Temperature affects the survival of probiotics. High heat of 113º to 131ºF in shipping, storage, or pasteurization can kill the bacteria. Low temperatures or freeze-drying during processing and storage help keep probiotic organisms stable, because colder temperatures hold less moisture and keep the organisms in an inert state.
Refrigeration inhibits the growth of organisms in fermented foods and probiotics, and exposure to warmth and moisture activates the organisms. Thawing and refreezing can damage cell membranes and be detrimental to probiotic survival, but once frozen, yogurt and other fermented foods can survive frozen for a long shelf life.
Water and temperature interact and affect the survival level of the probiotic bacteria. As temperature increases, the impact of moisture is magnified. The higher the moisture levels and water activity, the lower the survival of probiotics. If the product is dried, the bacteria viability can be maintained for 12 months or more in unrefrigerated storage. Chlorine in water can kill the bacteria, so use filtered, boiled, or spring water in processing fermented foods.
Oxygen can be detrimental to the survival and growth of most probiotic bacteria. Fermentation occurs best without oxygen, so that beneficial organisms can establish a strong colony. If oxygen is present, the fermentation process will be interrupted and ruined. Some fermentation methods use an airlock to create a water barrier; others use modified-atmosphere packaging. When making pickles, covering the vegetables or fruits with brine helps reduce the contact of the pickle with oxygen to prevent spoilage.
Bacteria, Yeasts, and Molds
There are three types of organisms that can start the fermentation process: bacteria, yeasts, and molds.
Bacteria are single-celled microorganisms with a cell wall, but no nucleus. They consume starches and sugars in food. They serve as the basis for both fermentation and infectious diseases. They can live in almost any type of environment—in soil, acidic hot springs, radioactive waste, seawater, deep in Earth’s crust, in the stratosphere, and in the bodies of other organisms. There are both beneficial and harmful bacteria in the human body.
Yeasts are single-celled fungi, which reproduce asexually by budding or dividing. Yeasts are in nature—in the air, soil, and in the intestinal tract of warm-blooded mammals. Like bacteria, yeasts can have beneficial and harmful effects in foods and in the body. Mushrooms and sourdough starter are beneficial fungi; thrush, a yeast infection of the mouth and throat, are harmful fungi.
Molds are multicelled fungi, and they grow in moist environments by multiplying spores. The spores of molds can be on many surfaces and will grow with the appropriate moisture content, nutrients, and temperature. Certain molds can have toxic effects when inhaled or ingested. Beneficial molds are used in food production and make enzymes that break down starches and make them more absorbable.
All bacteria require a certain pH level to survive, which can vary widely according to the bacteria, says Trudy Wassenaar of Argonne National Laboratory. Extreme changes in the pH balance of the local environment for bacteria tend to kill the bacteria. Bacteria cannot abide in extreme pH environments.They have a hard time growing in highly acidic fruits or very alkaline vegetables.
Meats have a neutral pH and are more likely to develop illness-causing bacteria, so they need more restrictions in food preparation requirements. Raising the pH level may kill certain bacteria, but new bacteria may grow in a new pH environment. Whey, lemon juice, and vinegar are easy ways to increase acidity.
In preparing fermented foods, acids are produced during the process that can react with the material that they contact. Reactive materials include: aluminum, brass, copper, iron, zinc, and some plastics. Nonreactive materials include: aluminum, enameled cast iron or steel, food-grade plastic, glass, nylon, and stainless steel. If using plastic, make sure that it is BPA-free, and that it does not break down easily.
Putting salt on vegetables or fruit, draws out the moisture from the food and creates a natural brine that begins the development of the Lactobacillus bacteria while curbing the growth of harmful bacteria. This works as a natural preservative and contributes flavor. You can also submerge the food in a brine made of saltwater that is nonchlorinated. Sometimes you may need a combination of dry-salting and brining if the food does not have enough moisture to make enough brine to cover the food.