Freshness is the lifeblood of fruit. This holds true for winemaking as well. Therefore, harvested grapes should be processed into wine as quickly as possible—every minute, every second counts. Many people may hold this image of winemaking.
The topic we explore today runs 180 degrees counter to such conventional thinking. It involves deliberately freezing fresh grapes. Let us examine this unconventional winemaking technique.
Freezing for Purposes Beyond Freshness Preservation
The concept of freezing fruits and vegetables while fresh would not seem particularly unusual to those living in Japan. In Japan, with its advanced freezing technology, various foods and prepared dishes are frozen and distributed. Even without citing such examples, it is common in daily life to store purchased vegetables and meat in the freezer. The primary purpose in these cases is to maintain freshness.
So, does freezing grapes harvested for winemaking also serve the same purpose of maintaining freshness? The answer is 30% correct and 70% incorrect.
Freezing grapes is indeed sometimes performed to maintain freshness and condition. This is particularly true at research institutions and urban wineries that import raw materials from overseas to conduct winemaking operations year-round. However, many wineries freeze grapes for purposes other than preservation.
Cryoextraction: Freezing for Enhanced Extraction
When we hear about freezing grapes in winemaking contexts, the first technique that comes to mind is cryoextraction.
Cryoextraction is a winemaking technique that involves artificially freezing harvested grapes. It is commonly known as an approach to artificially create the same product as ice wines found in Germany and Canada. The key difference lies in this: ice wine is produced from grapes that are harvested while still on the vine after natural freezing, whereas cryoextraction involves placing harvested grapes in a freezer to freeze them artificially.
It should be noted that some countries and regions distinguish a derivative technique called "supraextraction" as a variation of cryoextraction. The difference between these two methods lies in whether a waiting period is established for partial thawing of the frozen grapes before pressing. When specifically called supraextraction, the winemaking process includes a designated waiting period for partial thawing.
However, even cryoextraction that does not establish such waiting periods cannot press grapes while they remain completely frozen. Therefore, some degree of waiting time is necessary in practice. For this reason, most countries and regions classify both methods as cryoextraction without distinction.
Processing Temperatures for Cryoextraction
Because of the association with artificially creating ice wine, many people believe that cryoextraction must be performed at minus 7 degrees Celsius, the same temperature used for ice wine. This is incorrect.
Cryoextraction serves no purpose beyond being a method to freeze grapes and improve extraction efficiency. In other words, as long as the temperature is sufficient to freeze the grapes, the specific temperature used for the process is not problematic. Moreover, wines claiming to use cryoextraction are available in the market even when produced at temperatures where grapes do not achieve complete freezing.
As grapes ripen and become sweeter, they become more difficult to freeze due to a phenomenon called freezing point depression. Therefore, storing grapes at minus 7 degrees Celsius does not necessarily guarantee complete freezing in all cases.
Two Primary Purposes for Freezing Grapes
There are two main purposes for freezing grapes beyond preservation: concentration of sweetness and enhancement of extraction. Both relate to extraction from a certain perspective, and in that sense, both constitute cryoextraction. However, their specific mechanisms are not identical. Let us examine each approach.
The Sweetness Concentration Mechanism: Why Freezing Makes Grapes Sweeter
Concentrating sweetness by freezing grapes—while this may seem like a straightforward explanation, do you understand why freezing grapes makes them sweeter?
Grape juice consists predominantly of water. Various organic and inorganic compounds, including sugars and acids, are dissolved in this water. Water freezes at 0 degrees Celsius, but when non-volatile components like sugar dissolve in water, the temperature at which water becomes ice—the freezing point—decreases. In other words, the solution becomes more difficult to freeze.
The key lies in understanding what "more difficult to freeze" means. Grape juice does not fail to freeze at 0 degrees Celsius entirely. In fact, it does freeze partially at 0 degrees Celsius. However, only the water portion freezes at 0 degrees Celsius.
To reiterate, grape juice is essentially a large amount of water with dissolved sugars and acids. When placed in a 0-degree environment, the water portion of the juice begins to freeze. The sugar and other non-freezing components that were dissolved in the frozen water then migrate to and continue dissolving in the remaining unfrozen water. As this phenomenon continues, the sugar concentration in the unfrozen portion increases progressively. The higher the sugar concentration becomes, the more difficult that water becomes to freeze.
Eventually, a liquid so sweet that it remains liquid even at minus 7 degrees Celsius is left behind. Even if it does freeze, it reaches a state where it would melt immediately with the slightest temperature increase.
This is the mechanism of sugar concentration. The process resembles distillation, where temperatures are raised gradually in stages. Ultimately, by pressing and extracting this final unfrozen portion or the portion that melts immediately, extremely high-sugar juice can be obtained.
As this mechanism demonstrates, sugar concentration requires slow freezing at relatively high sub-zero temperatures. If the grapes were frozen instantaneously using something like liquid nitrogen, the gradual freezing of water would not occur, leaving the sugar distributed throughout the entire water content as it freezes.
