Module 6. Mastitic milk

Lesson 25

MASTITIC MILK – SUITABILITY FOR PROCESSING AND PUBLIC HEALTH SIGNIFICANCE

25.1 Introduction

Milk obtained from animals suffering from the infected udder is termed as ‘mastitis’. IDF defines mastitis as ‘an inflammation of udder, almost always of microbial origin. The mastitic milk has higher microbial count and somatic cell count and has altered composition accompanied by reduced yield.

Mastitis is a parenchymal inflammation of the mammary gland that is caused by microbes that invade the udder, multiply and produce toxins, which are harmful to the mammary gland. It is characterized by physical, chemical and usually bacteriological changes in milk and pathological changes in glandular tissues of the host animal. Mastitis is one of the most important deadly diseases of milch animals, responsible for heavy economic losses due to reduced milk yield, milk discard after treatment (9%) cost of veterinary services (7%) and premature culling. Mastitis is a global problem that adversely affects animal health, quantity and economics of milk production and huge financial losses. Unlike clinical mastitis, in sub clinical mastitis there are neither visual abnormalities in milk like blood clots, flecks etc. nor in mammary gland like swelling, hotness etc.

25.2 Different Forms of Mastitis

Mastitis can be classified based either on symptoms or on causative micro-organisms.

25.2.1 Classification based on symptoms

Swollen, hot, red and painful udders.

· Acute or clinical: Macroscopic changes to udder or milk, readily detectable by milker.

· Chronic: Little compositional changes with almost complete absence of pain in udder.

· Sub-acute/ sub-clinical: Most common form, udder and milk appear normal. Diagnosed by detecting pathogens and somatic cells and change in milk composition

25.3 Causative Microorganisms

Mastitis is caused as a result of udder infection with one or more of the causative micro-organisms. These microbes enter through the teat tip into the teat duct, where these get colonized due to the presence of left over milk and subsequently, spread throughout the udder causing infection.

25.3.1 Microorganisms associated with mastitis

· Most common causatives are Staphylococcus aureus and Streptococcus agalactiae.

· Coliforms : Escherichia coli , Enterobacter, Klebsiella, Citrobacter.

· Other Streptococci: Streptococcus uberis, Streptococcus dysgalactiae, Streptococcus faecalis and Streptococcus pyogenes.

· Other Staphylococci : Staphylococcus epidermidis and Staphylococcus albus.

· Corynebacteria : Corynebacterium bovis, Corynebacterium pyogenes.

· Ricketsia: Coxiella burnetii.

· Yeast: Cryptococcus neoformans, Candida pseudotropicalis.

· Molds: Asperigillus spp.

25.3.2 Classification based on causative microorganism

· Contagious mastitis: Streptococcus agalactiae (as natural inhabitant of udder)

· Common mastitis: Species of Streptococcus, Staphylococcus and Escherichia coli

· Summer mastitis: Corynebacterium pyogenes

· Environmental mastitis: Streptococcus uberis

25.4 Compositional Changes in Mastitic Milk

The colonization of mammary glands by mastitis causing microorganisms trigger a series of events that in turn causes major compositional alterations.

· Initial increased level of pathogenic bacteria occurs, which is closely followed by considerable increase in somatic cell count.

· Subsequently, there is a wide range of related effects like impaired synthetic ability of the secretary tissue causing lower milk yield and altered levels of major and minor milk constituents and increased infiltration of blood constituents i.e. serum proteins into milk.

Overall milk from the infected quarters in different cases of mastitis may have the following altered constituents:

25.4.1 Increased constituents

Total whey proteins (i.e. bovine serum albumin, immunoglobulins), sodium, chloride and other ions like Cu, Fe, Zn, various enzymes and certain glycoproteins increased significantly in mastitic milk. The pH of milk also increases.

25.4.2 Decreased constituents

Lactose, fat, total casein (i.e. alpha and beta fractions) decrease but gamma fraction increase, some whey proteins (i.e. alpha-lactalbumin and beta globulin), potassium and other minerals like calcium, magnesium and phosphorus decreased. On the whole mastitic milk in general has a lower SNF, fat, casein and lactose and higher serum proteins, chloride ions and pH.

