Yeast is an unicellular saprophytic fungus present in the air and on the surface of fruit. Yeast causes the spoilage of fruit, jam, honey, and is used in the production of bread, beer, and wine. It feeds on carbohydrate foods, flourishes at 25C-30C, is killed above 60C, requires moisture for growth, can live with or without oxygen, and requires time to grow. It reproduces asexually through budding, where a cell develops a bud, the nucleus moves towards the bud, the nucleus divides in two, a wall develops and divides the bud from the parent cell, and the bud separates from the cell. Yeast has granular cytoplasm, food vacuoles, vacuole, and a cell wall.
Food preservation involves long term storage, preventing enzyme activity in food, inhibiting the growth of microbes, preventing microbes from re-entering by sealing the food, and maintaining as much nutritive value, color, flavor, and texture as possible. Methods of food preservation include freezing, heat treatments, chemical preservation, and dehydration.
Freezing involves the use of low temperatures, which are unsuitable for enzyme activity and microbial growth. Micro-organisms are inactivated, not killed, and ice prevents microbial multiplication. Quick freezing at -25C results in small ice crystals causing less damage to cell walls, with better retention of nutritive value, texture, color, and flavor. Slow freezing at -6C to -18C results in larger ice crystals and greater loss of nutritive value. Packaging methods for freezing include freezer bags, polythene boxes, and aluminum containers. Blanching involves scalding food in boiling water to inactive enzymes, followed by plunging into water to stop the cooking process.
Heat treatments involve subjecting fruit and vegetables to very high temperatures. For example, in jam making, fruit is boiled with a high proportion of sugar to kill microbes and enzymes. Pectin and acid help the jam to set, while 65% sugar acts as a preservative. Chutney-making involves boiling fruit and vegetables with vinegar and spices.
In jam making, sugar forms a concentrated solution which dehydrates microbial cells, while in chutney making, sugar and vinegar lower the pH to an unacceptable level for microbial growth. Pickling involves using vinegar and salt to form a concentrated solution which dehydrates microbial cells.
Herbs can be dried by tying them and drying in a hot press. Microorganisms and enzymes require moisture and are inactive without it.