2.3.1 Nutrition sources

Cellular organisms require specific internal conditions for optimal growth and function. The state of this internal milieu is strongly influenced by chemical, physical and biological factors in the growth environment. Understanding yeast requirements is important for successfull cultivation of yeast in the laboratory but also for optimalization of industrial fermentation process (Walker, 1998). Elemental composition of yeast cell gives a broad indication as to the nutritional reguirements of the yeast cell. Yeasts acquire essential elements from their growth environment from simple food sources which need to be available at the macronutrient level (approx. lO3 M) in the case of C, H, O, N, P, K, Mg and S or at the micronutrient level (approx. 10-6 M) in the case of trace elements. Yeasts are chemoorganotrophs as they use organic compounds as a source of carbon and energy. Yeasts can use a wide variety of substances as nutrient sources. Decreasing availability of one substrate can, in many instances, be compensated by the utilisation of another (Xiao, 2005).

When a single essential nutrient becomes limiting and eventually absent, the cellular proliferative machinery is efficiently shut down and a survival program is launched. In the absence of any one of the essential nutrients, yeast cells enter a specific, non-proliferative state known as stationary phase, with the ultimate aim of surviving the starvation period. In the presence of a poor carbon source, starvation for nitrogen induces sporulation and in the presence of a good carbon source stimulates pseudohyphal growth (Gasch & Werner — Washburne, 2002). Starvation is a complex, albeit common, stress for microorganisms. The nutrients for which a cell can be starved include carbon and nitrogen, with other elements such as phosphate, sulphur, and metals being less commonly evaluated.

The environment presents for yeasts a source of nutrients and forms space for their growth and metabolism. On the other hand, yeast cells are continuously exposed to a myriad of changes in environmental conditions (referred to as environmental stress). These conditions determine the metabolic activity, growth and survival of yeasts. Basic knowledge of the effect of environmental factors on yeast is important for understanding the ecology and biodiversity of yeasts as well as to control the environmental factors in order to enhance the exploitation of yeasts or to inhibit or stop their harmful and deleterious activity (Rosa & Peter, 2005).

In order to improve the yield of carotenoid pigments and subsequently decrease the cost of this biotechnological process, diverse studies have been performed by optimizing the culture conditions including nutritional and physical factors. Factors such as nature and concentration of carbon and nitrogen sources, minerals, vitamins, pH, aeration, temperature, light and stress have a major influence on cell growth and yield of carotenoids. Because carotenoid biosynthesis is governed by the levels and activities of enzymes employed to the total carbon flux through the carotenoid synthesizing system, the efficient formation of carotenoids can also be achieved by construction of hyperproducing strains with mutagenesis and genetic/metabolic engineering (Frengova & Beshkova, 2009).

The efficiency of the carbon source conversion into biomass and metabolites, and the optimization of the growth medium with respect to its availability and price has been subject of intensive studies. Numerous sources including pentoses and hexoses, various disaccharides, glycerol, ethanol, methanol, oils, n-alkanes, or wide variety of wastes derived from agricultural have been considered as potential carbon sources for biotechnological production of carotenoids. Carotenoid pigment accumulation in most yeasts starts in the late logarithmic phase and continues in the stationary phase (typically for secondary metabolites), and the presence of a suitable carbon source is important for carotenoid biosynthesis during the nongrowth phase. Yeasts can synthesize carotenoids when cultivated in synthetic medium, containing various simple carbon sources, such as glucose, xylose, cellobiose, sucrose, glycerol and sorbitol. Studies on carotenogenesis have led to a growing interest in using natural substrates and waste products from agriculture and food industry: grape juice, grape must, peat extract and peat hydrolysate, date juice, hydrolyzed mustard waste isolates, hemicellulosic hydrolysates (Parajo et al., 1998), hydrolyzed mung bean waste flour, sugar cane juice, sugar cane and sugar-beet molasses, corn syrup, corn hydrolysate, milk whey. In recent years, raw materials and by-products of agro-industrial origin have been proposed as low-cost alternative carbohydrate sources for microbial metabolite production, with the view of also minimizing environmental and energetic problems related to their disposal (Frengova & Beshkova, 2009).

The chemical composition and concentration of nitrogen source in medium might also be means of physiological control and regulation of pigment metabolism in microorganisms. Several inorganic and organic nitorgen sources as well as flour extracts and protein hydrolysates have been studied for improvement of carotenoid production. However, it seems that variation in carotene content in yeasts with regard to N-source used in a medium and the rate of pigment production is influenced by the products of catabolism of the nitrogen source rather than being the results of direct stimulation by the nitrogen compound itself (Certik et al., 2009, Somashekar & Joseph, 2000).