- Milk forms some clumps or strings during the heating step. Some milk proteins may have jelled. Take the solids out with a slotted spoon or in difficult cases after cooking pour the milk mixture through a clean colander or cheesecloth before inoculation.
- Yogurt fails to coagulate (set) properly. Milk proteins will coagulate when the pH has dropped to 4.6. This is done by the culture growing and producing acids.
- Adding culture to very hot milk (+115°F) can kill bacteria–Use a thermometer to carefully control temperature.
- Too hot or too cold of an incubation temperature can slow down culture growth–Use a thermometer to carefully control temperature.
- The starter culture was of poor quality–Use a fresh, recently purchased culture from the grocery store each time you make yogurt.
- Yogurt tastes or smells bad.
- Starter culture is contaminated–Obtain new culture for the next batch.
- Yogurt has over-set or incubated too long–Refrigerate yogurt immediately after a firm coagulum has formed.
- Overheating or boiling of the milk causes an off-flavor–Use a thermometer to carefully control temperature.
- Whey collects on the surface of the yogurt. This is called syneresis. Some syneresis is natural. Excessive separation of whey, however, can be caused by incubating yogurt too long or by agitating the yogurt while it is setting.
Food safety, spoilage and shelf life
Yogurt provides two significant barriers to pathogen growth: (a) heat and (b) acidity (low pH). Both are necessary to ensure a safe product. Acidity alone has been questioned by recent outbreaks of food poisoning by E. coli O157:H7 that is acid-tolerant. E. coli O157:H7 is easily destroyed by pasteurization (heating). Therefore, always pasteurize milk or use commercially pasteurized milk to make yogurt.
Discard batches that fail to set properly, especially those due to culture errors. Yogurt generally has a 10-21 day shelf life when made and stored properly in the refrigerator below 40°F. Molds, yeasts and slow growing bacteria can spoil the yogurt during prolonged storage. Ingredients added to yogurt should be clean and of good quality. Introducing microorganisms from yogurt add-ins can reduce shelf life and result in quicker spoilage–“garbage in, garbage out”. Discard any yogurt samples with visible signs of microbial growth or any odors other than the acidity of fresh yogurt.
Always use clean and sanitized equipment and containers to ensure a long shelf life for your yogurt. Clean equipment and containers in hot detergent water, then rinse well. Allow to air dry.
When making this recipe in our test kitchen we used a saucepan instead of a double boiler. Despite constant stirring we still had some minor scorching. We took care not to stir or scrape the scorched area. During the cooking step milk proteins formed strings that we scooped out with a slotted spoon. We inoculated our entire batch of milk with starter and poured the mixture into separate containers. To some containers we added different amounts of honey or sugar stirring to dissolve the sweetener, while others we left plain. Our yogurt reached pH 4.7 in approx. four hours, pH 4.6 in approx. five hours and pH 4.5 in approx. six hours. The yogurt set was firm after six hours and the taste was mild. The yogurt was immediately refrigerated until the next day. On the following day we processed the yogurt into some of the variations listed above under “Yogurt Types”.
source: www.uga.edu, photo from biology.clc.uc.edu