After looking at the factors to be considered when sampling milk, it is now possible to draft a sampling plan. The sampling plan has a specific objective of ensuring that a the obtained sample will be reflective of the qualities that one is seeking to establish about the group.
The plan will clearly indicate the size of the sample and all the important features for examination under every group.
Some common features of the sampling plan include:
a) Sample size
- The variations in the populations
- Seriousness of the outcome if a bad sample is not detected
- Cost of analysis
- Variations of the populations
b) Sample location
No problem when dealing with homogenous population; however, heterogenous population requires consideration of the sampling plans e.g.
Random sampling – random picking of the samples from the population
Systematic sampling – you pick the samples following a given order.
Judgment sampling – you pick samples following past experience.
c) Sample collection
State clearly whether the sampling will be manual or by specialized mechanical devices
Preparation of lab samples
These are the steps to follow:
i) Make the sample homogenous
Most samples are heterogenous due to inter-unit variation or intra-unit variation.
Inter-unit variation – variation of properties in different units e.g. milk fat content variations that occur in different milk cans
Intra-unit variation – variation within individual units, e.g. fat content variation in one milk can.
ii) Reduce the sample size
Smaller sample sizes are easier to manage during analysis, which reduces the chances of sample contamination and cross-contamination.
iii) Prevent changes in the milk sample
Ensure the sample does not undergo physical, chemical, or enzymatic changes. Enzymatic changes should be countered by elimination or inactivation of the enzyme, which can be achieved through:
- Adding chemical preservatives
- Freezing the sample
- Heat treating the sample
Fat oxidation is the most likely chemical change to occur on fat based dairy products such as cream, ghee (products with unsaturated fatty acids). To reduce oxidation, limit such samples to light exposure, elevated temperature, oxygen, or from co-oxidants.
Prevent microbial growth through freezing, cold storage, heat treatment, and use of preservatives.
Physical changes likely to occur may include loss or gain of moisture, which can affect butter analysis. Another serious physical change may also include crystallization (as in the case of ice cream samples). You will be able to control most of the physical changes by monitoring the storage temperature and relative humidity.
iv) Sample identification
Carefully label all the laboratory samples so that in case of any problem, it is easy to trace the origin. The information that is usually used for the identification of laboratory samples includes:
- Sample description, i.e. what is it? (whether butter, cheese, ice cream, etc)
- Note the time of sampling
- The location of sampling
- Identity of the person who took the sample
- Method used to select the sample
- Record of the procedure used for each sample