Chapter 2: LITERATURE REVIEW
Goat milk production
In Africa, goats are reared first for their meat but can be a significant source of milk production (Jaitner, Njie, Corr and Dempfle, 2006). Goats contribute 15 % to the total milk supply compared to 69 % from cattle and 11 % from sheep in Southern and Eastern Africa (Degen, 2007). In South Africa, about 60 % of rural households of former homelands own goats while less than 30 % own cattle (Statistics South Africa, 1999; Shackleton, Shackleton, Netshiluvhi, Mathabela and Phiri, 1999). Countries like Somalia, Sudan, Kenya, Mali, Ethiopia, Namibia and Botswana have bigger pastoral communities that rely significantly on goat milk in the dry season (Degen, 2007).
Quantitative data for goat milk production in pastoral communities are scant because they are produced on a small-scale mainly for home consumption, and relatively small amounts of goat milk products enter the formal market (Shackleton, Shakleton and Cousins, 2001). Small-scale goat milk production in rural centers makes use of minimal infrastructure. Milk is harvested mainly by hand milking or via use of semi-intensive systems (Degen, 2007). Here, goat milk is consumed raw or processed into artisanal soured milk products. This demand for goat milk for home consumption is increasing due to increase in human population (Haenlein, 2004). Rural small-scale goat milk production has a promising potential to contribute significantly to global milk production and is thus an avenue that needs further development. A less popular type of goat milk production is practiced on a commercial scale in urban areas. Intensive systems are used to produce good quality goat milk under hygienic conditions to be used for dietetic purposes or for processing into connoisseur cheeses (Degen, 2007).
Goat milk has been compared to cow milk and considered superior in terms of its digestibility, medical advantage as a substitute for cow milk (Haelein, 2004 ), and its potential economic role in rural development (FAO, 2001).
Anti-allergenic properties of goat milk
The health benefits of goat milk are related to allergic reactions to cow milk proteins prevalent among children less than 4 years (El-Agamy, 2007). Several clinical studies have shown that the common cow milk proteins that give positive skin reactions in infants are α-lactalbumin, α-s-1-casein and β-casein (Haelein, 2004; El-Agamy, 2007). Although goat milk proteins are similar to cow milk proteins in their general classifications, they differ in their frequencies and genetic polymorphisms (Grosclaude, 1995). The α-s-1-casein is the major α-s-casein protein in cow milk and it is also the major cause of allergic reactions to cow milk. In goat milk, the α-s-2-casein variant is the dominant α-s-casein (Ambrosoli, De Stasio and Mazzoco, 1988). The α-s-2-casein does not give positive skin reactions and is more digestible compared to α-s-1-casein (Ambrosoli et al., 1988).
Nutritional properties of goat milk
Nutritional studies with Spanish rats that had malabsorption syndrome showed that these rats had significantly improved digestibility and improved copper and iron absorption when fed with goat milk compared to cow milk (Barrionuevo, Alferez, Lopez Aliaga, Sanz Sampelayo and Campos, 2002). In a similar study, Alferez, Barionuevo, Lopez Aliaga, Sanz Sampelayo, Lisbona, Robles and Campos (2001) showed that goat milk reduces total cholesterol levels due to the higher levels (36 % in goat milk versus 21 % in cow milk) of medium chain triglycerides (MCT). It is believed that the improved absorption of minerals in the digestive tract may be partly due to higher levels of essential amino acids in goat milk compared to cow milk. For example, the improved copper absorption is due to high cystine content in goat milk (Haelein, 2004). Table 1 shows differences in essential amino acids and the fatty acid contents of goat and cow milk. Goat milk contains higher MCT, monounsaturated fatty acids (MUFA) and polyunsaturated fatty acids (PUFA), all known to have beneficial health properties particularly for cardiovascular conditions and for treatment of gastrointestinal disorders (Haelein, 2004). Also, Le Jaouen (1981) reported that the higher content of small fat globules in goat milk compared to cow milk makes goat milk more digestible which gives it a nutritional advantage.
Goat milk products
Most dairy goat breeds reared on a small scale are dual purpose goats kept for their milk and meat. Goat milk is consumed raw, pasteurized or sterilized. It is also used in the production of connoisseur goat cheeses such as blue veined cheese, Feta and Manchego, which are highly patronized in developed countries (Harding, 1995; Haenlein, 2004). Other goat milk products include goat milk powder, yoghurt, butter oil and cream (Pandya and Ghodke, 2007). In other societies, left over milk is allowed to sour naturally in clay pots, calabashes, or any suitable container into several indigenous dairy products such as Madila (Ohiokpehai, 2003), peculiar to Botswana and Amasi (Gran, Gadaga and Narvhus, 2003), which has a wider consumer base in the Southern African region.
General bacterial quality of goat milk
Although goat milk has several benefits, the major factor limiting its production is high losses of raw milk due to souring at ambient temperatures. There have been thorough studies of the microbiological quality of cow milk. However, information on the assessment of the microbiological quality of raw and processed goat milk is limited. Spoilage of raw goat milk by bacterial fermenters naturally present in milk, in the surrounding atmosphere or through fecal contamination hours after milk collection is an economic problem to goat milk production at rural centers. Microorganisms that occur in milk are usually due to unhygienic milk handling rather than transmission from the goat (Thompson and Thompson, 1990). In their study, Thompson and Thompson (1990) observed that hand milking as well as the cleanliness of the milker and the milking parlour present opportunities for contamination of raw milk.
