Proximate Analysis of Fish Tissue by Mid-Infrared Transmission Spectroscopy

Mid-infrared spectroscopy was assessed as a means for rapidly determining the fat, protein, and moisture content of fish tissues. For fat and protein, a sample preparation protocol was developed for the conversion of fish tissue into a milk-like emulsion for analysis by a Multispec MK I infrared milk analyser. Fish (cod, tuna, and mackerel) were minced, dissolved, and emulsified in 0.2 M NaOH using a polytron and analysed for fat and protein using the conventional fat and protein wavelengths used for milk analysis. Instrument calibration for fat and protein was based on the Mojonnier and Kjeldahl procedures, respectively. Moisture was determined separately by extracting the water from the minced tissue using absolute methanol and measuring the absorbance of the resulting solution using a 5.56/6.02-μm filter pair. The instrument was calibrated using standardized water–methanol mixtures and the results compared with the standard vacuum oven method. Rapid analyses of fat and protein were possible giving individual results to within 1% of the standard chemical determinations. Moisture analysis, also worked well, being more accurate and rapid than the vacuum oven procedure. The limiting factor in its use for proximate analysis appears to be a combination of sample compositional variation and/or the general reproducibility of the standard methods. We conclude that the analysis of fish by mid-infrared transmission spectroscopy could serve as a means of rapidly carrying out the proximate analysis of fish and fish products for quality control purposes.