Yeasts are microscopic fungi which reproduce by budding. Most yeasts grow best in the presence of oxygen, although fermentative types may grow slowly anaerobically. The majority of yeasts prefer acid foods (pH 4 to 4.5) with a reasonable level available moisture. However, many yeasts will grow in high concentrations of sugar and salt. The optimum growth temperature for yeast is around 25°C to 30°C with a maximum of around 47°C. Some yeasts can grow slowly at 0°C and below.
Yeasts are used in the manufacture of foods such as bread, beer and vinegar. However, they cause spoilage of many foods including jam, fruit juice, honey, meats and wines.