Introduction
Yogurt is one of the most widely distributed dairy products. Yogurt in
different forms with diverse local names is made throughout the world.
(Tarakci and Erdogan, 2003). The use of yogurt dates back many centuries,
although there is no accurate record of the date when it was first made.
According to legend, yogurt was first made by the ancient Turkish people
in Asia, (Kurt, 1981). The uniqueness of yogurt is attributable to the
symbiotic fermentation involved in its manufacturing.The composition of
yogurt is dependent on the type and source of milk and a range of seasonal
factors. For example: whole milk or skimmed milk, season, lactation period
and the feeding mode. It is also significantly influenced by manufacturing
conditions (such as temperature and durationand equipment utilized) and
on the presence of other ingredients such as powdered milk or condensed
milk, (Blance, 1986). Yogurt is derived from Turkish word Jugurt
describing any fermented food with acidic taste. Its manufacture involves
the use of specific symbiotic/mixed culture of Lactobacillus bulgaricus
and Streptococcus thermophilus (Kon, 1959). Yogurt is a
coagulated milk product that results from the fermentation
of lactic acid in milk by Lactobacillus bulgaricus and
Streptococcus thermophilus (Bourlioux and Pochart, 1988).
It has a smooth textureand a mildly sour and pleasant flavor. It
is obtained from pasteurized or boiled milk soured by naturally occurring,
or lactic acid fermenting bacteria i.e. Lactobacillus bulgaricus and
Streptococcus thermophilus (FAO, 1977). The commercial yogurt of today
is usually made by fermenting milk with mixed culture of Lactobacillus
bulgaricus and Streptococcus thermophilus, each of these organisms
acidify milk and produce specific yogurt flavor and aroma (Almena et
al., 2005). The addition of probiotic bacteria is made not only because
of certain claimed health-promoting effects in the intestinal tract. Other
beneficial health effects suggested include enhancement of the immune
system, reduction of lactose intolerance, control of diarrhea and reduction
of LDL cholesterol (so called “bad cholesterol”) (Scheinbach,
1999). Product quality and consumer satisfaction are important for increasing
the sales of various types of yogurt products (Debbie et al., 1991).
Quality assessment encompasses specifications, sampling, testing procedures
and recording or reporting. Specifications are typically set by the manufacturer.
Ultimately, the consumer is the final judge of quality. Yogurt quality
is difficult to standardize because of many forms, varieties, manufacturing
methods, ingredients and consumer preferences that exist (Kroger, 1976).
This situation makes yogurt an interesting, challenging area to work in.
A practical approach towards the quality comparison of probiotic and natural
yogurt is to evaluate the different samples of probiotic and natural yogurt
available in UK supermarkets. As a first step, the physico-chemical and
microbiological properties of probiotic and natural yogurt were evaluated.
Then sensory analysis was conducted to check the overall acceptability
of different samples of probiotic and natural yogurt.
Materials and Methods
The research work of the project was conducted in the microbiology laboratory
in the Orion Building of the University of Teesside, Middlesborough.
Collection of samples: Fresh samples of probiotic and natural
yogurt were bought from various supermarkets in Middlesborough.
Product analysis
Physico-chemical Analysis
Total Titratable Acidity: Acidity was measured by potentiometric method
according to the BS 7142-5, 1997.
pH: The pH of probiotic and natural yogurt was determined by using
a Digital pH meter, Hana pH meter No. 211. The pH meter was standardized
using pH 4.0 and pH 7.0 buffer solutions. The yogurt samples were stirred
with a small of amount-distilled water before pH measurement. pH was measured
over several days.
Protein: Protein and nitrogen contents of yogurt samples were
determined by Kjeldahl method according to the BS 1741-5.2 (1990).
Fat: Fat was determined (on wet weight basis) by Soxhlet’s
method by using this formula, % Fat = g of fat in dry sample/g fat of
in wet sample* 100 (Suzanne, 2003a).
Total solid: Total solids were determined by following formula,
% Total solids (wt/wt) = wt. Of dry sample/ wt. of wet sample* 100 (Suzanne,
2003b)
Microbiological analysis: The microbiological analysis of probiotic
and natural yogurt was carried out for total viable count as described
by David and Fankhauser (2005).
Organoleptic evaluation: All the samples of probiotic and natural
yogurt were evaluated by ranking method for sensory characteristics and
overall acceptability by a panel of judges from the staff of University
of Teesside, Middlesborough using the method described by BS 5929-6 (1989).
Statistical analysis: The data was statistically analyzed according
to John (1995a,b). Students’ T-test was applied to compare the samples
of probiotic and natural yogurt as described by Daniel (2002). Significant
differences were determined at α= 0.05%.