Incidentally, when making products like sherbet, the fastest possible freezing is required. This approach is completely opposite to the concentration concept, as failure to freeze quickly would result in varying sweetness throughout different parts of the sherbet.
Enhanced Extraction Efficiency Through Freezing
Freeze extraction technology is also applied to red wines. The purpose is not, of course, to create extremely sweet wines, but to enhance the extraction efficiency of compounds such as phenolics.
Freezing grapes is believed to cause the water contained in grape tissue cells to crystallize as ice, increasing in volume and destroying cellular structures such as cell walls. This is considered to have a positive impact on extraction efficiency.
The process called maceration, which is fundamental to winemaking, is performed to transfer components contained in grape skins to the juice. The extraction efficiency of maceration is lowest when grapes remain as whole bunches and increases after mechanical force is applied through processes like destemming. This occurs because pre-existing damage to grape tissue accelerates component extraction from those damaged areas. In other words, pre-destroying cell walls through grape freezing can achieve the same effect.
Temperatures and Risks in Freeze Extraction
When the goal is to promote component extraction, the critical factor becomes the rate of tissue destruction accompanying freezing. Larger ice crystals formed within cells are believed to enhance extraction efficiency more effectively, but the optimal processing temperature varies according to different research findings. Some literature suggests that slower freezing produces larger ice crystals, while other sources favor near-instantaneous freezing. In reality, extraction tendencies appear to vary depending on grape varieties and target components, making it difficult to definitively establish an optimal processing temperature.
However, there is a clear caution associated with using grape freezing to promote extraction: total acid content decreases and juice pH increases.
The Deacidification Effect of Freezing
When extraction is performed on frozen grapes, the confirmed results show almost no clear consistency, with one notable exception: decreased acid content and increased pH.
The reason for this phenomenon is relatively clear. It is believed to result from increased extraction of potassium or calcium from cell walls destroyed during the freezing process.
Many people have likely seen transparent, glass-like fragments settled at the bottom of white wine bottles. These are not glass but rather organic compounds called tartrates—crystallized acids. They form when potassium or calcium present in wine combines with tartaric acid also dissolved in the wine. The higher the concentration of potassium or calcium in wine, the more likely they are to appear. Freezing grapes significantly increases this likelihood.
When acids precipitate as tartrates, the amount of acid dissolved in the juice decreases. This acid reduction promotes pH increases. While the trigger is tissue destruction from freezing treatment, the phenomenon that occurs is identical to the deacidification effect caused by increased extractives from skins and stems in whole-bunch fermentation. This deacidification effect is observed not only in red wine production but also in orange wine production.
How Freezing Methods Affect Wine Aroma
Grape freezing treatment was long believed to have no impact whatsoever on the winemaking process. While this differs when grapes are pressed without complete thawing for concentration purposes, using grapes that have been frozen and then completely thawed was thought to produce no concentration effects and therefore no impact on grape or wine quality. Indeed, when wines are made from grapes subjected to freezing treatment for preservation purposes, there is typically little significant difference in final alcohol concentration.
However, recent discoveries show that freezing grapes actually changes the extraction tendencies of various components.
The relationship between freezing treatment and extraction results is highly complex, with some components reportedly showing no changes from freezing. Nevertheless, similar to the changes in total acid content and pH already discussed, confirmed impacts on wine aroma are significant. Wines made from grapes subjected to freezing treatment tend to develop more floral aromas compared to those made without freezing, while simultaneously showing increased potential for more distinctive wine aromatics.
"Distinctive" in this context does not necessarily imply positive effects. While not quite qualifying as off-odors, cases have been confirmed where aromas not typically found in conventional wines emerge.
Conclusion: Does Grape Freezing Make Wine More Distinctive?
Grape freezing treatment represents one technology seeing increased adoption through responses to climate change, evolving quality consciousness, or changes in winery locations. However, it remains a relatively high-cost technology that cannot necessarily claim high environmental compatibility. Furthermore, the scope of its effects remains unclear in some areas. Nevertheless, considering changes in aroma characteristics, it can be viewed as a technology with potential to impart distinctive characteristics to wine, similar to carbonic maceration.
When implementing grape freezing, the required equipment varies significantly depending on the freezing method employed. Some approaches require large-scale freezers while others need no such equipment. The critical consideration is that different approaches may produce vastly different results. However, this also means that this technology offers numerous adjustable parameters.
While the technical difficulty of establishing optimal conditions is high, depending on the approach, it may be possible to alter wine characteristics without equipment upgrades. Although grape processing in winemaking faces legal challenges in regions like the EU, this could become a valuable winemaking technique in countries and regions without such regulations.
Grape freezing treatment is said to produce varying effects depending on freezing temperature and duration. Detailed discussion of which quality parameters might change and how will be covered in articles published in our online circle or magazine. Those interested are encouraged to consider joining our circle or subscribing to our magazine.
