Laboratory Session 1
Pre-Fermentation, getting the numbers right
September 11, 2018
1. Understanding potential variability in results when drawing and preparing a sample for testing:
a) Timing of sampling and duration of grape skin exposure before sampling.
Should be done after all maceration is complete as pH and (TA Titratable Acidity) change during this process. Duration of maceration depends on the variety, condition of grapes, ambient temperature and the onset of wild fermentation, which is not always a problem.
b) Clarity of the juice sample
This is really important. If solids are present any SG measurement will be suspect as will pH and TA. When juice or must is fully macerated one should collect a filtered sample to get a reliable SG. This will serve for the TA and pH measurements.
c) The effect of CO2 on measurements.
If there is any wild yeast fermentation in the sample that you are analyzing the C02 can significantly affect the accuracy of your measurements. Your can correct for this by heating the sample in a microwave and then take your measurements again after the sample has cooled to room temperature
d) Effect of enzymes
These will liberate tartrates and malates from the skins which will serve to raise the TA and pH. However if the sample is obtained after all maceration is completed then these TA and pH measures will be the correct values.
e) Frozen grapes
Sampling after thawing frozen grapes or juice and getting reliable parameters from frozen must/juice is problematic. Ideally rely on the producers figures if supplied. If not, a lot of time and heating and stirring is required to get the mixture homogeneous. As a check one could compare SG’s from samples taken at different heights in the must. Good luck.
2. Equipment and accuracy
f) Minimal equipment – indicator & titration solutions (Underhill) Phenolpthalein is the indicator of choice for TA measurement, changing colour to pink at the end-point which is around pH 8.2 –> 8.5. It is preferable to use a pH pen and titrate a 15 ml sample to pH 8.2 (not noticeably different from pH 8.5 so don’t worry about this). The titratable acidity measurement doesn’t have to be extremely accurate whether you titrate to pH 8.2 or 8.5. It doesn’t make a hill of beans.
g) A Hanna pH pen is adequate for wine making and better choice than indicator solutions but you must have decent equipment. pH meter maintenance is very important. These instruments are very delicate and need to be treated with respect. They need to be washed properly and not damaged against surfaces during use
h) Keeping a pH meter in good condition. Storage is important. NEVER LEAVE IT SOAKING IN WATER rather store the glass bulb wet with pH 7 buffer (or a dilute potassium chloride solution). If using a commercial storage solution match it to the manufacturer of the meter as there is more than one type of reference cell material in use today. If the storage solution has been allowed to dry out it should be soaked in buffer for at least 24 hours.
i) Keep the glass bulb clean at all times. The glass bulbs for the pH meter can get dirty; one of the biggest reasons is protein but there are other materials as well. The response will become slower. You can buy commercial cleaning solutions from Hannah (editors note: including one specifically for wine making) but that can be a bit expensive. Alternatively you could try using some laundry cleaners with enzymes to clean the electrode or meat tenderizer. Meat tenderizer is primarily papaya proteinase (papain) derived from the papaya plant. Some pH electrodes that have become unresponsive can be salvaged by soaking in a hot solution of KCl or HCl.
3. Between pH and TA what is more important to keep the fermentation healthy? The pH is more important. In wine making there is a magic figure of pH 3.55. Above that the pH of the must will rise during fermentation, and below it will drop. Aim for a starting pH 3.55 or less. There can be some confusion about pH and TA. PH depends on the ratio of unionized acid to ionized acid. TA is the sum of ionized and unionized acid. The two are related but behave independently
4. Tartaric acid additions After measuring the TA some Saanich Sommelier wine makers at times reduce the quantity of tartaric acid that they add to the must to an amount at is less than what is called for by the TA measurement. After fermentation additional tartaric acid is added if needed.
5. Perceived acidity in wine
It is a combination of pH and TA but there are other factors. One tool that can be used in creating a wine with the correct perceived level of acidity is the Acidity Index. It is the total acid (g/L) – pH. of a wine There is a the table that describes the acidity index for various wines. An attempt will be made to attach it to this article when it becomes available. One of the best measures of perceived acidity is your palate when completing the final blending prior to bottling. When tasting the pH of a wine has little do with the sensation of acidity. Rather it is the acids originating in the grapes such as tartaric acid, malic acid, citric acid and succinic acids have a much greater influence. Each by themselves have a different flavor but no odor. When added to wine to wine during final blending they have the potential to change the aroma profile of the wine as well.
6. High Brix musts and juice when starting fermentation
If attempting to mitigate the potential for excessive alcohol content in wine it has been a practice on occasion to add acidulated water to musts from high Brix grapes. Success has been reported using water additions where the final quality of the wine has not been affected. When adding tap water be aware that chlorine is present in the water. SO2 present in the must or added to the water will neutralize the effects of the chlorine.