Results and Discussion
Physico-chemical analysis
Total titratible acidity: The total tirtatable acidity of probiotic
yogurt and natural yogurt is shown in Table 1. The average
TTA of probiotic yogurt was 1.41 percent with a standard deviation of
0.03. The average TTA acidity of natural yogurt was 1.44 with a standard
deviation of 0.01. These results are in line with findings Tarakci and
Erdogan (2003) in which acidity increased over the storage period. Guler
and Mutlu (2005) also observed an increase in Total Titratable Acidity
during the storage period.
|
| Fig. 1: |
Comparison of TTA of probiotic and natural yogurt |
|
| Fig. 2: |
Comparison of pH of probiotic and natural yogurt |
Kroger (1976) reported that in probiotic yogurt the Lactobacilli
will grow continuously in between pH 4.0 and 4.4, since they are also
capable of producing acid, so ultimately the acidity of probiotic yogurt
tends to increase. But in case of natural yogurt there is no bio-live
culture, so a decrease in TA is expected.
pH: The pH of probiotic yogurt and natural yogurt is shown in
Table 2. In both cases, pH increased during storage.
These results are in line with findings of Salji et al. (1985)
and Sutherland and Varnam (1994) who reported yogurt pH as 4.50. Probiotic
have higher pH then natural yogurt (Fig. 2), possibly
due to the low total viable count of probiotic bacteria over the storage
period. Nighswonger et al. (1996) also reported decling counts
of L. acidophilus in yogurt over the storage period.
Protein: The protein content of probiotic yogurt and natural yogurt
is shown in Table 1. The average protein content of
probiotic yogurt was 5.4 with a standard deviation of 0.003. While the
average protein content of natural yogurt was 5.3 with a standard deviation
of 0.005. These results are in line with findings of Janhoj et al.
(2006) who reported the protein contents of low-fat stirred yogurt ranged
from 3.4 to 6.0%.
| Table 1: |
Physico-chemical analysis (Mean±SD) pH of probiotic
and natural yogurt collected during summer season (2006) |
|
| Table 2: |
Microbiological analysis (Mean±SD) pH of probiotic
and natural yogurt collected during summer season (2006) |
|
| Table 3: |
Organoleptic Evaluation (Mean±SD) of probiotic
and natural yogurt collected during summer season (2006) |
|
Fat: The fat content of probiotic yogurt and natural yogurt is
shown in Table 1. The average fat content of probiotic
yogurt was 0.76 with a standard deviation of 0.005. While the average
fat content of natural yogurt was 0.29 with a standard deviation of 0.001.
These results are in line with findings of Janhoj et al. (2006)
who reported that fat contents ranged from 0.3 to 3.5% for a low-fat stirred
yogurt, but these results are totally different from the Mutlu et al.
(2005) who reported the fat content of bio-yogurt made from goat’s
milk i.e. 3.1%. As the fat contents of natural yogurt are lower as compared
to probiotic yogurt so ultimately that will affect the quality of the
yogurt.
Total solids: The total solids content of probiotic yogurt and
natural yogurt is shown in Table 1. The average total
solids content of probiotic yogurt was 17.75 with a standard deviation
of 0.006. While the average total solids content of natural yogurt was
of 19.2 with a standard deviation of 0.035. As regards the probiotic yogurt
these results are in line with findings of Muhammad et al. (2005)
who reported the highest range of total solids in yogurt was17.1%, but
in case of natural yogurt these results are significantly different.
Solid- not-fat (SNF): The SNF content of probiotic yogurt and
natural yogurt is shown in Table 1. The average SNF
content of probiotic yogurt was 1.85 with a standard deviation of 0.05.
While the average total solids content of natural yogurt was 1.78 with
standard deviation of 0.06.
Microbiological analysis
Total viable count: The total viable count of probiotic and natural
yogurt is shown in Table 2. The average total viable
count of probiotic yogurt was 4.04 x 108 with a standard deviation
of 0.93. The average total viable count of natural ranged was 4.6 and
standard deviation 1.22. These results are in line with findings of Lopez
et al. (1997) who reported log aerobic mesophilic count from <
1.0-5.38 and from 4.87-6.67 per ml in natural yogurt. There are found
no significant variation in total viable count of probiotic and natural
yogurt because defined starter culture is used (Kon, 1959) under proper
conditions of fermentation for manufacture of yogurt.
Organoleptic evaluation
Overall acceptability: The overall acceptability scores of probiotic
and natural yogurts are shown in Table 3. The average
values of overall acceptability of probiotic and natural yogurt were 4.95
and 4.20 with a standard deviation of 1.52 and 1.66 respectively. Colour,
taste and aroma are the important factors, which determine the acceptance
or rejection of a food article. Among probiotic and natural yogurt samples,
the average overall acceptability score was highest for the probiotic
yogurt as compared to Natural yogurt. Comparing the overall acceptability
of probiotic and natural yogurts by using T-test assuming unequal variances
showed that there was a significant difference among overall acceptability
of different samples of probiotic and natural yogurt.
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