1 Principals of laboratory control of milk 2
1 - Principals of laboratory control of milk. 2 - Sampling of milk. 3 - Examination of milk. * Physical examination. * Chemical examination. 4 - Adulteration of milk. 5 - Sanitary and Keeping quality of milk. 6 - Detection of heat treatment.
-Milk is the lacteal secretion of mammary gand practically free from colostrums & obtained by complete milking of one or more healthy dairy animals. -Obtained mainly from buffalos, cows, sheep, goats. -In Egypt, It is mainly obtained from buffaloes. -Mixing of more than one type is illegal.
1 - must be of Normal Composition: *free from adulteration by addition of water. partial skimming. or both. *free from non-permissible material such as preservatives.
2 - must be Clean: *free from visible dirt and sediments. *of low total bacterial count (500, 000 non pathogenic bacteria/ml). 3 - must be Safe & Wholesome(good for health): *free from contamination with pathogenic M. os or it‘s pathological secretions.
Sampling of milk • Aim of sampling: samples are usually collected to be examined: 1 - Physically and Chemically. 2 - Microbiology. 3 - or together. - The samples taken for analysis must be truly representative of the product to be analyzed. - The proper sampling of fluid milk or any dairy product is the first step in obtaining an accurate analysis.
• Precautions during sampling: All precautions should be taken in consideration during sampling to prevent any contamination and adulteration from outside.
Type of examination -Physical examination Precaution Sample should be protected from heat, light & stored in cool place especially when examined for flavor. -Chemical examination Sample must be thoroughly mixed and all equipment must be clean, dry. -Microbiological examination ( Bacteriological ) Sample must be thoroughly mixed and all equipment must be clean, dry and sterile.
Equipments used in examination Test tube Burette Dipping measure
Pipette Plunger Milk stirrer
Milk sample bottle
Dipper Scoop Cleaning brush
*Milk sample bottle: - - Capacity: 250 – 500 cc. - Filling: filled completely with milk to avoid shaking of milk which result in separation of fat during transportation. - Description: - shoulder must be slope in order to not store any milk in the edge which may form milk stone & thus lead to accumulation and multiplication of M. Os. - has narrow neck. - wide sand area. - covered with cork or rubber stopper.
- has label to denote number, date, time, place, name of hygienists and other remarks. * Cleaning and Sterilizing equipment: Cleaning -All equipment are rinsed in tap water immediately after use & cleaned with suitable detergent solutions, then rinsed thoroughly to remove foreign residues.
Sterilizing -Differ according to nature, shape, size and equipment. 1)- Hot air oven: • 160 – 170 ᵒC for 2 hours. • 180 ᵒC for 1 hour. • used to glass and metal equipment.
2)- Autoclave: • At 121ᵒC for not less than 15 min. with 1. 5 Ib. • used to glass, metal equipment and media. N. B Hot air oven & Autoclave, the equipments can be used for 2 days sterilized. 3)- Steaming: • At 100 ᵒC for 1 hour. • Equipment used immediately.
4)- Immersion in boiling water bath : • in 100 ᵒC for 30 min. • Equipment used immediately. • Used for rubber stopper. 4)- Immersion in alcohol 70%: • immersion in alcohol 70% then flamed • Equipment used immediately.
Technique of Sampling ( Sampling procedure) Collection of sample Transportation Identification of sample
(A) Collection of sample 1 - From individual animal: Ø immediately before obtaining the sample, wash the flank region and udder then dry them directly. Ø Normally the entire milking including stripping are placed in one container and thoroughly mixed by plungers or number of transfers. Ø In some cases ( as in mastitis), milk should be taken from each quarter in a separate container.
2 - From bulk tank milk: Ø Thoroughly mixing of milk by plunger, If the amount is small enough, It is better to pour milk from one vessel to another one. Ø Take proportional amount to the total bulk of milk. 3 - From bottles or cans: Ø Take one or more of them as a random sample. 4 - From Factories: Ø Take sample from different stages of manufacture.
(B) Transportation - It differ according to type of examination. For physical examination: • The sample should be protected from heat, light to prevent losses of ascorbic acid and riboflavin (Vit B 2) • Examine immediately. For chemical examination: - • add 0. 1% formalin as preservatives.
For microbiological examination: • Sample either tested immediately or iced in refrigerator at temp. not exceed 4ᵒC until examined to prevent growth & multiplication of M. Os
(C) Identification of sample - Each sample should be labeled to denote: Ø Number, date, Time of sampling. Ø Name, Address of the holder of license. Ø Aim of sampling. Ø Place from which sample is taken.
