Cheese is the generic name for a group of fermented milk based food products, produced in a great range of flavours and forms throughout the world (Fox, 1993). Processed cheese may normally consider as being a stable products with a reasonable shelf life (Kristensen and Skibsted, 1999; Schar and Bosset, 2002). The cooking process helps to destroy spoilage microorganisms and improve the shelf life of the processed cheese (Siew et al., 2004). In cooked hard cheese Staphylococcus aureus is usually inactivated during the first day of manufacture due to the high cooking temperature (Zangerl and Ginzinger, 2001). Yeast and molds counts (88.9%) and the coliform counts (96.6%) were found in the 90 samples studied, moreover 4 lots (22.2%) were rejected according to International Commission on Microbiological Specifications for Food (Mercado and Rivas, 1986).
Caric and Kalab (1993) reported that processed cheese spreads are made by selecting
suitable cheese according to age, flavour, body and texture. They also added
that proper selection of natural cheese is the most importance for the successful
production of processed cheese. Moreover it was concluded that locally produced
soft cheese in Iraq could be used instead of imported semi-hard cheese to make
processed cheese of acceptable quality (Hanna and Nader, 1996).
Packing protects food during storage, transportation and distribution against deterioration which may be physical or biological (Anandaswamy et al., 1980). Processed cheese is usually packed and wrapped in lacquered foil, tubes cups, cans cardboard or plastic, cartons and occasionally in glass jars (Stanelyscharow et al., 1970). Pre-sterilized cans were used to pack processed cheese spread made from cheddar cheese prepared from buffalo milk (Tewari et al., 1996). Metwally et al. (1996) studied the effect of packaging materials on the keeping quality of processed cheese by using glass jars, locally made polymeric laminated materials and imported poly amide sheets. They recommended the use of glass for packaging processed cheese due to it inertness and its availability in Egypt. They also reported that storage temperature had a greater effect on quality compared to packaging materials.
This study was a trial to produce processed cheese from traditional Sudanese white cheese with different fat levels at different ripening periods and to estimate the hygienic quality of the resulted cheeses. Also the shelf life and package materials for the processed cheese were evaluated.
Materials and Methods
Sources of Milk Rennet and Salts
The present study was done at the Department of Dairy Production, Faculty
of Animal Production, University of Khartoum during the period of November 2002
to February 2003. Fresh cows milk was brought from Khartoum University
farm, Rennet tablets were obtained from Chr-Hansens Lab (Denmark), the
salt was purchased from the local market and di-sodium phosphate was a product
of Sigma Chemical Company.
Manufacture of Sudanese White Cheese
The Sudanese white cheese was made after adjustment of the milk into two
different fat levels (4.4 and 2.2%) using the cream separator. The technology
of Sudanese white cheese manufacturing was done as described previously by Osman
(1987). Then both cheeses that made from low and high fat content of milk were
stored at room temperature for ripening to both 15 and 30 days.
Manufacture of The Processed Cheese
The cheese was cut into small pieces by using stainless steel. Then 3% of
di-sodium phosphate and 50% of distilled water were added. It was then mixed
using a mixture at water bath steam (80°C for 5 min). Processed cheese were
packed into two types of packaging (plastic pack and glass pack) and stored
The samples were examined for total bacteria count, coliform count, yeast
and molds count and psychrophilic counts. All media were obtained in dehydrated
forms and they were prepared according to the manufactures instruction.
Plate count agar was used for the total bacteria count and psychrophilic count.
MacConkey agar is a selective differential media used for coliform counts and
Malt extract agar was used for enumeration of yeast and molds. Sterilization,
preparation of the samples, serial dilution, incubation and counting were done
according to Harrigan and MacCance (1976).
The data of the present study were analyzed using the complete randomized
design. ANOVA test and Duncan Multiple Range test were used to determine the
differences between means using SAS (1989) programs.
Sudanese white cheese and the processed cheese showed mean counts for psychrophilic
of 4.9x107 and 2.1x103 cfu g-1, respectively
(Table 1 and 2). The different fat level
revealed high significant differences (p<0.001) for psychrophilic count for
the Sudanese white cheese (4.5x107 and 5.2x107 cfu g-1
for 2.2 and 4.4% fat, respectively). However non significant differences (p>0.05)
were obtained for the processed cheese (2.1x103 and 2.1x103
cfu g-1 for 2.2 and 4.4% fat, respectively). Significant increases
(p<0.05) in psychrophilic counts were also found during the storage of the
processed cheeses and due to variation of packaging materials as shown in Table
3. The ripening time and its interaction with the different fat % of milk,
from which the Sudanese white cheese was made, showed high significant differences
(p<0.001) for psychrophilic counts in both cheeses (Table 4).
