Lesson 2.INTRODUCTION TO FOOD MICROBIOLOGY: PART-II: YEAST, MOLD AND VIRUS
Module 1. The trajectory of food microbiology
INTRODUCTION TO FOOD MICROBIOLOGY: PART-II: YEAST, MOLD AND VIRUS
2.1 Food Borne Yeasts
Yeasts have been associated with foods since earliest times, both as beneficial agents and as major causes of spoilage and economic loss. Current losses to the food and dairy industry caused by yeast spoilage are estimated at several billion dollars. As new food ingredients and new food manufacturing technologies are introduced, novel food spoilage yeasts are emerging to present additional problems. To date over 70 biological species of yeasts have been described and thousands of different varieties have been shown to exist in all kinds of natural and artificial habitats.
Yeasts may be viewed as being unicellular fungi in contrast to the molds, which are multi-cellular. Yeasts can be differentiated from bacteria by their larger cell size and their oval, elongate, elliptical, or spherical cell shapes. Typical yeast cells range from 5 to 8 um in diameter, with some being even larger. Older yeast cultures tend to have smaller cells. Most of those of importance in foods divide by budding or fission.
Yeasts can grow in presence of various types of organic acids such as lactic, citric and tartaric acid etc and also over a wide range of acid pH and in up to 18% ethanol. Many grow in the presence of 55-60% sucrose. Many colours are produced by yeasts, ranging from creamy to pink to red. The asco- and arthrospores of some are quite heat resistant.
Members of the Candida genus form shining white colonies and cells contain no carotenoid pigments. Candida tropicalis is the most prevalent in foods in general Some members are involved in the fermentation of cocoa beans, as a component of kefir grains, and many other products, including beers, and fruit juices.
Debaromyces is one of the most prevalent yeast genera in the dairy products. It can grow in 24% NaCl and at an aw as low as 0.65.
Kluyveromyces spp. produces β-galctosidase and are vigorous fermenters of sugars including lactose. K. marxianus is one of the two most prevalent yeasts in dairy products, kefir grain and causes cheese spoilage.
The genus Rhodotorula contains many psychrotrophic species that are found on fresh poultry, shrimp, fish and beef. Some grow on the surface of butter.
Fig. 2.1 Food Borne Yeasts (a) Saccharomyces spp. (b) Candida spp.
Torulaspora multiplies by lateral budding. They are strong fermenters of sugars. Torula delbrueckii is the most prevalent species.
2.2 Food-Borne Molds
Molds are filamentous fungi that grow in the form of tangled mass that spreads rapidly and may cover several inches of area in a very short period. It is also referred to as mycelial growth. Mycelium is composed of branches of filaments referred to as hyphae. The molds of great importance in foods multiply by ascospores or conidia. The ascospores of some of the mold genera are notable for their extreme degrees of heat resistance.
Alternaria spp. form septate mycelia with conidiophores and large brown conidia are produced. They cause brown to black rots of fruits, apples, and figs. Some species produce mycotoxins.
The Aspergillus spp. appear yellow to green to black on a large number of foods. Some species cause spoilage of oils. A. niger produces β-galactosidase, glucoamylase, invertase, lipase and pectinase. A. oryzae produces a -amylase. Two species A. flavus and A. parasiticus produce aflatoxins, and others produce ochratoxin A and sterigmatocystin.
The yeast like fungi, Geotrichum are also referred to as dairy mold.
2.2.4 Mucor and Rhizopus
Mucor species that produce non-septate hyphae are prominent food spoilers. Similarly, Rhizopus spp. also produce non septate hyphae but give rise to stolons and rhizoids. R. stolonifer is by far the most common species in foods and is also referred to as “bread mold”.
2.3 Food Borne Viruses