Effect of Temperature and Modified Vacuum Packaging on Microbial Quality of Wara A West African Soft Cheese
This study introduced vacuum packaging into wara a West African soft cheese storage.Wara was vacuum packaged and stored in whey. Samples were taken during 5 and 21 day storage period at 15 and 28°C to determine populations of total aerobes(TA), anaerobes, Enterobacteriacea, psychrotrophs, as well as molds and yeasts(M/Y) in Log10 CFU g-1. TA increased from 2.25 and <1.00 to 7.67-8.16 and 5.82-8.33 respectively for Calotropis procera processed cheese (CPPC) and Lemon Processed Cheese(LPC) stored in whey at 28 and 15°C during the 5 day storage. Enterobacteriacea were undetectable (<1.00) during the 5 day storage at both temperatures. Anaerobes increased from 2.43 for CPPC and undetectable levels for LPC on 1day of storage to 6.91-8.68 and 5.67-9.01, respectively at 15 and 28°C storage in whey. Population s of M/Y remained undetectable until the 5d when the M/Y increased to 6.16-8.04 and 4.82-7.8, respectively for the CPPC and LPC at 28 and 15°C storage temperatures. In vacuum packaged cheese TA increased from 2.25 and <1.00 to 5.45-6.80 and 4.73-6.45, respectively for CPPC and LPC stored in whey at 28 and 15°C during the 21 day storage. Enterobacteriacea and M/Y were undetectable at the 1 day and at the end of 21 day storage at both temperatures. Anaerobes increased from 2.43 for CPPC and undetectable levels for LPC on 1 day of storage to 4.68-6.76 and 4.8-6.24, respectively at 15 and 28°C at the end of 21 day storage. The study suggests vacuum packaging can be introduced into wara storage to further reduce the microbial population.
In Nigeria appropriately 90% of the dairy cattle belong to the Fulani pastoralist
who process surplus fresh milk to various nutritious milk products such as nono
(sour milk) kindirmo (local yoghurt), manshanu (local butter), and wara (West
African soft cheese). Wara is an un-ripened cheese consumed in several parts
of West Africa. The cheese is prepared by coagulating fresh cow milk with the
rennet extract of Sodom apple (Calotropis procera) or pawpaw (Carica
papaya). The preferred coagulant is from C. procera because the cheese
made with this coagulant has a sweeter flavor then that of the cheese made with
the other coagulant. The ingredient in the leaves of C. procera useful
for cheese production is calotropin, an enzyme that curdles milk proteins (Belewu
and Aina, 2000). The extract of C. procera has also been shown to
have some antimicrobial and antifungal properties, respectively by Ahmed
et al. (2006) and Hassan et al. (2006).
Wara has an average shelf life of 2-3 day when stored in whey at ambient temperature
(approximately 28°C) (Adegoke et al., 1992;
Umoh and Solomon, 2001; Belewu et
al., 2005) although its storage at lower temperatures had only a little
effect on the microbial counts (Adetunji and Chen, 2009)
and the concern in refrigerated food is still listeriosis and botulism. Some
kinds of Clostridium botulinum grow at refrigeration temperature (Garcia
et al., 1987). The wara cheese is usually dip fried in vegetable oil
near the end of its shelf life in order to extend the shelf life. Attempts has
been made in the recent past to include starter cultures or various preservatives
such as propionic acid, sodium benzoate, and sorbic acid in the production of
wara (Aworh and Egounlety, 1985; Joseph
and Akinyosoye, 1997; Sanni and Onilude, 1999; Belewu
et al., 2005). Some of these preservatives have been shown to be
effective in inhibiting mesophilic and psychrotrophic bacteria as well as coliforms.
However, these preservatives may not be easily accessible to the local cheese
processors in West Africa.
Previous laboratory studies have confirmed the presence of pathogenic organisms
like Staphylococcus auerus and Listeria monocytogenes in milk
and milk products processed in Nigeria including ice-cream, fermented milk and
local butter (Adetunji et al., 2003).
