Cold stabilisation: wine’s invisible quality test

How to keep crystals out of bottle and why the choice of method matters

Tartrate crystals may be harmless, but consumers rarely see them that way. Along with protein haze, they account for roughly half of calls to the Wine Australia-funded AWRI Helpdesk, making stabilisation before bottling one of the least glamorous and most consequential steps in winemaking. The good news is that producers now have more options than ever. The harder question is which ones best suit your wine, winery and balance sheet.

Why wines become unstable

In grape juice and wine, tartaric acid (H₂T), bitartrate ion (HT^-) and tartrate ion (T^2-) are in equilibrium but that can shift depending on a number of factors, including:

• Alcohol content
• Temperature
• pH
• Concentration of other proteins, polyphenols and polysaccharides
• Concentration of potassium ions in the juice/wine

Alter one of the key variables and that balance moves. Sometimes the result is greater stability. Sometimes it pushes the wine into supersaturation, creating the conditions for potassium bitartrate to precipitate instantly and potentially later — often after bottling when it is least welcome. The AWRI covers the factors and science behind potassium instability in more detail.

Whilst not discussing calcium instability in detail here, some of the tools to treat potassium instability will also help with calcium, though some have little to no effect. Further information can be found on the AWRI’s page on calcium instability.

How your cold stability choices can affect your business

Along with the reputational risks of unstable wine in the market, the process and/or products you use will affect other aspects of your business. Much of this falls into LEAN manufacturing which you can read more about in Wine Australia’s The LEAN guide. Some of the ‘wastes’ and therefore costs to your business can be reduced by understanding the impact of your current practices and potential alternatives, including: 

Defects 

• Understand the cost to recall a product
• Ensure your laboratory testing is robust and to standard – the AWRI’s cold stability page is a useful resource for the testing options available

Overprocessing 

• Are you performing steps that could be removed from the process?
• Understand the impact on tank use, tank cleaning, laboratory testing and re-testing, water use and issues created with wastewater with each method

Waiting 

• The traditional cold stabilisation method requires extended time at a low temperature, which means running plant for prolonged periods which is also costly

Motion 

• Understand how many product movements are in your current process. Would a change in process reduce this? What impact would that have on cost and quality? 
• Each move costs time and has the potential to decrease quality

Cost considerations – to invest or not?

Whilst LEAN production might sound daunting and not for you, understanding your current processes and costs of doing business are essential to making informed decisions on what methods are best for your product and business. Start by considering your current expenditure for: 

• Energy
• Labour in the winery and lab
• Water
• Wastewater management
• Wine additives such as tartaric acid

Once these are understood you can better assess the available options for cold stabilisation, how they affect the above costs and what impacts they may have on your wine style. It then becomes a payoff between increased capital versus lower operational costs. Many businesses have clear rules on payback expectations that will often make the decision for you. 

Assuming your winery has sufficient cooling capacity to be running traditional cold stability, capital expenditure is rated as low (Table 1). If you need to replace your cooling plant due to age, capacity or expansion this rating should be considered as high (Table 1). 

Changing cold stabilisation methods will never result in zero refrigeration needs, but significant capital savings can be achieved from a combination of:

• Reducing capacity needed – capital savings
• Efficiency of new machines – operational savings
• Running at a more efficient setpoint – operational savings

You can find more information and resources on improving winery refrigeration efficiency and cooling technology here: 

• Winery refrigeration efficiency guide – part of a Wine Australia-funded project by the AWRI which looked at the impact of brine temperature on plant efficiency along with other useful tips to reduce energy use
• Cloud-based cooling control technology – Frigid.Cloud – a project funded through Wine Australia’s Agtech Program. See also the following video: 

Investing in solar and/or batteries could also be considered if the main driver is reducing energy costs. It can also provide practical benefits and improvements to operations, like balancing the power load so that you’re supported through peak periods. The case studies below look at small to medium sized wineries that have invested in solar and battery technology and are part of Wine Australia’s video series showcasing wine sector leadership in sustainability:

• Powering stability: Henschke's journey to energy independence 
• Carbon neutral for growth: Ross Hill's smart, low-input model 
• Less in, more out: Silver Spoon Estate's low-input winemaking edge
• Batteries on wheels: Ballycroft's vehicle-to-grid advantage

Stability toolbox for winemakers

There is no ‘one solution fits all’ for tartrate stability, but all options fall into two broader methods:

• Methods that remove potassium or related ions
• Methods that add compounds to inhibit crystals from forming

Each method will have varying degrees of effectiveness, but each one will also have resulting effects on: 

• Wine chemistry, changes in pH and TA
• Costs from energy use, labour to operate/clean and in additives needed
• Timing – when can it be used in your process and can it add other benefits?
• Water use and wastewater generation

This AWRI research project, funded by Wine Australia, looked at both protein and potassium stability, including a promising trial using zeolites to remove potassium from wines, developing new surfaces to facilitate crystallisation and reducing energy needs. It also includes testing of some of the stabilising products covered here.

A comparison of various methods across relative costs, water use and wine chemistry is provided in Table 1, with further information on each method detailed below.

