Question

During our recent fat analysis for whole round sprats the machine was showing quite high values (sometimes – “out of range”) so we sent some samples to the lab for verification.  There is big divergence between ours and lab results. Each sample send to the lab contained the same 8 fish checked by fatmeter, and I asked  to blend all of the fish before sampling. Please can you tell us why the results are such different?

Answer

There are wild fluctuations in the fat content of the EIGHT samples that make up the average, in your samples… A, B, & C.

Sample A…  29.5, 30.3, 39.6, 23.8, 34.6, 12.3, 11.2, 37.7    Average = 27.3%

Sample B…  31.7, 19.5, 10.3, 31.9, 22.2, 24.0, 17.6, 30.2    Average = 23.4%

Sample C…   11.7. 9.0, 18.9 23.3, 21.5, 25.2, 29.0, 24.2      Average = 20.3%

I would make the following comments…

Measurement preparation…

In the above samples, I have marked the suspect values in RED.  In my opinion these readings are the subject of Operator / Measurement Procedure faults, and some of the lesser values, in the sample are also suspect to a lesser degree.

It is important to group the fish into different grades, depending upon the Fat Content.  This can be done quickly by screening the fish using the Fatmeter.  Segregate the fish into different groups of fat content, as follows…

5- 10%

11 -15%

16-20%

20% +

The Fatmeter microwave energy penetrates into the fish to varying degrees, dependent upon the fat content / water content relationship, of the fish being measured… the greater the fat content (less water), the further the microwave energy penetrates through the fish (remember that the microwave energy is attenuated much more rapidly, when there is a high % of water present in the sample).

As a guide…

At  ~ 7%  Fat Content (~ 71% Water)... the microwave energy will only penetrate about 7-10mm through the skin and into the fish muscle.

At 15% Fat Content (~ 63% Water)… the microwave energy will only penetrate about  12 – 15mm through the skin and into the fish muscle.

At 25% Fat Content (~ 52% Water)… the microwave energy will penetrate about 20 mm through the skin and into the fish muscle.

Therefore it is IMPORTANT to ensure that when measuring high fat content Sprat, that the minimum thickness is 25mm.  If the fish are thinner than this, then you must “double up” the fish (one fish on top of another, to achieve the minimum thickness), measuring on one side of each doubled-up fish sample.

It is very seldom that you get Sprat Fat Contents greater than…25%. The reason for the large measurement values is that you are measuring through the fish and onto the underlying surface (it is always best NOT TO MEASURE on a Metallic Surface, as this will give a higher degree of microwave signal reflection). This is causing erroneous readings, caused by  reflections from the underlying surface.

You must always GROUP the Fish according to Fat Contents, and “double up” fish samples with a higher Fat Content depending upon the thickness of the fish samples.