Excerpt
2
Introduction
In the food production industry, food spoilage by microorganisms poses an immense challenge to
food sustainability and health safety. Most of the preservation methods are aimed at destroying
the microbial populations that are present in the raw materials, preventing contamination,
improving the texture of the manufactured food products, and creating desired flavours (Walstra
2013). For instance, Ultra-High Temperature pasteurization, commonly referred to as UHT
pasteurization is a preservation method that is carried out to decontaminate raw food products.
This preservation method is used predominantly in milk processing, unlike its alternative
processing method, High Temperature/Short-Time pasteurization which is used to process an
array of raw food materials. In most cases, raw milk is usually contaminated with bacteria,
moulds and yeast microbes. However, bacteria constitute the greatest percentage of microbial
populations in raw milk based on studies which have found different pathogenic bacteria in raw
milk, especially L. monocytogenes, Salmonella spp, Campylobacter spp and E. coli (Tremonte et
al. 2014). Therefore, this article provides a comprehensive discussion on how the microbial flora
changes during Ultra-High Temperature pasteurization.
Control of Contamination during Preparation Stage
Foremost, UHT pasteurization involves the preparation of raw milk prior to the actual
pasteurization process. In most cases, Mycobacterium spp are usually present in raw milk from
cows because cattle serve as hosts for these mycobacteria especially M. bovis which has been the
concern in international trade of most animal products (Franco et al. 2013). However, most of the
bacteria which are involved in milk spoilage are from the environment. Some of the main
sources of contamination of milk during UHT pasteurization, as well as other milk processing
methods are the udder and hygiene (Cempírková 2007). During milking, milk is likely to become
contaminated within the teat canal epithelium. Teat canal and the udder are usually inhabited by
an array of normal skin microflora comprising of streptococci and micrococci species (Hantsis-
Zacharov & Halpern 2007). Therefore, contamination of milk with these bacteria is controlled
through the adoption of safe milking practices such as washing of the udder before milking.
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Transfer of Raw Milk to Heat Exchangers
The second step in milk preparation during UHT pasteurization involves the transfer of milk
from the milking plant to the preservation apparatus. In most cases, milk contamination occurs
during this step due to poor handling of raw milk. Evidence shows that vessels such as milk cans
which are used to hold milk before processing contain microorganisms. Some of the bacteria that
are commonly found on the surfaces of milk cans include Brucella spp, Enterobacter and
Clostridium spp (Scott 2008). Therefore, milk handling prior to the process of pasteurization
determines the degree of raw milk contamination. In order to minimize contamination at this
stage, high standards of hygiene are recommended to ensure raw milk is not contaminated
through handling. One of the most appropriate control measures is disinfecting milk holding
vessels to destroy biofilms which may be present in the equipments (Scott 2008).
However, any contamination during the steps before UHT pasteurization is controlled during the
actual preservation in heat exchangers which eliminate all microorganisms in raw milk, as well
as destroying bacteria toxins and spores. In UHT pasteurization, milk is heated to 138
0
to 150
0
C
(280
0
to 302
0
F) for 1 to 2 seconds. As a result, temperature changes during the heating process
coincide with changes in the microbial flora in raw milk. Ordinarily, most bacteria in raw milk
exhibit optimal activity between 20
0
C and 30
0
C. However, it is worth noting that bacteria can
ferment milk at low temperatures, even below 7
0
C as it has been observed with psychrotrophic
bacteria (Munsch-Alatossava & Alatossava 2006).
Decontamination at 63
0
C to 72
0
C
Once milk is placed in heat exchangers for UHT pasteurization, temperatures are raised over
time to reach the ideal temperature for destroying all microbial flora including microbial proteins
such toxins. As the temperature increases from 30
0
C, the optimal temperate upon which bacteria
growth occurs to 63
0
C, most non-spore-forming bacteria are eliminated through destruction by
heat. In practice, temperatures between 63
0
C and 72
0
C have been found to be ideal in
destroying microbial flora in milk (Scott 2008). This is the concept applied in other
pasteurization techniques used in processing milk in which milk is heated to 63
0
C to 72
0
C for
some time, in order to destroy bacteria. Therefore, raising temperature up to 72
0
C ensures all
non-sporeforming bacteria are destroyed. Bacteria species which are destroyed by temperatures
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include Pseudomonas spp, Pantoea spp, Leuconostoc mesenteroides, Acinebacter spp,
Lactococcus spp, Enterobacter spp, and Psychrobacter spp. Other non-sporeforming bacteria
species which are destroyed by heat increase to 72
0
C are Kurthia gibsonii and Exiguobacterium
spp (Anzueto 2014). It is also worth noting that all bacteria species of bovine origin such as
Mycobacterium tuberculosis and Brucella spp are destroyed at this temperature range.
Non-sporeforming bacteria remaining are usually destroyed as the heat exchanger temperatures
rise to 100
0
C. It is also worth noting that some spore-forming bacteria species are destroyed at
pasteurization temperatures below 100
0
C. However, most spore-forming bacteria have been
found to withstand high temperatures, and this is attributable to their adaptive traits, primarily the
formation of heat-resistant spores. Therefore, almost all heat-resistant bacteria in milk are
destroyed by the time temperatures within the heat exchangers increase to 138
0
C. In practice,
UHT pasteurization occurs by heating milk between 138
0
C and 150
0
C for 1 to 2 seconds. This
temperature is ideal for destroying all bacteria spores which remain after active cells are
destroyed at lower temperatures during the heat treatment process. Therefore, bacteria spores
which are present in milk are destroyed at this level. Microbial floras which are destroyed at this
step include Bacillus spp, Clostridium botulinum, Brevibacillus spp, Paenisporosarcina,
Paenibacillus spp, and Lysinibacillus spp. In most cases, Bacillus spp including Bacillus
licheniformis, Bacillus aerophilus, Bacillus safensis, and Bacillus subtilis are known to be the
most heat-resistant bacteria species causing milk spoilage (Anzueto 2014).
Decontamination at 73
0
C to 150
0
C
Ideally, increasing the temperatures above 72
0
C up to 15
0
C eliminates heat-resistant microbial
floras in their order of heat susceptibility. For instance, all spore-forming microbial floras are
eliminated under temperatures between 72
0
C and 120
0
C including Bacillus cereas (Scott 2008).
Above the temperature of 120
0
C, only five bacteria species exist in milk during UHT
pasteurization. These microbial populations include Bacillus licheniformis, Brevibacillus
borstelensis, Bacillus subtilis, Paenibacillus, and Aneurinibacillus (Scheldeman et al. 2005).
Therefore, temperatures for UHT pasteurization (138
0
C to 150
0
C) ensures all heat-resistant
bacteria are destroyed. In addition, this temperature destroys microbial toxins that may contribute
to food spoilage. For instance, microbial toxins such as Botulinum neurotoxin are inactivated by
Excerpt out of 7 pages
- Quote paper
- Patrick Kimuyu (Author), 2018, Microbial Flora Changes during UHT Pasteurization of Milk, Munich, GRIN Verlag, https://www.grin.com/document/388524
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