Table 25.1 Comparison of composition of normal and mastitic milk

Constituent(s)	Normal	Mastitic
Fat (%)	3.45	3.2
Protein (%)	3.61	3.56
Lactose (%)	4.5 to 5.3	3.3-4.9
Bovine serum albumin (mg/ml)	0.82 to1.29	4.3-21.5
Lactoferrin (mg/ml)	0.1 to 0.2	6.2
Sodium (mg/100ml)	57.0	104.6
Potassium (mg/100ml)	172.5	157.3
Chloride (mg/100ml)	80 to 103	7250
pH	6.65	6.9-7.0
Somatic cell (x 10²/ml)	20 to 1000	100-5000
Catalase (µM O₂/min/ml)	0.08	1.84
Lactic dehydrogenase (milliunits/ml)	300 to 500	Up to 5525
Alkaline phosphatase (units/ml)	191	712
Acid phosphatase (µM/ml)	0.063	0.081
Lysozyme (µg/ml)	0.0	5.6

25.5 Significance of Mastitic Milk

Mastitis in lactating animals affects the yield, quality and public health aspects of milk. With severe clinical mastitis, abnormalities of milk are easily observed and milk is discarded. Such milk normally would not enter the milk chain. But when milk of cows with sub-clinical mastitis, i.e. with no visible changes, is accidentally mixed into bulk milk, it enters food chain and can be dangerous to consumer. Although, pasteurization destroys all human pathogens, there is concern, when raw milk is consumed or when pasteurization is incomplete or faulty. Milk and other dairy products are frequently infected with S. aureus. Milk of infected animals is the main source of enterotoxigenic S. aureus of animal origin. For example, certain S. aureus strains produce heat-resistant enterotoxins that cause nausea, vomiting and abdominal cramps, when ingested by humans and are responsible for staphylococcal food poisoning. Toxins are produced due to improper cooling of milk, during cheese making from raw milk and also due to post-processing contamination. These toxins cannot be destroyed by heating. The bovine mammary gland can be a significant reservoir of enterotoxigenic strains of S. aureus.

S. agalactiae is an important bovine pathogen, especially as a cause of both clinical and sub-clinical mastitis in dairy animals. Mastitis constitutes a source of economic loss for the dairy industry due to its effects on milk quality. It not only lowers the quality of cheese and other milk products and decreases milk yield. It also reduces nutritive value of milk due to the changes in its composition, increases processing problems and off-flavours. It also decreases the shelf life of fluid milk products, due to the growth of spoilage causing bacteria. Moreover, the impact of mastitis involves the additional cost of therapeutic strategies and veterinary services. S. agalactiae is considered a major cause of increased somatic cell counts as related to standards in bulk tank milk. SCC increases in milk because of mastitis, hence, milk quality, and lactose and casein contents decreases. Milk yield of a cow with an infected quarter may to the extent of 40%, while animal does not show any apparent clinical signs of mastitis. A reduction in milk quality ultimately leads to loss of income to the dairy farmers.

Another public health concern regarding mastitis is antibiotic residues in milk due to extensive use of antibiotics in the treatment and control of disease. Antibiotic residues in foods can lead to severe reactions in people allergic to antibiotics at low levels, cause sensitization of normal individuals and development of antibiotic-resistant strains of bacteria. Compliance with recommended withholding time helps minimizing the risk of antibiotic residues to occur in milk and meat which is the producers’ responsibility.

25.5.1 Milk yield

The milk yield and also the productive life of milch animal are adversely affected due to mastitis.

25.5.2 Milk quality

The quality of milk as said above is lowered due to a number of compositional changes.

25.5.3 Suitability for fermented dairy products

Mastitic milk is not suitable for making fermented milk products. In cheese making the following effects are noticed in product on while using mastitic milk.

25.5.3.1 Lower product yield

This might be due to increased fat losses in whey and reduced starter activity.

25.5.3.2 Poorer product quality

Rennet clotting time is increased causing decreased curd firmness and a loose final texture of cheese. Also results in lack of adequate flavor development due to retarded starter activity.

25.5.4 Other fermented products

Mastitic milk is not suitable for preparing fermented milks like dahi, yoghurt, cultured butter milk, kefir etc. where product quality is directly dependent on starter cultures. The starters are affected in terms of their growth and activity (acid and flavor production) in sub clinical mastitic milk. For example, both lactobacilli and lactic streptococci are inhibited in milk from Streptococcal and Staphylococcal cases of clinical mastitis. Both the rate of acid production and final acidity attained in mastitic milk are lower than that in normal milk. Even at less than 5% level of mastitic milk in the pooled milk, the aroma, taste and consistency of end product are adversely affected. Diacetyl production by Lactococcus lactis ssp. lactis biovar diacetylactis is lower in mastitic milk.

25.5.5 Public health significance

Some of the mastitis causing microbes which are excreted in milk is also pathogenic to humans. For example, certain strains of Staphylococci associated with mastitic milk cause food poisoning due to enterotoxin production. The storage of mastitic milk under favorable temperatures leads to elaboration of enterotoxins that are not inactivated even during pasteurization and spray drying of milk. Other pathogens and food poisoning causing bacteria like Salmonella spp and enteropathogenic strains of certain coliforms (E. coli and Klebsiella spp) might also be associated with public health hazards through mastitic milk.