Examination of milk • 1 - Physical examination. • 2 - Chemical examination. • 3 -Micrebiological (bacteriological) examination.
I. Physical Examination • • 1 - Flavor. 2 - Color. 3 -Boiling point of milk. 4 -Chemical reaction of milk. 5 -Specific gravity of milk. 6 -Specific gravity of milk serum. 7 -Freezing point of milk. 8 - Refractive index of milk serum.
Flavor (odor & taste) - It must be performed before exposure to heat or light. a)- odor: - - Normal milk has a characteristic odor but not pronounced. - characteristic odor disappear by : * long standing. * following cooling & aeriation.
b)- Taste: - Normal taste of milk is slightly sweet (due to combination of sweetness of lactose and saltness of chloride). - Taste is more pronounced when milk is slightly warm.
** Factors affecting Flavor: 1 - Type of food consumed by the dairy animal. ‘’ garlic, onion, turnips, fish meal. 2 - Physiological condition of the dairy animal ‘’ diseased or not ‘’ 3 - Decomposition of one or more constituent of milk either chemically or bacteriologically. 4 - Adsorption of other odor from the surrounding during production, processing. ‘’ Kerosene, fly spray, disinfectant.
1 - Cooked flavor: • Due to heating of milk above 80ᵒC which activate the (C-SH ) sulfhydryl group that originate from Blactoglobulin and protein associated with fat globules membranes. 2 - Burnt or Caramel flavor: • Due to overheating of milk above 100ᵒC. • or Due to microbial action as Streptococcus Lactis var maltigenes.
3 - Rancid flavor: • Due to occurrence of hydrolytic rancidity (rancidity of fat by Lipoprotein lipase(LPL) enzyme presents in milk serum). • or Due to microbial lipase enzyme produced by: - Pseudomonas fluorescens - Achromobacter lipolyticum. Ø The flavor is due to the short chain fatty acids produced.
4 - Salty flavor: • Due to physiological changes as: * High content of chloride. * Late lactation period. * Colostrum. • Or Due to Pathological changes as in case of mastitis.
5 - Fishy flavor: • Due to Dissociation of lecithin by lecithinase enzyme producing trimethylamine. • Or due to chemical changes ‘’ choline peptonised into Trimethylamine. • Or due to feeding of animals on fish meal by mistake. • Absorption from surroundings.
6 - Sour flavor: • Occurs as a result of increased acidity by lactic acid producing bacteria, which ferment lactose producing lactic acid. 7 - Fruity flavor: • Due to the presence of yeasts and moulds or may arise due to psychrotrophs such as Pseudomonas fragi.
8 - Bitter flavor: - Bitterness usually results from: a)- Proteolytic bacteria which produce protease as Micrococcus, Pseudomonas, Alcaligenes, Bacillus. b)- Lactose fermenters as coliforms or sporogeneous yeasts. c)- Feeding of dairy animal on foreign matters. d)- Milk obtained from cows in late lactation period.
Color - Normal color of milk: Ranged from Bluish white to yellowish white. *White color: is due to: 1 - Reflection of light on the dispersed fat globules. 2 - Presence of calcium caseinate. 3 - Presence of colloidal phosphate.
* Yellow color: is due to the presence of carotene pigments (readily soluble in milk fat). * Blue color: is due to the presence of Lactoflavin (main constituent of vit B 2 ) Or Lactochrome pigments. N. B blue color is the normal color of whey
Normal color depends on: 1– Breed and Species of the dairy animals. 2 - Types of feed consumed by the dairy animals. 3 - Amounts of solids and fats present in milk.
Mainly due to microbial action: 1 - Blue color: Due to the growth of Pseudomonas syncyanea and Pseudomonas pyogenea. 2 - Yellow color: Due to the growth of Sarcina lutea or Flavobacterium synxanthium.
3 - Green color: Due to the presence of Pseudomonas fluorescens when milk is hold at low temp. for long time. 4 - Red color: Due to either the presence of: a)- Blood as a result of: Masitis – Injuries in udder. - Milk is secreted red directly during milking.
b) the growth and multiplication of bacteria as: * Serratia marcescens. * Micrococcus roseus. * Torula glutins. - In which milk secreted from the udder white in color but after a time becomes red due to growth and multiplication of exopigments producing microorganisms.
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