Tài liệu Practical Food Microbiology 3rd Edition - Part 7 docx

Milk and dairy products

7.1 Pasteurized milk

7.2 Untreated milk

7.3 Ultra heat treated milk and sterilized milk

7.4 Dairy products

The tests and methods given in this section are based mainly on those for milk

and dairy products stipulated in European and UK legislation. EC Directive

92/46/EEC [1] lays down health rules for the production and placing on the

market of raw milk, heat treated milk and milk-based products. This Directive

was incorporated into UK law as the Dairy Products (Hygiene) Regulations 1995

[2], Code of Practice number 18 of the Food Safety Act 1990 [3] and associated

Guidance Notes [4]. The Regulations and Guidance Notes contain microbiolog￾ical standards and guidelines for products sampled at any point in a production,

holding or heat treatment establishment. The methods to be used for examina￾tion of liquid milk are described in Commission Decision 91/180/EEC [5].

Methods for other dairy products are specified in the UK Regulations. All meth￾ods specified are recognized internationally; the legislation also states that any

other internationally recognized method that gives equivalent results may be

used. The regulations apply to milk and milk products of any animal origin (cow,

goat, sheep, buffalo).

Pasteurized milk

The Dairy Products (Hygiene) Regulations 1995 [2] specify tests for coliforms,

pre-incubated plate count, phosphatase and peroxidase for pasteurized milk. In

addition, the regulations require the absence of pathogens in 25mL of product

but do not specify which organisms should be investigated. However, Commis￾sion Decision 91/180/EEC [5] states that if the other tests are satisfactory, a spe￾cific test for pathogens is only necessary if the milk is thought to be associated

with an outbreak of food poisoning.

Sampling

Conditions for sampling, transport and storage of samples can be found in Com￾mission Decision 91/180/EEC [5]. Sample units of pasteurized milk in complete

sealed packages should be taken from the packaging machine or cold room as

soon as possible after processing and on the same day as processing. For routine

testing, three separate samples should be taken:

• Sample 1 —to measure temperature on receipt at the laboratory.

7.1

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Milk and dairy products 193

• Sample 2 —to be used for the coliform, phosphatase and peroxidase tests.

• Sample 3 —to be kept intact at 6°C for the pre-incubated plate count test.

For statutory purposes each test is performed on five separate samples; therefore

at least 12 separate samples are required for the coliform and pre-incubated plate

count tests, to allow for one bottle per insulated sample transport container for

temperature monitoring.

Samples should be transported to the testing laboratory in an insulated con￾tainer with the least possible delay and should be transported and stored be￾tween 0°C and 4°C. The time between sampling and examination should be as

short as possible and should not exceed 24h.

Colony count

A colony count (referred to in the legislation as a plate count) is no longer speci￾fied in UK legislation for the testing of pasteurized milk as this was not a require￾ment of Directive 92/46/EEC [1]. However, this test can be a useful tool for

quality assurance purposes. The standard specified in the 1989 UK Dairy Regula￾tions [6] was 2.0¥104/mL. In practice freshly pasteurized milk usually has a

colony count below 104/mL. The methods described in Sections 5.3–5.6 are suit￾able for performing colony counts but milk plate count agar should be used with

incubation at 30°C for 72h. Dilutions to 10-3 may be required.

194 Section seven

Method 1 Pre-incubated colony count

This method is described in Commission Decision 91/180/EEC [5].

Equipment

Incubator at 6±0.5°C

Incubator at 21±1°C

Water bath at 44–47°C

Pipettes or pipettors and sterile tips, to deliver 1mL.

Media

Milk plate count agar

Peptone saline solution (maximum recovery diluent).

Procedure

(a) On arrival in the laboratory, incubate the sample (consisting of an intact con￾tainer) at a temperature of 6±0.5°C for 120±2h (i.e. 5 days) together with an

continued

Special note

For routine purposes, other methods of colony counting such as spiral plating

(Section 5.4) are acceptable. If results are required for referee purposes the pour plate

method described below should be used.