22 food hygiene & safety
Table 1. N/Protein determination in cereals.
☞
protein determination by combustion include AOAC default protein factor of 6.25 (5.70 for rice). The protein
Method 992.23 and AACC Method 46-30, 1999. These factor can be changed in accordance with the food type.
methods indicate that the suitable fineness of grind must Temperature of the left reactor was set at 950°C and
be determined for each different material analysed to 840°C for the right reactor. Temperature of the oven was
achieve precision that gives RSD (equal or less) of 50°C. A 500 mg EDTA (9.59 per cent nitrogen) standard
two per cent for 10 successive determinations of was used and a sample weight of 600mg-1.6g. The
nitrogen. oxygen amount necessary for the complete combustion
The AACC 46-30 crude protein combustion method of samples was calculated automatically by the OxyTune
specifies that nitrogen, freed by pyrolysis and subsequent function present in the Eager Xperience software.
combustion at high temperature in pure oxygen, is Table 1 shows the nitrogen and protein
quantified by thermal conductivity detection and determination in cereals samples. Barley and rice were
converted to equivalent protein by appropriate homogenised to 1mm and 2mm particle sizes. Data
numerical factor. This generic combustion method is obtained through ten consecutive determinations
applicable to all flours, cereal grains, oilseeds and demonstrated excellent reproducibility. In all cases, the
animal feeds. Different conversion factors are used for relative standard deviation was less than two per cent
various cereal grains and oilseeds. The AOAC 990.03 according to the official methods. No memory effect was
protein in animal feed combustion method is applicable observed when changing the type of sample, indicating
to the determination of nitrogen in all types of forages. the complete detection of nitrogen present in the
A Thermo Scientific FLASH 4000 nitrogen/protein sample. No significant differences in the results were
analyser was used to validate the combustion method in observed changing the sample particle size from 2 to
terms of accuracy and reproducibility according with the 1mm.
pre-treatment of the sample (Fig. 1). Different cereals Table 2 shows the nitrogen and protein
and seeds were chosen for this analysis. determination in sunflower seeds. Due to their sample
Protein content was calculated automatically using nature and high content of fat, oil-seeds require a
the Thermo Scientific Eager Xperience software with proper optimisation of the oxygen amount needed for
combustion to obtain accurate data.
The sample was homogenised at 2mm particle size.
Data were reproducible with a RSD percentage as per
official method requirements. The left part shows the
reproducibility of ten determinations using a weight of
around 500mg, while the right part indicates the
reproducibility of ten determinations in a range from
700–1000mg of the same sunflower seed sample. No
memory effect was observed when changing sample
weight. p
The authors work for Thermo Fisher Scientific in Milan, Italy.
Dr Liliana Krotz is OEA Product Specialist, Dr Elena Ciceri is
OEA Analytical Product Development and Dr Guido Giazzi is
Table 2. N/Protein determination in sunflower seeds OEA Product Manager.
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