Table 1: Overview of removal and addition methods for cold stability

Removal methods

The following removal methods work by either forcing tartrate precipitation and removal of them, or by removing potassium ions from the product driving the equilibrium away from tartrate formation.

Traditional in-tank cold stabilisation method

The traditional approach is to chill wine to -4°C, seed it with cream of tartar and force potassium bitartrate to crystalise out of the solution, then settle and filter before bottling. It does little for calcium stability unless also seeding with calcium tartrate crystals but it remains the benchmark against which the alternatives will be judged. It is a dependable, well-understood method but will significantly increase your energy usage and costs. 

Continuous tartrate removal technology

This uses the same chemistry as traditional cold stabilisation, but the batch in-tank process is replaced by a continuous flow method. At the right scale and with specialised plant, this can cut operating costs in three key ways:

• Lower power costs, made possible by the specific plant design that treats a smaller volume of wine in a continuous process using the outgoing filtered wine stream to pre-cool the incoming wine, saving energy. 
• Reduced labour costs, arising from eliminating the need to clean tartrates from tanks and testing and replacing it with an automated, continuous process.
• It also filters the wine, which reduces an additional step in your process. 

Its attraction is less about new science and more about operational efficiency and reduced energy costs. For wineries with enough throughput to justify the capital investment, this can be a great option.

Ion/resin exchange

Ion exchange removes positively charged ions (such as potassium and calcium) by passing juice or wine through a charged resin bed where positively charged ions are swapped for hydrogen ions. The ability to treat juice (<200 Nephelometric Turbidity Units (NTU)) and the added effect of a reduction in pH with only minor effect to TA, means this technique can also be used to offset significant tartaric acid additions and, therefore, costs early in a wine’s life. 

With only around 20% of the product needing to be treated and then blended back to be effective, it can also be used on red must, where a portion is bled off and treated before being put back in the ferment.

Note: It may not always deliver complete stability when used only at juice stage but will be enough to reduce tartrate precipitation in ferment tanks, resulting in major savings in time and water required for additional cleaning. Ion exchange can then be used again post fermentation to improve stability or used in conjunction with additives like KPA as part of a two-prong strategy.

Electrodialysis

Electrodialysis uses charged membranes and an electrical field to pull ions such as potassium and calcium out of wine and into a water solution on the other side of the membranes. Wine needs to be clarified (<20 NTU) to stop fouling of membranes, so it doesn’t lend itself to early use in a wine’s life.  Care needs to be taken to ensure wines are slightly higher in pH than desired so they can handle the resulting pH reduction.

Waiting to treat wine means tartrates through ferment and storage will still occur, meaning tank cleaning savings are limited only to the final stabilisation process. Its main drawback is cost to purchase, though hire options make it easier to evaluate its effect on your wines prior to capital commitment. 

Addition methods

Instead of removing ions or unstable material, the following additions are aimed at stopping the crystals from forming, either by interfering with nucleation or crystal growth. They are quicker and cheaper to apply, but their performance can be affected by other wine chemistry, and as a result their effectiveness has limitations. 

In practice, they can be highly useful if you understand the limitations. They are often better at managing risk than eliminating it altogether and many can increase mouthfeel which can also be beneficial to some wine styles.

Potassium Polyaspartate (KPA)

KPA is the new kid on the block and, of all the addition methods, is the one that can achieve potassium stability on its own. It works by inhibiting crystal formation. Wines should be protein stable first because the treatment can also encourage protein precipitation. They are used at a similar stage to when we would normally cold-stabilise wine, meaning no changes to other processes are needed. It also passes through filters unlike some other additives. 

The FSANZ Food Standards Code limits the amount that can be added, meaning you need to ensure that the maximum addition of 200 ppm is enough to create a stable wine. This is a consideration in quick-to-market wine products that have not had time to drop some potassium tartrate naturally in tank prior to use. 

Manufacturers generally claim little or no effect on calcium stability. Its appeal lies in:

• Convenience
• Reduced energy 
• No effect on pH or TA 

Carboxymethyl cellulose (CMC)

CMC adsorbs onto growing crystals and slows or stops their development. It can be effective for modest instabilities, especially close to bottling. CMCs are a broad collection of similar molecules, therefore different products may not be interchangeable and may affect colour stability, mouthfeel and/or filtration in different ways. 

Yeast mannoproteins

Yeast mannoproteins can inhibit crystal growth while also contributing to mouthfeel and other sensory properties. That may be a benefit or a drawback, depending on the style of your wine. Their performance can weaken under extreme temperature conditions, so they are better suited to addressing slight instability rather than doing the whole job alone. In the right wine though, they may offer a useful dual function.

Gums

Gums extracted from plant material inhibit nucleation and crystal growth. Their effect is usually modest, though they can be useful alongside other products, especially where colour and tartrate interactions are also in play. They are helpful but rarely the whole answer, and their value tends to depend on what else is happening in the wine. 

Metatartaric acid

Metatartaric acid was once a standard addition, but its weakness is durability. It can prevent tartrate growth in the very short term but degrades in bottle and loses effectiveness within weeks to months, resulting in crystals forming in transit and on shelves. Given the advances in the additives described above it should be used with caution.