Foschino, Invernizzi, Barucco and Stradiotto (2002) studied the general bacterial quality of raw goat milk in Bermago, Italy. In their study, they isolated several pathogens including Escherichia coli, Listeria monocytogenes, Salmonella spp., Staphylococcus aureus and Staphylococcus caprae. Lactic acid bacteria dominated the natural microflora of raw milk. These were composed mainly of lactobacilli and lactococci. Other bacteria such as enterococci, Micrococcus, coliforms and yeasts were also isolated (Foshino et al., 2002). In another study conducted on bulk tank goat and ewe milk from 403 different farms in Switzerland, Enterobacteriaceae was isolated from 61.6 % of goat milk samples, S. aureus was detected in 31.7 % of goat milk, 23.0 % of the goat milk samples were positive for Mycobacterium avium subsp. paratuberculosis, and 16.3 % were positive for Shiga toxin-producing E. coli (Muehlherr, Zweifel, Corti, Blanco and Stephan, 2003).
Several lactic cultures and yeast have also been isolated from artisanal goat cheeses from around the world. These comprise mainly of the genus groups Lactobacillus, Lactococcus, Enterococcus, Streptococcus, Micrococcus, Leuconostoc, and Candida (Tornadijo, Ferenso, Bernardo, Sarmiento and Carbello, 1995; Sablé, Portrait, Gautier, Letellier and Cottenceau, 1997). The origin of milk (i.e. the dairy farm) has been identified as a major factor affecting the variability of bacterial composition of milk (Foschino et al., 2002; Oliver, Jayarao and Almeida, 2005). The interplay of several elements including composition of feed and contamination during milk collection determines the bacterial quality of raw milk (Foschino et al., 2002).
Significance of Escherichia coli O157:H7 as a foodborne pathogen
The occurrence of bacterial pathogens in milk and milk products is of significant public health concern. Prevalence of pathogens such as S. aureus, L. monocytogenes, Campylobacter jejuni and E. coli in milk has been well established over the years; however, little is known about the occurrence of shiga toxin-producing E. coli (STEC) in milk (Oliver et al., 2005). Jayarao and Henning (2001) isolated several pathogens from bulk tank milk including STEC. Enteroheamorrhagic E. coli (EHEC), which is a subtype of STEC, is of particular importance due to the severity of disease, with most EHEC infections caused by E. coli O157:H7 (Oliver et al., 2005; Chung et al., 2006).
According to Perna, Mayhew, Posati and Blattner (2001), more than 75,000 cases of E. coli O157:H7 foodborne infections occur annually. E. coli O157:H7 is of critical public health significance because it has a low infectious dose of up to 100 cells and therefore is highly pathogenic. It causes acute illnesses including diarrhea associated haemorrhagic colitis (HC) characterized by severe abdominal cramps, watery diarrhea and subsequently bloody diarrhea with little or no fever (Riley et al., 1983). Karmali, Petric, Lim, Fleming and Steele (1983) also reported that E. coli O157:H7 causes haemolytic uremic syndrome (HUS) manifested by acute renal failure, thrombocytopenia and microangiopathic hemolytic anemia which follow bloody diarrhea. Non-bloody diarrhea, with long term sequelae is also typical of E. coli O157:H7 (Karmali, 1989; Paton and Paton, 1998).
Many ruminants including healthy cattle, sheep and goats naturally harbour E. coli O157:H7 in their intestinal tract (Oliver et al., 2005). These ruminants shed E. coli O157:H7 suggesting that they provide a specific niche for them (Griffin and Tauxe, 1991; Hancock, Besser, Rice, Ebel, Herriott and Carenter, 1998). Transmission of E. coli O157:H7 occurs mainly by food and water, but also occurs by person-to-person contact and occupational exposure (Mead and Griffin, 1998). Foods such as unpasteurized milk and dairy products, undercooked hamburgers, apple juice and vegetables have been implicated in E. coli O157:H7 outbreaks (Steele, Murphy and Rance, 1982; Doyle, 1991; Oliver et al., 2005). A list of milk, fermented milk and milk contact surfaces from which EHEC has been isolated is presented in Table 2.
LIST OF TABLES
LIST OF FIGURES.
LIST OF ABBREVIATIONS
Chapter 1: INTRODUCTION
1.2 Problem statement
Chapter 2: LITERATURE REVIEW
2.1 Goat milk production
2.2 Significance of Escherichia coli O157:H7 as a foodborne pathogen
2.3 Preservation technologies applied in dairy processing
2.4 The lactoperoxidase system
2.5 E. coli general stress response
2.6 E. coli response to acid stress
2.7 E. coli tolerance to lactoperoxidase system.
2.8 Cross-protection of acid adapted E. coli
Chapter 3:RELATIVE GENE EXPRESSION IN ACID-ADAPTED ESCHERICHIA COLI O157:H7 DURING LACTOPEROXIDASE AND LACTIC ACID CHALLENGE IN TRYPTONE SOY BROTH
3.2 Materials and Methods
Chapter 4:THE INFLUENCE OF LACTOPEROXIDASE, HEAT AND LOW PH ON SURVIVAL OF ACID-ADAPTED AND NONADAPTED ESCHERICHIA COLI O157:H7 IN GOAT MILK
4.2 Materials and Methods
4.5 Conclusion .
Chapter 5:EFFECT OF LACTOPEROXIDASE SYSTEM AND ESCHERICHIA COLI O157:H7 GROWTH ON ACIDPRODUCTION BY SINGLE STRAIN AND INDIGENOUSnLACTIC ACID BACTERIA IN GOAT MILK
5.2 Materials and Methods
Chapter 6: GENERAL DISCUSSION
6.1 Review of Methodology
6.2 Comparative acid-resistance of E. coli O157:H7 in Tryptone Soy Broth, goat milk and fermented goat milk
Chapter 7: CONCLUSIONS AND RECOMMENDATIONS.
Chapter 8: REFERENCES
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