By carefully massing each ingredient, many quantitative experiments may be performed, such as measuring the volume of gas produced, the percent alcohol produced, or the sugar used. Any fruit can be substituted for the grapes. A variable could be the temperature, to determine the best temperature for yeast enzyme activity. Another modification might be to keep the experiment for a day, then open the bag and allow air to reach the juice. Test pH with another indicator strip.
Yeasts use fruit sugars as a substrate for cellular respiration.
In the absence of oxygen, glycolysis continues as fermentation, resulting in alcohol production. Opening the bag to air borne Acetobacter bacteria to complete the conversion of alcohol to vinegar. Measure 250ml of fresh milk and pour into the perforated bag. Place in a warm place overnight. Observe coagulation of milk.