Frozen Grapes How To

What You Should Know About Frozen Grapes

  • Grapes before freezing
  • Receiving your grapes
  • Thawing your grapes
  • Preparing grapes/juice for fermentation
  • Acid adjustment
  • SO2 addition
  • Sugar adjustment
  • Rehydration of High Sugar varietals
  • Yeast and malolactic acid additions

From the Vine to Freezer: Black grapes are hand harvested into 1/2 ton bins in the evening or early morning. Brehm Vineyards and the grower mutually agree to the time & date of harvest. The selection of the vineyard, the rows of the vineyard and the timing of the harvest are the most important wine making decisions. As any cook knows, the ingredients are the key to the meal. You can mess them up, but their quality dictates the dish. The grapes are taken from the vineyard to BV’s processing station upon harvest. The cool harvested fruit is immediately refrigerated. At dawn the following day the grapes are destemmed, with the least maceration possible. Five gallons of must are placed in six-gallon pails. The pails are transported to a freezer and frozen. On occasion SO2 or another wine ingredient will be added to the grapes. This is rare. When an ingredient is added, it and the dosage will be posted on BV’s website. As the grapes are destemmed, a representative sample of the grapes will be taken. That evening the sample will be tested for brix, pH, and total acidity. Some samples may be sent to a professional lab where results may also include malic acid, nitrogen levels for fermentation-YAN, potassium, glucose & fructose. These tests will also be posted on BV’s web site.

White grapes are harvested in the same manner as black grapes. From the vineyard they go directly, whole berry, into a horizontal, pneumatic bladder press. They are pressed at low pressure and pumped into stainless steel tanks. They are chilled and held at or below 40°f for two to three days. Once the gross sediment has fallen, 20 liters (5+ gallons) are pumped into 6-gallon pails, taken to freezer and frozen. SO2 is usually added to Riesling grape juice. Other juices will only have ingredients added as deemed necessary. Any ingredient added will be noted on the web site of Brehm Vineyards. Brehm Vineyards takes care to minimize any chemical addition to the juice, believing that this is the winemaker’s option. The absence of SO2 to the juice will cause it to oxidize, turn brown. It is important that you not add SO2 to this juice when you receive it. The fermentation of the juice requires oxygen. The yeast will consume the oxygen in the juice and the oxidized phenolics will fall out before the end of fermentation, to reveal a golden/green wine. An addition of SO2 before this is accomplished risks maintaining these brown oxidized components in the wine. You must add SO2 at the end of yeast or malolactic fermentation, not before the end of sugar/yeast fermentation.

The freezing of the grape must/juice within the pail gradually happens from the outside to the center of the pail. As the cold enters from the sides, it freezes the water and drives the acids and sugars to the center of the pail. The freezing process actually concentrates the sugars and acids, while purifying the water. This separation is the basis of frozen orange juice.

Receiving Your Grapes It has been our experience over the years that occasionally a pail of frozen grapes will slightly implode during the defrosting cycle causing the pail to appear dented. There is nothing wrong with the pail or the grapes within. This is the natural consequence of the ice thawing. However, you should not leave these pails upside down. Swollen pails, pails venting CO2, leaking pails (probably not usable), and any other serious conditions as a result of the shipment of the grapes should be received from the transport agent “with exception” to the specific condition. You should write a detailed description of the condition and have the driver, or agent write theirname, date, and recognition of the condition on the bill of lading or air waybill BEFORE you leave with the pails. Please notify your supplier, or Brehm Vineyards immediately of the problem.

How to Thaw Your Grapes Depending on the method used to ship the grapes, you will need to let the pails of grapes thaw for 1-3 days. It is important that you thaw your grapes in an environment at a constant temperature. You want to thaw the grapes quickly and evenly. Room temperature, 60° – 70°F / 21°C, is ideal for thawing, as well as for conducting the primary sugar fermentation.

Under no circumstances should you let your grapes thaw slowly in a refrigerated environment. If you let the grapes thaw out slowly, you greatly increase your chance of allowing a bacteria, fungi or mold from taking hold and contaminating your grape must. If you have no alternative, in the case of red grapes, the addition of SO2 may be beneficial -30 to 40 ppm.

