YEASTS

Yeasts

Mostly unicellular

Importance of yeasts in food

Agents of spoilage production of alcoholic beverages and bread, sources of nutrients, enzymes, vitamins and fats

How to enumerate and isolate?

Surface plating

Cultural characteristics of yeasts

Pigment formation, colony appearance, film or pellicle formation

Morphology of yeasts

Spheroidal to oval, elongated or cylindrical, apiculate, with true mycelia and pseudomycelia

Asexual production

By budding or fission

Sexual reproduction

Ascospore formation

Oxidative yeasts

O2 available, forms cells co2, h2o

Fermentative yeasts

No O2, alcohol product

Use of C or N sources

Hydrolytic enzymes are produced, substrate is transported across membrane

Saccharomyces

Yeast for Food and beverage production

Zygoaccharomyces rouxii

Yeasts that are xerophilic (Aw 0.62)

Zygosaccharomyces baili

Xerophilic yeasts (pH 1.8)

Schizosaccharomyces

Osmophilic yeasts

Osmophiles

Adapted to environments generating high osmotic pressures (high salt or sugar concentrations)

Candida

Beef and poultry spoilage, cacao seeds formation

Kluyveromyces

Yeast most prevalent in dairy products - lactose as CHO source - lactase production

Lactase production

Enzyme that breaks down lactose into glucose and galactose

Debaromyces

Prevalent in dairy products - juice concentrate and yoghurt spoilage

Debaryomyces hansenii

Halophilic yeasts (24% NaCl, Aw 0.65)

Pichia

Causes spoilage or pickles, hat-shaped ascospore

Hanseniaspoea

Teleomorph (sexual reproduction stage or morph) of Kloeckera, apiculate cells

Rhodotorula

Pink, red, or orange yeasts - psychrotrophs - spoilage of refrigerated foods

Yeasts

Facultative anaerobes - anaerobic conditions help them ferment alcohol