ISOLATION AND CHARACTERIZATION OF YEASTS OF SOME FRUITS AND FRUIT PRODUCTS OF SRI LANKA

Abstract: Thirty six strains of yeasts isolated from some fruits and fruit products were identified using their biochemical and morphological characteristics. These yeasts belonged to six genera viz: Candida (Sixteen strains), Kloeckera (nine strains), Hanseniaspora (three strains), Pichia (four strains), Saccharomyces (three strains) and Tomlopsis (one strain). The most predominant species was Candida krusei (Ten strains).


Introduction
Microbial spoilage of food is commonly the result of the combined activities of yeasts, moulds and bacteria.However, depending upon the environment, one of these groups of microbes may prevail over the others.Yeasts ordinarily do not compete well in mixed populations, and therefore, cause spoilage only under conditions which are favourable for their growth, but unfavourable for growth of most bacteria.'' The most important factors which determine the ability of yeasts to compete with moulds and bacteria are the number and types of contaminating yeasts, available nutrients, pH, redox potential and water a ~t i v i t y .~ Spoilage of fresh fruits and vegetables by yeasts usually result from their fermentative activity rather than degradation of plant tissue with degradative enzymes.However, some yeasts are reported to be capable of producing such enzymes and cause ~poilage.~Sri Lanka produces a variety of tropical fruits, and also, possesses a rapidly develeping fruit processing industry.However, a considerable amount of the produce is lost due to post harvest deterioration where microbial attack is suspected to be one of the predominant causes.In the present study an attempt was made to isolate and characterize the yeast flora of some local fruits and fruit products which may play a role in their spoilage.

Experimental
Random samples (about 10 each) of fresh ripe fruits (except dates and raisins, which were dried), canned fruits, juices, syrups and cordials were obtained from retail sellers, canneries and supermarkets of Colombo.These samples included fruits of papaw, grape, date, tomato, passion fruit, lime, pineapple, nelli, raisin, plum, banana, amberella, grapefruit, tangerine, lawulu and mango, and papaw juice, grape-juice, pineapple-juice, nelli-juice, mango-juice and passion fruit cordial (Botanical names of fruits are given in Table 1).

Carica papaya Vitis vinifera Phoenix dactylifera Lycopersicum esculentam Passiflora edulis Citrus aurantifolia Ananas sativus Philanthus ernblica Vitis uinifera Prunus domestica Musa sapienturn Spondias rnangifera Citrus decumana Citrus n o bilis Chrysophylla sp. Mangifera indica
Yeasts were isolated by plating on a medium which had the following composition in g litre-' of distilled water : glucose 20.0, yeast extract 3.0, peptone 5.0 and agar 20.0.Plating was done by streaking a loopful from enrichment cultures prepared by inoculating small amounts (about 1-2 g) of the sample into tubes containing broth of the same medium less agar.Cultures were incubated at room temperature for 2-4 days depending on growth.Methods used to study the biochemical and morphological characteristics were same.4 The following criteria were used to characterize and identify the isolated yeasts: fermentation pattern, assimilation pattern, morphology and sporulation.Some of these criteria were represented by codes.'

Results and Discussion
In this study thirty six strains of yeasts were isolated and identified.These thirty six strains of yeasts belonged to the six genera Candida, Kloeckera, Hanseniaspora, Pichia, Saccharomyces and Torulopsis.The predominant species were Candida krusei (ten strains) and Kloeckera apiculata (seven strains).The morphological and biochemical characteristics of these strains and their sources of isolation together with their completed identification codes13 are given in Table 2.
Candida krusei the most predominantly encountered in this study was isolated on eight occasions from fresh fruits and in two occasions from dried fruits ( a a t e and Raisin).Mrak et a17 and Miller and Mrak12 has listed A Candida krusei as one of the most numerous yeasts in spoiled figs.A strain of Candida tamarandei which is synonymous with Candida krusei has been isolited from fermenting date.14Another species, Candida guilliermondii two strains of which were isolated from papaw juice and grape juice in this study has been earlier isolated from fig wasps.14Candida mesenterica (three strains) and Candida peliculosa (one strain) which also were encountered in this study have not been isolated from fruits earlier.
Kloeckera apiculata, of which seven strains were isolated from fruits in this study, has been earlier reported8 t o cause softening in strawberries.This also has .-been r e ~o r t e d as the only yeast isolated from blackcurrents picked at the end of the harvesting periodm3-~l s o , ~l o e c k e r a apiculata has been reported much earlier under the synonym Saccharomyces apiculatus as an organism widely distributed in fruits and orchards.1° Next to Saccharomyces cerevisiae this species has been reported t o be the most prominent in wine grape^.^Another species Kloeckera afizcana, isolated from Lawulu (two strains) has earlier been isolated from figs.9 -. ..-Saccharomyces cereu;siae which is generally a common contaminant in many food products was encountered only once in this study (one strain from Tomato sauce).Two other species, Saccharomyces exiguus and Saccharomyces italicus a strain each of which was isolated from nelli juice and mango juice respectively has earlier been reported as a contaminant in grape must.'Among the other yeasts encountered in this study which have bein reported earlier are Hanseniaspora valbyeanGs6 (wine g a p e ) and Pichia fermantansl (orange juice).None of the other yeasts (Pichia polymorpha, Pichia kuriavzevii and Torulopsis inconspicua) have been reported as contaminants of fruits earlier.

Table 1 -
Botanical names of fruits Papaw

Table 2 -
Identification of Yeasts from Fruits &