ISOLATION, CHARACTERISATION AND MODIFICATION OF SOME YEAST STRAINS FROM CITRUS JUICE FOR ENHANCED FERMENTATIVE ABILITY
ISOLATION, CHARACTERISATION AND MODIFICATION OF SOME YEAST STRAINS FROM CITRUS JUICE FOR ENHANCED FERMENTATIVE ABILITY
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Date
2014-12
Authors
OBASI, BLESSING CHIDI
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Abstract
ABSTRACT
Yeast species present in sweet orange juice were isolated from healthy fermented single strength
orange juice (FSSOJ) and defective single strength orange juice (DSSOJ). The characterisation
of the indigenous yeast flora was carried out using analytical profile index techniques (API AUX
20C kit), physiological characterisation and genetic modification by mutation (nitrous acid and
UV light irradiation treatment). The physiological characterisation involved the testing of several
parameters required for ethanol production and dough leavening ability for bakery products
(bread). These parameters were temperature, ethanol concentration tolerance, flocculation rate
and sugar fermentative activity. The isolation rate of different yeast strains obtained using (API
kit) in this research showed that yeast varies in their morphological and physiological
characteristics. A total of 51 species were identified, representing 4 genera; 32 from FSSOJ, 19
species from DSSOJ. The isolation and prevalence rate of the yeast strains were Candida krusei
(29)(57%),Rhodotorula minuta(10)(20%), Candida zeylanoides (4)(8%), Candida parapsilosis
(3)(6%), Geotrichumcapitatum(2)(4%), Candida lusitaniae, Candida norvegensis and Kodamea
ohmeri (1)(1%) respectively. The ability of some of the isolates:Candida lusitaniae, Candida
parapsilosis and Rhodotorula minuta to metabolise xylose showed that they possessed the genes
responsible for xylose fermentation. Thus, these made them suitable for ethanolic fermentation
of hydrolysates of cellulosic materials. All the wild yeast strains obtained in this research
combined four attributes namely; flocculation ability, tolerance to ethanol 8-18% (v/v), growth
at 37 - 420C, and ability to ferment 10-40% (w/v) glucose except Geotrichum capitatum. Yeast
possessing such combination of attributes appeared to have considerable potential for industrial
application. Based on the physiological attributes and mutagenicity of the wild yeast strains for
ethanol, dough leavening ability and the evaluation of the bread quality made with them,the
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following results were obtained. The mutants of nitrous acid-induced and UV-irradiated(NAM
and UVM:2F36,2F8,2F9,DS6 and DS20) selected for ethanol production showed an appreciable
increase in ethanol yields which ranged from 14.78-18.18% v/v within two to five days, while
the wild yeast strains(2F36,2F8,2F9,DS6 andDS20)ranged from 10.93-17.05% v/v within five to
seven days of fermentation period respectively. The ethanol yields were obtained from
fermentation of 5% sorghum starch after distillation, using titrimetric dichromate method. The
result obtained from the dough leavening capacity showed that the mutants of sweetened dough
had a better leavening ability than that of the unsweetened dough. However, some species of the
wild strains, Kodamea ohmeri,Candida norvegensis, and Rhodotorula minuta obtained in this
research had an outstanding dough leavening capacity both in the sweetened and unsweetened
dough when compared with the commercial baker‟s yeast. The results of the hedonic rating of
the bread samples showed that the panelists preferred the wheat bread made using the mutant
strains of yeast in the sweetened bakery product, while those samples made using the wild strains
were preferred in the unsweetened bread samples. This study enabled the acquisition of data on
the basic physiological and mutagenic pattern of yeasts required for the industrial production of
higher ethanol concentration and dough leavening capacity with regard to their use in bakery
products.
Description
A DISSERTATION SUBMITTED TO THE SCHOOL OF POSTGRADUATE
STUDIES AHMADU BELLO UNIVERSITY, ZARIA
IN PARTIAL FULFILLMENT OF THE REQUIREMENTS FOR THE AWARD
OF A DOCTOR OF PHILOSOPHY DEGREE IN MICROBIOLOGY
DEPARTMENT OF MICROBIOLOGY,
FACULTY OF SCIENCE,
AHMADU BELLO UNIVERSITY, ZARIA
NIGERIA.
Keywords
ISOLATION, CHARACTERISATION, MODIFICATION, YEAST, FERMENTATIVE, CITRUS JUICE