01 February 2016
Measuring the colour of milk, cheese and other dairy products is a fairly straightforward procedure that should be able to monitor the quality of the product whilst being able to keep up with the quantity of milk being processed. With around 11 billion litres of milk being produced each year in Britain alone, keeping quality levels high on such a scale is a task that needs accurate and simple control processes.
The first point to think of when considering analysing milk and dairy products is what are you actually looking for? What is the physical or chemical characteristic that needs controlling and monitoring? What is the purpose of the analysis?
In a great deal of manufacturing and production processes, consistency is carefully monitored throughout in order to maintain a stable appearance and, as colour is a key indicator of appearance anomalies, the use of colour measurement data is extremely useful. For the determination of certain other properties within the product, colour and appearance analysis can measure and record data that indicates the presence of such properties.
For quantifying the amount of beta carotene found in certain food products, there are established methods for doing so using an analytical spectrophotometer. The beta carotene would need to be extracted into a solvent and measured on such an instrument and compared to known beta-carotene standards that represent the limit of low and high beta-carotene concentrations.
Quantifying beta-carotene levels using colour analysis would work in a similar way. As beta-carotene is responsible for the yellow/orange colour in cheese, it stands to reason that variations of degrees of yellowness in these products would be an indication of the variations in beta-carotene concentrations.
Colour analysis of both milk and cheese products can be accomplished using the same type of colorimetric spectrophotometer. To achieve data that correctly and accurately represents the colour of the samples, the instrument to use would be a spectrophotometer that utilises 45°/0° geometry.
Some key factors to think of are
• How thorough does your testing need to be?
For a thorough analysis of the product, data should represent the batch as a whole as far as is able. For the sake of time, a single measurement of a sample can be taken and recorded however, for a more detailed representation, an average of each sample can be taken. A higher degree of accurate data will take more time to quantify whereas a one-off measurement of a sample can be done quickly.
• How much of the product is available for a single sample?
Do you have access to as much as is needed or is the supply for analysis limited? Strictly speaking, the more sample analysed, the more accurate the data collected.
• How many samples of a batch can be taken?
To give a more accurate depiction of the product in its current state, multiple samples from across the entire batch would ideally be taken. The amount would depend on the size of the batch and the time available for batch testing.
As an example, the ColorFlex EZ could be used for this colour analysis. This benchtop colorimetric spectrophotometer is used with a port up function that allows glass sample cups to be placed on top for analysis. Typically, for the colour measurement of milk samples, the sample cup would be filled approximately halfway or to a pre-determined mark with the liquid sample and a white backing disc placed on top using a ring and disc set. Backing with a white disc allows all of the emitted light to be reflected back to the detector and used in the measurement rather than being wasted. Data would be received from a single measurement which can then be analysed or multiple measurements can be taken over a number of samples of the same batch to create an average set of results best depicting the product as a whole.
For colour measurements of cheese, the process is similar however a sample cup need not be used. If we assume that the cheese samples are solid and opaque, the samples should be placed at the sample port and a measurement taken. Similar to measuring milk, singular or multiple samples can be taken depending on how accurate the data needs to be in relation to a batch.
Using the methods detailed above briefly, the surface area of the milk sample measured would be 64mm or 2.5 inches. For cheese samples, using a standard measurement port, 1.25 inches of the sample is illuminated with 1 inch being measured, allowing for a larger surface area to be analysed therefore increasing accuracy.
Following the measurements, spectral data would be received which can then be converted into the relevant colour scales and indices. For this application, the main focus would be on CIE L*a*b*, in particular the b* value which denotes yellowness, and also a yellowness index, YI E313.
This data can either be noted from the instrument itself or be analysed more thoroughly using EasyMatch QC software. Using software can give a more visual representation of how samples compare to a standard.
With these results and data, correlations can then be found between beta-carotene levels determined through analytical spectrophotometry and the degrees of yellowness quantified in the samples. Standard colour data and tolerances can be paired with known beta-carotene levels meaning any future samples measured can be compared to standards of relevant data depicting the amount of beta-carotene in the sample.