see in beer production at lower temperatures. This is usually due to having a much lower protein concentration on average than other beverages that suffer chill hazing problems. Excessive heat however will eventually force a denaturation of proteins in wine, similar to the way an egg hardens in a pan when you cook it, and the large decrease in solubility will crash the proteins out of solution. This is precisely what occurs when wines are heat unstable and throw a heat haze.
Treatments for Heat Instability in Wine
The way to prevent this haze from occurring is to remove the unstable proteins from the wine, or to induce conditions that prevent the proteins from becoming insoluble. In the latter case, you may see wines treated with yeast cell wall mannoproteins that bind with the wine proteins and form stable colloids that resist flocculation. As for removal, one potential method is tannin fining, usually with chestnut gall tannins that provide limited sensory impact to the wine.
Far and away though, the process generally performed for heat stability treatments is bentonite fining to remove proteins. Bentonite is an aluminum-silicate based earthen clay formed from volcanic ash that will swell in liquid and form a colloid with an overall net negative charge. The negative charges on the bentonite will interact with the generally positively charged protein molecules in solution, forming weak electrical bonds. As the bentonite colloid is too heavy to remain in suspension, it will slowly precipitate, and in the process remove the interacting proteins along with it.
There are a few issues with bentonite fining to keep in mind. The first is finding the proper dosing rate to make the wine heat stable, while not overdosing, as bentonite will strip other compounds from the wine as well, particularly desirable aroma and flavor molecules. The second is that the bentonite must be removed from the wine by filtration. This is commonly done through a plate and frame operation as bentonite may clog other filter types, especially crossflow filters. Lastly, bentonite lees requires proper disposal. Simply running lees down the drain system will clog drains over time, as well as aeration ponds at larger wineries. Bentonite lees is often ported to dedicated composting areas in the vineyard or to municipal waste sites for disposal from smaller facilities, which requires a portable holding tank of some sort. Bentonite volumes from larger wineries often requires capture and removal offsite, commonly by companies involved in commercial tartrate extraction.
Bench Testing Heat Stability in Wine
As with cold stability, there are methods for testing whether wines need heat stability treatments or not. These methods also come with an added advantage of being able to determine optimum dosing rates for bentonite to remove the unstable proteins while avoiding overdosing and decreasing wine quality.
The crude test that many wineries use is simply to bake the wines in an oven, forcing any heat labile proteins to denature and form a haze. If any haze forms, samples of the base wine can be treated with different dosing rates of bentonite, filtered, then the bake test is repeated for each sample to determine at what concentration of bentonite addition the wines no longer form hazes. While fairly effective, this test is more of a shotgun method that covers a range of proteins which may not be problematic under normal wine heating conditions, and can result in overestimation of bentonite dosing rates.
The preferred lab method is to treat wines with an agent that forces protein instability, then measure the results by use of a turbidity meter. The two common forcing agents are 100% ethanol or a phosphomolybdic acid mixture.
Remember that increases in alcohol lead to decreases in wine stability. A 50/50 mixture of wine with ethanol will force any unstable proteins under wine conditions to denature and form a haze. Pure ethanol must be used as impurities from other alcohols will cloud when diluted in water and skew turbidity readings.
Phosphomolybdic acid mixtures cause protein denaturing and haze formation through cross-linkages with molybdenum ions.
Turbidity Meters In Testing Heat Stability in Wine
With any of the three methods, samples can be analyzed with much more precision than visual tests by use of a turbidity meter. A baseline value of the untreated wine is performed, then wines are given the force treatment to induce haze. The treated wines are then measured for turbidity to determine any delta value between the baseline and the treated wine. In general, if the wines have a difference of 5 NTUs or less, they are considered stable versus hazing. Some testing protocols use a more stringent value of 3 or 2 NTUs by spectrophotometer to declare stability, but at some point the fine values are approaching the error limits of the measurement itself.
Wines with delta values above 5 NTUs can then be treated at different bentonite dosing rates, filtered, then force tested again. A zero-dose rate is included in the trial as a baseline. The different dosing rates will form a decay curve if you plot NTU values over dosing rates. The point at which the decay curve flattens out becomes the ideal dosing rate for bentonite. The ideal rate as shown in the example curve for instance would be about 450 mg/L of bentonite.
Solutions for Heat Stability Testing In Wine From Hanna Instruments
Hanna Instruments offers two solutions for your heat stability testing needs. Our HI83749 Turbidity and Bentonite Check Meter provides a simple, portable solution for turbidity measurements and force testing by the phosphomolybdic acid method.
We also have the HI88703 should a dedicated benchtop unit better suit your testing needs.
Empowering your wine lab with either option will allow you to better control your heat stability demands in the cellar, reducing costs over time by determining if wines need treatment or not, and by optimizing your bentonite usage. Paired with control of your cold stability testing needs, your winery should notice an increase in quality as well by avoiding or limiting excess wine processing.
For more information regarding how Hanna Instruments can help you with measuring your wine, contact us, as email@example.com or 1-800-426-6287.
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