- Is there a higher risk of contamination of
milk from machine or hand milking?
- Referring to the above definitions, what are
the key screening tests that small scale milk processors should
regularly perform on raw milk deliveries? And which are the most cost
efficient testing procedures?
- What are the best methods for milk processors
to create awareness amongst small milk producers in developing countries
of the need to always deliver good quality, unadulterated milk?
- Some field reports have shown that the results
of alcohol test for buffalo milk are not reliable. What are your
MILK TESTING, QUALITY CONTROL, HYGIENE AND SAFETY
By: Roberto Giangiacomo - Istituto
Sperimentale Lattiero Caseario - Lodi, Italy
The subjects mentioned in the title
suggest two separate approaches for
improving milk utilisation. Up to ten years ago, milk testing and quality
control mainly addressed chemical composition, in particular fat content,
and the suitability for processing milk into cheese. In the last decade,
due to the increase in the international milktrade, the interpretation of
these terms has been expanded to include hygienic control.
1. Milk composition
Milk composition is a basic requirement
for the evaluation of the efficiency
of transformation of feed into milk and determines the suitability of milk
to be processed into further products. A large amount of literature is
available on this subject, and it continues to be important, due to the
numerous factors involved in determining the amount of milk produced by
animals, the protein and fat content, and its hygienic quality.
The main factor determining milk
composition is the breed of the lactating
animals, but also the environment and pastures play an important role. For
example in Italy, which is a country with a large variety of geographical
environments, milk composition can vary significantly with the season and
with the location. On flatlands, such as the Province of Cremona, which has
the highest number of lactating cows in the country, the protein content is
on average 3.26% and the fat content is 3.56%. In the province of Bolzano,
mainly mountainous, the average fat content is 3.42 and 4.05% respectively.
In southern regions, like Sicily, the average is 3.24 and 3.44%
respectively. The production for a single cow also varies significantly: an
average of 8.5 t per lactation in Cremona, versus an average 4.2 t in
2. Milk testing and quality
2.1 Milk testing and quality
control at farm level
Farmers do not carry out direct
measurements on the composition of milk.
However, they are able to evaluate the sanitary conditions of the animals
and should be able to detect the presence of mastitis, the major enemy on
the dairy farm. Mastitis results in a reduction in fat and protein and
farmer can see and feel a variation of the density of secreted milk. The
infection often comes with some secretion of blood, which results in the
colour becoming pale pink. Attention should be paid also to changes in
normal colour due to the ingestion of particular feeds, plants, or
by-products from other food processing which impair anomalous pigmentation.
These same substances might lead also to changes in flavour, for example fish
odour from fish protein concentrates, with aromatic substances remaining
entrapped on the fat globules.
The farmer is the key quality controller
on the farm. This will include
screening of milk containing residual drugs, for example antibiotics or
sulphamides, careful rinsing of the milk tanks to avoid the presence of
residual detergents. These residues and additions can easily be detected and
reduce the economic value of the milk.
2.2 Milk testing and quality
control by collection or processing centres
Centres collecting milk from farms
generally carry out the analysis of
antibiotics and sulpha compounds on each lorry. The most well known and
easily applied method is the colourimetric response to the growth of Bacillus
stearothermophilus var. calidolactis in a solid agar medium after
incubation. At each delivery the freezing point of milk is also measured for
detection of added water. Commercial instrumentation is available for this
analysis and its cost is generally repaid by the decrease in the amount paid
for adulterated milk. Instruments provide values in °H (Hortvet) or in °C
(Centigrade). Conversion formulas can switch from one system to the other.
Values above -0,520 °C (i.e. closer to 0 °C) are suspect, but the normal
interval of freezing points in the region has to be known . Formulas can
convert the variation in freezing point, as a consequence of variation in
the electrolytes content, into the amount of added water.
Similar information is usually given by
the measure of density using a
lactometer at 15 or 20 °C, which generally is between 1028 and 1034 g/L
at 15 °C. Values lower than 1028 g/L generally indicates presence of added
water, as a consequence of the variation in the fat and protein content by
dilution. Formulas can permit to compute from the density value an estimate
of the total solids.
pH is the measure of the global amount of
dissociated H+ and therefore a
rough estimate of the acidity of the milk. pH is a very easy measure, if the
instrument is well calibrated, and provides an immediate indication on the
condition of the milk. The normal values for milk are 6,6-6,8. Lower values
generally mean an acidification process due to development of bacteria;
higher values generally mean the presence of mastitis.
A more accurate measure of the degree of
acidification is given by the
titration of milk. Titration is carried out by adding through a burette a
solution of NaOH. Depending on the type of system in use the normality (N) of
sodium hydroxide changes: 0.25N for Soxlet Henkel (°SH), N/9 in Dornic (°D).
Current values are 7-7,8 °SH. Higher values generally mean an acidification
process due to development of lactic acids by bacteria.
Two simple and rapid methods can provide an estimate of the suitability of
milk to be consumed or processed: the stability of milk to ethanol 68% and
the alizarin-alcohol test. The first method is based on the behaviour of
milk when mixed to an equal volume of ethanol 68%: if the milk does not form
floccules, it is normal; if it does, it means that the milk is generally not
suitable to further processing.
The second method is more accurate and is
based on the change in colour of the equivolumetric mixture of milk with
alizarin-alcohol. According to a
colourimetric scale and the eventual presence of floccules it is possible to
define the normality, the degree of acidification or the presence of
abnormal milk (colostrum, mastitic milk). The above mentioned analyses are
carried out at each delivery and do not require specially trained staff.