Your goal is to thaw your grapes quickly and evenly so that the grapes get to proper fermentation temperature as soon as possible. Once you add your yeast and the primary sugar fermentation begins, the bubbling CO2 released acts as a natural barrier against bacteria, fungi or molds. Take advantage of this CO2 barrier by keeping the fermenting must always covered (not sealed) while in an open fermentor, or always sealed with a fermentation airlock when in carboys. When you receive your grapes, take the lids off and inspect the grapes for any freezer burned or moldy grapes. Remove any grapes that are questionable. Make sure your hands are clean and then go ahead and mix the grapes in the bucket thoroughly. If the must is completely thawed, you will be able to move the entire must around. The temperature of the must might still be cold. Stirring the grape must does help speed the thawing process.

Once your pails of grapes are completely thawed, pour the contents of each pail into your fermentor. Make sure that you scrape out everything that remains in the pail, especially the syrupy ‘stuff’ at the bottom (which is a combination of sugars and cream of tartar that settle on the bottom of the pail during the freezing / thawing process). Add this to the fermentor. Add a half-gallon of must back into the shipping pail. Swirl this must in the pail until the pail is clean. Your goal is to dissolve any acids and sugar residue into the must or juice. Add all must from the shipping pails back into the fermentor. Now mix the must together thoroughly. Cover your fermentor so that no contaminants can fall into the must. The cover should not be air tight as to allow the CO2 to leak out.

Preparing grapes/juice for Fermentation All additions to juice or grapes are based on the quantity of the resulting wine – not the skins. With BV’s pails that should be, conservatively: 12 liters of red wine and 20 liters of white wine.

Acid adjustment: While acid addition is ultimately taste (yours) regulated, it does have an important impact on the pH of the wine. The pH of the wine has a direct impact on the bugs that can thrive in the wine and the effectiveness of SO2 additions to the wine.

Generally I would advocate a beginning total acid of 0.70 % or above if naturally provided by the grapes. When the acid level exceeds 0.90 I would start to consider acid reduction. Knowing the malic acid content would be a great help in this instance (beware as malic acid is often expressed on a different basis). It is difficult to obtain an accurate reading of sugar, TA and pH from freshly thawed juice or grapes. This is caused by the differentiation of the sugars and acids during the freezing and thawing process.

White juice figures noted on BV’s web site are quite accurate. These white samples are taken from the tank before freezing. It is a uniform sample. Red must is periodically sampled during destemming. The process we employ achieves a reasonably accurate proportion of skins and juice. Each individual sample only represents a few vines. Many of these samples are combined for analysis. Each pail is different from the next, as are each sample. On average, BV’s published analysis is close to accurate for all the pails. BV’s web statistics of the particular grape offers our clients a starting point, one that should be observed – initially.

One (1) gram of tartaric acid will increase the acid of the juce by 0.1%. Acid addition should be employed with tartaric acid only.

A decrease in acid may be accomplished in a variety of ways. During the course of fermentation the acidity of wine will decrease. If the acidity does increase you probably have vinegar. Wine is not considered stable until the sugar content is reduced to 0.2% and the malic acid content is almost completely consumed. The most common acid reducer is accomplished with malolactic fermentation. High acid content is often caused by a high malic acid content.White Riesling wine is noted for its high acid. It traditionally does not undergo malolactic fermentation. The acid, if too high, is masked with sweetness. Sweetness is certainly a method of tempering a wine with excessive acid.

A unique way to lower acid is available with frozen juice. Instead of trying to include the cream of tarter at the bottom of the grape pail, you can endeavor to withhold it from your juice. Minimize any stirring of the juice. Siphon the juice from the sediment. Take the sediment, the juice at the very bottom of the pail, and put it all in a half-gallon jug. Keep it cold, and let it settle. Rack the sugar off once again. Adjust your acid to the desired level with how much of the sediment you add back. Be aware that this acid is tartaric acid in the form of cream of tarter. The UNSTABLE malic acid will remain in the juice.

