Cultivation of Candida utilis on Cassava Peel Hydrolysates for Single

Abstract: The growth of Candida utilis NRRL Y-1084 in acid and enzymatic hydrolysates of cassava peel and on glucose in a mineral salts medium was investigated in aerobic submerged cultivation. Kinetic and stoichiometric parameters for growth were determined. The cardinal temperatures of this yeast strain were 14 °C， 33 °C and 41 °C. C. utilis exhibited no absolute requirement for growth factors， although its maximum specific growth rate (μmax) was higher in the mineral salts medium with yeast extract than without， but its biomass yield coefficient (Yx/s) did not differ much in these two media. In the enzymatic hydrolysate， its Yx/s value on sugar was 0.44 with a μmax of 0.35 h-1， whereas the corresponding values were 0.52 and 0.48 h-1 in the acid hydrolysate and 0.50 and 0.37 h-1 in the mineral salts medium without yeast extract. The crude protein content of biomass grown in the glucose medium and the acid and enzymatic hydrolysates were 47.5%， 49.1% and 56.7%， respectively. The amino acid profile of the yeast biomass compared favourably with the FAO standard. Cassava peel hydrolysate has potential as a cheap carbohydrate feedstock for the production of yeast single cell protein by using C. utilis.

Key words: Candida utilis， yeast， cassava peel hydrolysate， amino acid profile， single-cell protein (SCP).

1. Introduction

Single cell protein (SCP) refers to the microbial biomass， or proteins extracted from there， obtained from processes in which bacteria， yeasts， filamentous fungi or algae are cultivated in large quantities as a protein supplement in animal feed or in human nutrition [1]. The protein can be consumed directly as part of the cells， particularly in animal feed formulations， or it can be extracted and processed into fibres or meat-like products for nutritious human food[2]. SCP， unlike the production of conventional plant and animal protein resources， does not require agricultural land and is not limited by sunlight because the micro-organisms can be grown in large bioreactor vessels or other suitable large scale cultivation systems. Other advantages of SCP production include the high growth rate of microbial cells， their ease of genetic manipulation and the high protein content of micro-organisms. Furthermore， SCP can be produced from a wide range of substrates， including agricultural and industrial wastes that constitute serious environmental problems.

Micro-organisms have the ability to upgrade low protein plant material to high protein feed [3]. Large scale utilization of methanol， starch and molasses as carbon feedstocks has proved economically viable for the production of animal feed and human food [4-6]. A continuous aerobic process has been successfully used for the production of SCP from cheese whey using the yeast Kluyveromyces fragilis [7-11]. In another instance， the suitability of deproteinized sweet and sour cheese whey concentrates as substrates for the production of SCP with Kluyveromyces marxianus was investigated [12]. Analysis of the amino acid composition of the SCP showed a distinct increase in eight out of ten essential amino acids compared to sweet and sour protein and exceeded the World Health Organization (WHO) guideline for valine， leucine， isoleucine， threonine， phenylalanine and tyrosine [12].

Yeasts are a rich source of not only proteins but also B-complex vitamins. They have been used as a supplement in animal feed to compensate the amino acid and vitamin deficiencies of cereals and are recommended as a substitute for soybean oil in diets for fowls [13]. It has been shown that the common carp can obtain a high portion of its dietary protein from the yeasts Candida tropicalis， C. utilis and C. lipolytica， with better results than with soybean or meat and bone meals [14]. In addition， yeast biomass is considered a cheap dietary supplement as it is easily produced on industrial scale from a number of by-products such as citrus pulp， molasses， paper industry wastes and fruit waste， as well as from hydrocarbons. Despite their vast potential， however， the use of yeast biomass as a protein source is not extensive and has been limited largely to the feeding of molluscs and as live feed in aquaculture [15]. Apparently sulphur amino acid deficiency restricts the use of yeasts， though there may also be other factors limiting their use， such as their high carbohydrate and nucleotide content [16].

Interest in the recovery of waste or by-products have been increasing for both economic and ecological reasons as well as for nutritional reasons [17]. In the last two decades in Nigeria there have been concerted efforts in finding ways of complete utilization of agro-industrial by-products， which sometimes constitute environmental hazards [18]. Cassava(Manihot esculenta Crantz syn. Manihot utilissima Pohl)， a staple food of the majority of people in tropical Africa， Central and South America [19， 20]， is subjected to various fermentations in the different countries to produce similar or different products [21]. In Brazil， Costa Rica and Bolivia， farina is often the end product， whereas in tropical Africa gari， fufu， lafun， chiwangue and myondo are produced from cassava [20， 22]. The various production processes are usually accompanied by some waste products that act as environmental pollutants [23]. Cassava peels， leaves and starch residues constitute 25% of the cassava plant[18]. These are usually discarded as wastes after harvesting and processing， with limited utilization due to their low protein， high crude fibre and cyanide contents [24]. The peel amounts to about 10%-20% of the root mass and is available all year round in Nigeria with an annual yield of approximately 4 million metric tonnes from the processing of cassava roots [25]. Little attention has been paid to the handling of the large quantity of cassava wastes that are generated. SCP production is a potential route for converting such wastes to a useful and valuable product.

The objectives of this study were to determine the growth kinetics of C. utilis in acid and enzymatic hydrolysates of cassava peel and to evaluate the chemical composition and amino acid profile of the resultant biomass with a view to its utilization as a food or feed protein supplement.

Acknowledgments

This work was supported by the John D. and Catherine T. MacArthur Foundation Grant awarded to O.O. Ezekiel and was undertaken in the Department of Microbial， Biochemical and Food Biotechnology， University of the Free State， Bloemfontein， South Africa.

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