Leave extract of C. procera has been shown to introduce the microorganisms
naturally associated with them into wara cheese (Adetunji
and Chen, 2009). In this study vacuum packaging was introduced into to the
storage of wara in order to improve the microbial quality and shelf life of
MATERIALS AND METHODS
Study site: This study was conducted between January to June 2006 at the Department of Food Science and Technology, The University of Georgia, Griffin GA, USA.
Preparation of milk for wara processing: Pasteurized whole milk was purchased from a store at Griffin Georgia. The milk was maintained at 4°C in a cooler and transported to the laboratory where it was stored at 4°C until use. Three liters of this milk each was put in two sterile pots for wara soft cheese processing (Fig. 1).
Samples, packaging and storage: Cheese from the above processing was divided into 16 pieces of 10 g each and divided into two equal groups for 28 and 4°C storage. Four in each group was inserted aseptically into a vacuum bag and sealed with a vacuum machine. The remaining four in each group is then aseptically inserted into sterile nylon bags which are not vacuum packaged; these four served as the control at each storage temperature. The two groups were then stored at the respective temperatures. Another group of cheese was stored in whey at the same storage temperatures as the vacuum packaged cheese.
|| Flow chart of wara processing
Microbiological sampling: Sampling was done every other day for a period of 5 days for cheese stored in whey and 21 day storage for vacuum packaged cheese. At each sampling day, one bag each of vacuum packaged cheese, one nylon packaged cheese and whey cheese containing 10 g cheese each was used. The pH was taken using a VWR scientific model 1800 electrode pH meter.
The samples were then homogenized within the bags with 10 mL of phosphate buffered saline with Seward Stomacher Lab System on the first sampling day. On subsequent sampling days cheese was homogenized with 20 mL of phosphate buffered saline. Serial dilution was made with 0.1% peptone water for ease of counts. Appropriate dilutions were surface plated on MacConkey agar (Becton, Dickinson and Company) for Enterobacteriaceae counts, potato dextrose agar (PDA) for yeast and molds counts, tryptose soy agar (tsa) for total aerobic bacteria, psychrotrophic and anaerobic bacteria counts. Plates for Eenterobacteriacea, molds and yeast and aerobic bacteria were incubated aerobically at 37°C while psychrotrophic bacteria plates were incubated at 6-7°C for 5-7 days. Plates for anaerobic bacteria were incubated anaerobically at 37°C in a GasPak system.
Statistical analysis: The study was performed in two replicates, each
with appropriate duplications. All microbiological data were transformed into
Log10 CFU mL-1 or Log10 CFU g before comparison
of means. Analysis of data was accomplished using the Fishers least significant
difference (Glimm et al., 1997) of means of bacterial
populations calculated with the General Linear Model (GLM) procedure of SAS
based on a 95% confidence level (SAS, 2000).
RESULTS AND DISCUSSION
In wara cheese stored in whey, general the population of total aerobes, anaerobes, molds and yeasts counts increased with storage days except on two occasions in whey stored wara cheese.
|| Microbial counts and pH of Calotropis procera and
Lemon juice cheese stored in whey
|C.P = Calotropis procera cheese ; D= storage days,
Values followed by same letters in a same row are not significantly different
with respect to storage temperature.
Enterobacteriacea counts were undetectable through out storage and pH dropped
with extension of storage period in whey stored cheese (Table
1). In vacuum packaged wara cheese molds and yeast counts which were initially
undetectable became detectable by the 4 to the 7 day of storage and then remained
undetectable to the end of the 21 day storage period. Enterobacteriacea counts
were undetectable throughout storage except on few occasions on days 7 and 11
of storage. pH which decreased from 5.65-6.14 on day 1 to 4.97-6.22 on day 11
also decreased from 5.23- 6.26 on day 14 to 4.84- 6.17 on day 21 (Table
2). A significance difference occurred between storage temperatures and
C. procera and Lemon juice processed cheese.
Some types of whey cheeses are supposed to be consumed within a short time
upon manufacture (e.g., Ricotta, Requeijão and Manouri), whereas others
bear a longer shelf life (e.g., Gjetost, Mysost and Myzithra). Whey cheeses
are significantly different from one another in terms of chemical composition,
which is mainly due to variations in the source and type of whey, as well as
to the processing practices followed (Pintado et al.,
|| Microbial counts and pH of vacuum packaged Calotropis
procera cheese and lemon juice cheese for 21 day storage at 28 and 15°C
|CP = vaccum Calotropis cheese; Values followed by same letters
in a same row are not significantly different with respect to storage temperature(p<0.05).