Determination of protein and fat content
require more sophisticated
instrumentation and trained staff. Official methods exist, issued by Codex
and by IDF. For determination of these milk constituents on a large scale,
automatic instruments are available. A mid-infrared radiation is filtered by
selected filters permitting the passage of those wavelengths corresponding
to the absorption of the chemical bonds characteristic for protein, fat, and
lactose. A calibration curve with known samples permits the quantitative
determination of the three constituents simultaneously. If such an
instrument is available, the milk composition is assessed at each delivery
and makes possible a milk payment system based on quality. If traditional
wet chemistry has to be used for these determinations, the analyses are
generally carried out every two weeks. Determinations on the Total bacterial
count, using a standard plate count are also carried out every two weeks.
More information on analytical procedures
carried out in collection centres
can be found in the FAO
small scale dairy farming manual.
In 1992 the EU issued the Directive 92/46
establishing the hygienic
requirements for the production and marketing of raw milk, liquid milk for
consumption, milk for the preparation of dairy products, and dairy products.
In the application of the Directive, cow milk destined for the production of
liquid milk for consumption must comply with some chemical and physiochemical
parameters, e.g. minimum protein content of 28 g/L, non-fat dry matter
minimum 8,50%, weight not less than 1028 g/L, freezing point lower or equal
The Directive gives great emphasis to the
precision of the analytical
measurements and to the purpose establishes that member countries designate a
central laboratory, co-ordinated by a European Central Laboratory, to provide
analytical methods, to organise comparative tests, to promote new methods, to
improve personnel skill, etc.
Under the umbrella of these national
laboratories farmers' organisation, the
performances of the own laboratories of industrial organisations or any
other organisation, can be kept under control. After the initial investment
costs for equipment, the analyses are quite inexpensive and do not request
solvents or any other reagent which must be disposed of in a safe manner.
This procedure facilitates the continuous
monitoring of milk trade, the
payment of milk by quality (see discussion paper 1.4) and result in the
improvement of the relationship between sellers and buyers. Moreover, the
assurance of the correctness in analytical performance permits the
development of specific programs aiming at improving feeding for a higher
animal productivity, both qualitatively and quantitatively.
The establishment of national networks of laboratories also allows
monitoring and control of milk composition in different areas, often with
different climatic conditions. In addition it permits easy data collection
for statistical purposes, contributing to the basis for national and
3. Hygiene and Safety
In the last decade more and more emphasis
was put world-wide on foodborne diseases. WHO and FAO are already involved in
developing programs aiming at monitoring these diseases and minimising their
effects (FAO/WHO Food Standards Program). However, the type of diseases, the
agents, the epidemiology, the type of products involved and the technologies
necessary to minimise the risks associated with the consumption of those
products, etc. are not well known.
The EU has issued provisions concerning
hygiene in milk production, milk
collection and processing. Raw milk from cows and buffalo must be produced by
animals officially free from tuberculosis and brucellosis, which do not
present symptoms of infectious diseases transmissible to humans through milk
and in good general sanitary conditions. Cows should produce at least 2
litres of milk per day. Raw milk from sheep and goat must be produced by
animals officially free from brucellosis, unless destined for the production
of cheese with over 60 days ripening.
Milk must be free from residual substances
as veterinary drugs or
detergents. The Directive provides also the maximum limit of total bacterial
count for the milk of the various species marketed in EU. The combination of
hygienic provisions for the animals, of environmental characteristics in the
milking areas, in the milk collection points, and during transport, of heat
treatment, of hygienic processing and of a self-monitoring programme by
the factory minimises the risk of marketing contaminated products potentially
transferring diseases to the consumers.
Other micro-organisms are emerging,
capturing the attention of scientists
and analysts. Research in dairy products often shows the detection of
bacteria which before were never analysed, but very likely present also in
the past. The development of modern analytical tools reveals the presence of
an unexpected number of different biotypes of the same species. For several
reasons micro-organisms change, become more virulent, more resistant to
certain antibiotics, the number of people susceptible to being infected
increases, etc. All this can contribute, together with the increased
capacity of monitoring and keeping record of infections transmitted through
dairy products, to an increased number of cases of foodborne diseases.
In developing countries, there are some
reports on the presence, with an
incidence ranging 10-20% of the samples, of harmful organisms such as E.
coli O157: H7, Brucella melitensis, Bacillus cereus, Yersinia
enterocolitica, Aeromonas hydrophila, Pseudomonas pseudomallei, Lysteria
monocytogenes, Staphylococcus aureus, etc. Samples were isolated
from products consumed directly and deriving from cow, sheep, goat, camel,
buffalo milks. It is also clear that there is a high risk of contracting
diseases by consumption of these products. However, we make this statement on
the basis of our limited European experiences which were recorded in a
totally different environment, on a different population with very different
food consumption habits, in very different sanitary conditions, etc.
There is no evidence that the same number
of a bacterial species can induce
disease independently of the biotype, of the product in which it is present,
of the characteristics of the population consuming that product.
The adoption of sanitary provisions requires
an in-depth knowledge in each country, or homogenous geographical area, of
the real incidence of foodborne diseases, which diseases are transmitted
through dairy products, in which products do the virulent agents persist
after processing, and which
provisions are necessary to minimise the risk of the presence of these
organisms. But this issue would deserve a specific paper.
Another issue that would need special
attention and discussion is the
presence of aflatoxins, in particular aflatoxin M1. These toxins are quite
often found in milk and milk products, as consequence of the growth of fungi
on feed, a phenomenon facilitated by the environmental conditions in several
developing countries. Due to the carcinogenic properties of some toxins the
control of aflatoxins is highly advisable for the benefit of consumer health.