Professional winemakers reduce acid by taking out all the acid in a small portion of the juice – before fermentation. The desired acid reduction is calculated in terms of the liters of juice, of the total lot, that will be completely deacidified. The addition of acidex or other base strips all the acid out. Once the juice is devoid of acid, it is added back to the batch to be fermented.

SO2 Additions: Brehm Vineyards tries to provide you with grapes or juice with no adjustments. You are the winemaker; we are the harvest and crush pad professionals. BV will add SO2 to Riesling juice and botrytised juice. BV will add SO2 to juice or grapes when it is believed to be beneficial. SO2 addition is rare.

In the case of white juice, if we have not added SO2, you should not add it until after your sugar, and or malolactic fermentation. Addition of SO2 to the oxidized juice may condemn the wine to an oxidized tinge. Without SO2 the brown, oxidized color of the juice will completely drop near the end of sugar fermentation – if no SO2 is added. The less SO2 added minimizes the later requirements of SO2, reduces the total SO2 in the wine and allows for more successful malolactic fermentations. You MUST add SO2 at the end of malolactic fermentation, or if ml is not desired, at the end of sugar fermentation. The amount of SO2 required is dependent on the wine’s pH. There is a good explanation of the how much, why on BV’s web site. As a crude rule of thumb: if the wine has a pH of 3.4, add 40 ppm; if 3.89, add 89 ppm.

Red grapes may have SO2 added when thawing. If cold temperatures or other circumstances cause the must to remain unfermented for more than 3 to 4 days, an SO2 addition will help inhibit the growth of undesirable lactic acid bacteria. Beyond possible inhibition of malolactic fermentation, reasonable additions of SO2 are not detrimental to the red wine.

Sugar Adjustments: The literature is complete about the addition of sugar to grapes – chapitalization. This is extremely rare with Brehm Vineyards’ grapes. The more common occurrence is too much sugar. The BV statistics will alert you that your grapes may have excessive sugar. This is quite common with ultra premium grapes in California. The ripeness of the grapes is not achieved by sugar alone. Today it is back to taste, the maturity of the skin, the absence of any green character. While professional winemakers may remove excessive alcohol by reverse osmosis or spinning cone technology, we are left with dilution. Have no fear, it does work. The ratio of skins to juice may be maintained if the amount of water added is withdrawn as juice before fermentation, a bleeding of the must. This will be a rosé to be enjoyed while bottling the remainder of the wine.MAKE SURE that the defrosted must is extremely well mixed. Test the sugar of your must. If your sugar reading is above 25° brix, you will need to add water and tartaric acid. Must rehydration is practiced by the finest wineries in California.

Adding Acidulated Water for Sugar Dilution (Rehydration): High brix levels can pose problems during primary fermentation and secondary fermentation. Stuck primary fermentations are common because many yeast strains are inhibited at high alcohol levels. These conditions can cause wines with residual sugars of between 1-4%. High alcohol (high sugar) levels do inhibit malolactic (secondary) fermentation. The idea is to dilute the brix down to a more manageable level of about 25° – 24.5° brix. If you simply add water to your must or juice, you will not only dilute your sugar concentration, you will also dilute your total acidity. For this reason, unless the must/juice already has excessive acid, it is important to use water that is acidulated with tartaric acid to perform your dilution. The acidulated water will not only dilute the sugar concentration, but it will keep your total acidity and pH constant. The common practice is to add 7 grams (or 1/4 ounce) of tartaric acid to 1 liter of distilled water to make up your acidulated water dilution solution. (This solution is equivalent to a total acidity of 0.70 g/100 ml or 7 g/L.) This solution of tartaric acid will be used to dilute your high sugar must or juice. (Note: You may need more than 1 liter of acidulated water. See below to determine the amount of acidulated water you will need for your volume of wine.)

