Examination of raw milk

Physical and chemical properties of raw milk

Examination of milk purity

During milking different contaminating materials may get into milk (epithelium, fur, feed etc) so it has to be filtered.

Dirt →indicate high microbial contamination 1.) Sedimentation test

Pour 1 litre milk, let it stand for 1 hour flotation sedimentation

2.) Filter test

Examination of milk density The density of milk: is the mass of a certain volume of milk at 15oC

(g/ml) Quick method for testing milk alteration: →watering ↓ →skimming ↑ Unit: Lactodensimeter degree, (oLd) 32oLd = 1,032 g/ml Standard: 1,029-1,034 g/ml at 15oC Instrument: dipping lactodensimeter combined with thermometer Pour milk into a glass cylinder put the lactodensimeter into the milk read results it refers milk at 15oC If warmer →dips more →lower result→ +0,2/oC If colder →dips less →higher result → -0,2/oC   At least 5 hours after milking

Determination of freezing point

Most sensitive method for detecting foreign water in milk. Freezing point depends on the lactose content and salt concentration. (breed: - 0,540oC; holstein: - 0,528oC)

- 0,520oC or less 0,003-0,005oC changing in the temperature

means 1% water 0,5-1% foreign water can be acceptable

It can be determined by cryoscope and an instrument with thermistor.

Determination of total solids

This method is used when the chemical content of milk is examined or when there is a suspicion of alteration.

Fat free total solids = solids-non-fat-content: 8,5 g/100cm3, 8,2 g/100g

1. Drying method : It means the remains of the milk after drying at 102+2oC until its weight will be constant.(g/100g)

  Method: We measure 30g of sand into a pot 30 minutes in a drying box at 102+2oC 10g milk into the pot, (mix with sand) 102+2oC until its weight will be constant (check in every hour)

Determination of total solids

Infrared absorption method Determination by calculation

a.) total solids ts = 1,2*f + 2,665*(100-100/d) g/100 cm3

b.) solids-non-fat-content  snf = ts-f g/100 g 

c. calculation of solids-non-fat-content by Fleischmann table

Determination of protein content Importance: as a quality parameters of milk (buying up

price, cheese production, etc.) The expected total protein content should be at least 3,3%

(casein: 2,7%, albumin + globulin: 0,6%)

Kjeldahl-method: : Classical method: accurate, but long procedure

Formoltitration by Schulz Principal of the method: we bind the free amino groups of

amphoter proteins in the milk with formaldehyde. Then we titrate the released carboxyl groups with alkaline and by the consumption of it we determine the protein content

Milk getting sour can give higher results.

Infrared absorption method and colour binding chemical reaction are also used

Detection of the potential acid degree Potential acid degree: the whole amount of alkaline binding substances Soxhlet-Henkel method

The acid degree by Soxhlet-Henkel of milk is (OSH): the required ml-s of NaOH solution of 0,25 M to neutralise 100 ml milk at the presence of phenolphtalein indicator

Standard acid degree of raw milk: 6,0 – 7,2 OSH

a, 50 ml milk 2 ml 2% alcoholic phenolphtalein indicator 0,25 M NaOH (light pink colour) The used ml-s of NaOH x 2 = OSH of milk. b, 20 ml milk1 ml 2% alcoholic phenolphtalein indicator0,1 M NaOH (light pink colour) The used ml-s of NaOH x 2 = OSH of milk.

Determination of the actual acid degree (pH value): The actual acid degree means the amount of the

hydrogen ions of milk. Standard pH value: 6,6 – 6,75 In case of mastitis: neutral or alkalic Colostrum: a bit acidic Indicator paper: informative but not accurate Electrometric way with pH detecting instrument

Determination of freshness of the milk

Alcohol test: Principal: when the milk is getting sour (the OSH is higher), or its

content abnormal, the balance of the colloid system changes and the proteins by the effect of 68 %alcohol will be precipitated. (above 8 OSH)

2 ml milk 2 ml 68 % alcohol (60,5 mass%, at 20oC 0,894 density) mix together In sour milk casein precipitate forming little flakes. There is a

relationship between the fineness of the precipitate and the acid degree of the milk.

At higher temperature the precipitation is faster and rougher.

Determination of freshness of the milk

Boiling test Principal: the casein precipitates by the effect of heat

below a certain pH value. 5 cm3 milk in a test tube

boiling in flame or in a water bath of 100oC for 2

minutes Casein precipitates can be seen: + Milk having 10 – 12 OSH or higher coagulates. Because of high globulin-content of colostrum it can

show this reaction too.!!!

Control of heat treatment of milk and milk product

The peroxidase and phosphatase enzymes presented in the milk get decomposed at certain temperature and time.Long lasting pasteurisation: 62-65oC, 30 minutesQuick pasteurisation: 72-78oC, 15-40sFlash pasteurisation: 78-100oC

Detection of phosphatase Principle: In milk non heat treated or heat treated but mixed with raw milk

or milk product phosphatase can be found and it releases 3,3-dimethyl-phenolphtalein from 3,3-dimethyl-phenolphtalein-hydrogen-phosphate which gives PURPLE colour in presence of alkaline. 5 ml milk2 ml phosphatase reagent

15 minutes, water bath of 37oC

0,5 ml 1 M KOH or NaOHsolution

Consideration: Purple colour presence of phosphatase raw milk

or pasteurised milk mixed with 2-5% raw milkNo colour changing the heat treatment inactivated phosphatase

Detection of peroxidase by the Storch test Principle: The peroxidase is sensitive temperature

above 80oC. The enzyme release atomic oxygen from H2O2 which oxydates N,N-dimethyl-1,4-phenylen-diamine-hydrochlorid to bluish grey compound.

6 ml milk 1 drop 1-3% H2O2 solution (H2O2 H2O + ’O’) (Storch

1) 2 drop 2%paraphenylen-diamine solution (Storch 2) Consideration: after a few minutes it turns to a

bluish grey colour insufficient heat treatment Keeps it original white colour appropriate

heat treatment

Fast microbiological methods

Reductase-tests Indicator material + microbe-dehydrogenase

enzym→ colour changes Raw milk: only in degrees, not accurate result Contaminating microflore decrease the redox

potencial of the media →redox indicators → colour changes

Fast microbiological methods

The methylene blue reduction test is based on the fact that the color imparted to milk by the

addition of a dye such as methylene blue will disappear more or less quickly. The removal of the oxygen from milk and the formation of reducing substances during bacterial metabolism causes the color to disappear.

Though certain species of bacteria have considerably more influence than others, it is generally assumed that the greater the number of bacteria in milk, the quicker will the oxygen be consumed, and in turn the sooner will the color disappear. Thus, the time of reduction is taken as a measure of the number of organisms in milk although actually it is likely that it is more truly a measure of the total metabolic reactions proceeding at the cell surface of the bacteria.

Fast microbiological methods

The resazurin test is conducted similar to the methylene blue reduction test with the judgement of quality based either on the color produced after a stated period of incubation or on the time required to reduce the dye to a given end-point.