Moreover the different storage periods, the types of packaging materials and
their interactions showed highly significant differences (p<0.001) with psychrophilic
count for processed cheeses as shown in Table 4.
|| Effect of different fat level of milk and ripening time on
hygienic quality of Sudanese white cheese
|In this and the following tables: SL = significant level,
NS = non significant (p>0.05) * = (p<0.05), ** = (p<0.01),
*** = (p<0.001), The same superscript letter in rows indicated significant
|| Effect of different fat level and ripening time on hygienic
quality of the processed cheese
|The same superscript letter in columns indicated significant
|| Effect of storage periods and packaging material on hygienic
quality of processed cheese
|The same superscript letter in columns indicated significant
Total Bacterial Counts
The total bacterial count of Sudanese white cheese and processed cheese
showed mean counts of 2.9x108 and 2.9x107 cfu g-1,
respectively (Table 1 and 2). The total
bacterial count of Sudanese white revealed 3.0x108 and 2.9x108
cfu g-1 for 2.2 and 4.4% fat, respectively, which were not significantly
different (p>0.05) as shown in Table 1. However highly
significant increase (p<0.001) was obtained when comparing means for the
different ripening time (Table 2). Similarly the total bacterial
count of the processed cheese showed highly significant differences (p<0.001)
with different fat % of milk (4.2x107 and 1.5x107 cfu
g-1 for 2.2 and 4.4% fat, respectively). Moreover the interaction
of the ripening time and the storage periods revealed highly significant differences
(Table 3 and 4). However the total bacterial
counts of the processed cheese revealed non significant differences (p>0.05)
when kept at different types of packaging (Table 3).
The mean coliform count of Sudanese white cheese and the processed cheese
showed mean coliform counts of 3.1x 106 and 6.5x 106 cfu
g-1, respectively (Table 1 and 2.)
Comparison of the different ripening time of Sudanese white cheese showed highly
significance differences (p<0.001) as shown in Table 1.
Coliform counts of the processed cheese revealed 2.6x 106 and 3.6x
106 cfu g-1 for 2.2 and 4.4% fat, respectively. Different
fat% of the milk, ripening time of white cheese from which processed cheese
was made, the different storage periods, the types of packaging and their interactions
showed highly significant differences (p<0.001) for coliform counts (Table
Yeast and Molds Counts
The mean counts o f yeast and molds of the Sudanese white cheese showed
4x105 cfu g-1, while the mean value for yeast and molds
counts of the processed cheese was 2.4x105 cfu g-1 (Table
1 and 2). The different ripening time of Sudanese white
cheese showed highly significant increase (p<0.001) with yeast and molds
counts (Table 3 and 4). Yeast and molds
counts of the processed cheese were found to be 4.1x105 and 3.8x105
cfu g-1 for 2.2 and 4.4% fat, respectively. Different ripening time
of the Sudanese white cheese from which processed cheese was made, the different
storage periods and the types of packaging of the processed cheese showed highly
significant differences (p<0.001) for yeast and molds counts (Table
2- 4). Similarly the interaction of the different combinations
of fat % of the milk, ripening time of Sudanese white cheese from which the
processed cheese was made, the storage periods and types of packaging of the
processed cheese showed highly significant differences (p<0.001) with yeast
and molds counts as shown in Table 4.
|| Effect of some factors on the compositional and hygienic
quality of Sudanese white cheese (variation of mean squares)
Sudanese white cheese falls into the family of soft and semi-soft pickled cheese
of east European countries, the East Mediterranean region and North Africa (Abdalla,
1992). The microbiological examination of the Sudanese white cheese and the
processed cheese made out of it, revealed improvement in the hygienic quality
for the processed cheese as shown by the decrease bacterial counts (Table
1 and 4). This supported Schär and Bosset (2002)
who reported that the processed cheese is often expected to be a stable product
with a very long shelf-life. This might be due to the heating of the processed
cheese (80°C for 5 min) at time of processing. This finding supported the
objectives of pasteurization that stated by IDF (1994). Moreover Viljen (2001)
added that only part of the primary microflora survives under the selective
pressures exerted by the intrinsic and extrinsic biotic factors present, processing
procedures and preservatives.
The counts of yeast and molds of the processed cheese showed lower values compared
to those from the Sudanese white cheese which indicated the improvement in the
quality (Table 1 and 2). This supported
Viljoen (2001) as he reported that the general environment from which raw dairy
products originate and the microbiological quality of the products in its processed
state inevitably admit yeast growth and spoilage. Also the psychrophilic count
of the processed cheese revealed the same improvement (Table 1
and 4). Moreover, the storage periods were also improved after
processing the Sudanese white cheese (Table 3) and that gives
chances of the possibility of using processed cheese in Sudan as one of food
industries in the future, particularly where pasteurization of milk for the
Sudanese white cheese making is not practiced. Moreover superior quality processed
cheese can be produced if the milk is pasteurized before cheese processing to
eliminate the original microflora of milk. This study supported Schar and Bosset
(2002) who reported that the changes with age of processed cheese are influenced
by four main factors: product composition, processing, packaging and storage
conditions (time and temperature).
The total bacterial counts, coliform counts and yeast and molds counts of the processed cheese were increased during the storage periods (Table 3). After 3 months the processed cheeses show spoilage, which might be due to the high initial total and coliform bacteria. However Schar and Bosset (2002) reported that even products without any bacteriological contamination retain their high quality only for a few months at room temperature. The processed cheese made from Sudanese white cheese after 30 days ripening showed higher bacterial growth compared to that made after 15 days ripening. This might be due to the effect of ripening time and the pasteurization (IDF, 1994). The present result also agreed with Aly et al. (1995) who reported that the increase in the level of ripening tended to increase the numbers of total bacteria. However, the psychrophilic count showed slight increase in the studied processed cheeses during storage periods (Table 3). This might be due to the refrigeration storage of the processed cheeses.
The present study recommended that hygienic handling during milking, processing of cheese, storage (packaging and preservation) and marketing should be improved and controlled. Further work is needed and recommended on different conditions to produce the processed cheese from Sudanese white cheese and to understand the microbiological characteristic of both chesses in more details.