The most common mistake made is adding acidulated water based on the volume of your must – crushed grapes, not your final volume of the pressed, finished wine. You must first determine how much finished wine you will produce before you dilute your must or juice. The same principle holds when adding sugar to chapitalize must or juice. For white grape juice, your yield is roughly the same as your starting volume. In general, for red grape musts, the yield is 3.2 gallons or 12 liters of finished, pressed wine per 5 U.S. gallons of fermented must. This will vary based on the skin to juice ratio. Bordeaux varietals average a little over 3 U.S. gallons finished, pressed wine per 5 U.S gallons of fermented must. Rhone varietals average closer to 3-1/2 U.S. gallons or 13 liters of finished, pressed wine per 5 U.S. gallons of fermented must. Note: Since we are measuring total acidity in metric units, it is important to convert your volume units from the US system to the Metric system. (Note: 28 grams = 1 ounce). Another method that may be conducted incrementally and checked by hydrometer/refractometer is done algebraically. In general, the formula is as follows: Let: O = Original Brix of must or Juice L1 = volume (in liters) of finished wine from undiluted must/juice B = Brix you want to dilute must/juice to L2 = volume (in liters) of finished wine from diluted must/juice Y = volume (in liters) of acidulated water to add to must or juice to dilute to desired level, B Equation 1: (L1) x (O) / (B) = (L2) Equation 2: (L2) – (L1) = Y For Example: We have 5 U.S. gallons of red must at a brix of 26.5°. How much acidulated water do you add to lower the brix to 24.5°? O = 26.5, L1 = 12 liters (5 U.S. gallons of red must = 3 gallons finished wine) B = 24.5 L2 = Do equation 1 to determine L2 Y = Do equation 2 to determine Y (12 liters) x (26.5 brix) / (24.5 brix) = L2 L2 = 13 liters (13 liters) – (12 liters) = Y . Y = 1.0 liters Therefore, we must add 1.0 liters of acidulated water to our 5 U.S. gallons of red grape must to dilute the brix down to 24.5 brix.

Add 7 grams of tartaric acid to every liter of water to be added, unless the original acid is above 0.8%.


Yeast & Malolactic Additions: It is debatable where natural yeast and bacteria enter the wine must. The process of freezing definitely reduces the microbe load in the must. It is HIGHLY recommended that the wine be inoculated with commercial wine yeast. There are hundreds of yeast that are available which will turn your must / juice into wine. It is most important that you select one that can withstand the potential alcohol you anticipate; a yeast that will withstand the temperature you are to ferment at and a yeast that will not consume an extraordinary amount of nutrients while it works. As a fall back, when in doubt, I recommend Lallamand’s DV10.

No matter what yeast is used; there are two points, not limited to frozen grapes, to keep in mind.

The temperature of the yeast. The rehydration of the yeast should take place in warm, 104°F water. It should be left undisturbed for approximately 20 minutes. Gradually, slowly, add small amounts of the juice from the grapes to be fermented. Make a yeast starter in a bottle with vented seal -cotton or fermentation lock. Once this starts to foam and work, add more juice. Repeat this process. You are building up the number of yeast to add to your thawing must and you want to gradually raise the temperature of the must and the yeast starter until they are within 10°f of each other. This will minimize the shock on the yeast and get the wine started sooner.

Food for the yeast. Once the yeast has been added to your batch and you see signs of fermentation, add a good yeast food. More than just diammonium phosphate, add a food with vitamins and nutrients. Between 15° and 12° brix add a second dose of yeast food. Many of the grapes sold are grown on rocky soils, harvested late, or in other ways low in nutrients – YAN. It is good insurance, and with some grapes mandatory!

Malolactic fermentation is as natural as your hair turning gray. It may be inhibited by adding lysozyme, cold temperatures, high SO2 content of the wine and ultimately before bottling by membrane (0.45 micron) filtration.

Riesling and some other aromatic wines are desirable without malolactic fermentation. Most other wines benefit and let you sleep better at night with acompleted malolactic fermentation – reduction of malic acid. Once the sugar fermentation is complete, while the wine is still warm and dirty, add a commercial malolactic bacterium.