Generally the populations of total aerobes and anaerobes, increased with extension
of storage time at 15 and 28°C and the effect of storage temperature on
total aerobes and anerobes was significant (p<0.05) in this study. The populations
of total aerobes and anaerobes increased more rapidly in wara cheese stored
at 28°C as compared to 15°C. The populations of these two groups of
microorganisms were significantly different in cheese samples stored at 28°C
compared to the samples stored at 15°C (p<0.05). This is probably because
the temperature of 28°C is more close to the optimal growth temperatures
of these organisms. Lower storage temperatures such as 15°C slow down microbial
metabolic activities therefore, retarding the proliferation of microbial cells
(Jay, 2000). Enterobacteriacea were undectected throughout
storage and molds and yeast counts were undetec until the 5 day of storage in
whey cheese. This probably due to a low level contamination of of these microbes
in the raw milk used for processing. The slight decrease in pH along the storage
days is likely due to increase in the number acidproducing bacteria.
The effect of vacuum packaging was greatest on molds and yeast and enterobactericea
counts which were undetec by the end of 21 day storage. Similar report were
made in Requeijao a traditional Portuguese whey cheese which was packaged under
vacuum, Yeasts were severely inhibited. Packaging under vacuum decreased yeast
growth rates and also reduced the populations attained at the end of the exponential
phase of growth, resulting in a shelf-life extension of the Stracchino cheese
of over 28 days (Sarais et al., 1996). A similar
reduction in coliform counts along storage days of cheese has earlier been reported
(Alalade and Adeneye, 2006). This report is similar
to entero bacteriacea counts, which were undetec in this study. Similarly an
extension in shelf-life was reported in vacuum packaged gas flushed bananas
(Chauhan et al., 2006). The inhibition of microbes
in vacuum packaged cheese is because air is evacuated from gas-impermeable pouches
followed by sealing (Jay, 2000). But the count of total
aerobes despite the vacuum packaging is an indication that wara is porous and
still has some air stored within its matter thereby encouraging multiplication
of aerobes. The presence of anaerobes throughout the storage is expected because
the wara was stored in an anaerobic condition. These findings are particularly
important in terms of improving wara and other dairy products. Moisture content
and pH of whey cheeses are usually high and favor microorganism growth (molds,
yeasts, lactic acid bacteria and Enterobacteriaceae accounting for the dominant
microflora in these cheeses). A positive correlation was earlier reported between
coliform bacterial count and moisture content (Alalade and
Adeneye, 2007). The use of lemon juice as a sanitizer and bacteriocidal
agent against pathogens has been reported (Sengun and Karapinar,
2005; Sengun and Karapinar, 2004) this explains
the reason for the significant difference in all counts between the Calotropis
procera and lemon juice processed cheese.
Our findings elucidated the limitation of shelf-life duration in wara cheese in a developing country (Nigeria). Overall our data indicate that vacuum packaging suppressed molds, yeast and enterobacteriacea. The use of lemon juice in wara processing also suppressed all the microbes in this study. This study suggests that wara cheese (as well as other whey cheeses and dairy products) could be vacuum packaged to reduce the risks of food poisoning bacteria and extend the shelf-life of these products.
Adequate packaging of whey cheeses should be provided, and legislation should be prepared to fix standard characteristics of each type of whey cheese.
This study was funded in part by FY-2005/2006 Junior Staff Development Fulbright Scholarship of the United States Institute of International Education.
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