Objective measurement of colour (CIE L*, a* and b*) was performed at the surface of meat samples using a reflectance spectrophotometer, 30 min after package opening.
Chroma (C*) and hue-angle (h) were calculated by the following formulae: C*=(a*2+b*2)1/2, h=tan−1(b*/a*).
Each value was the mean of 30 determinations.
2.4. Metmyoglobin percentage
Metmyoglobin percentage was estimated spectrophotometrically by measuring the reflectance at 525 and 572 nm
The maximum value of the quotient between K/S572 and K/S525 at the beginning of the experiment was fixed as 0% MetMb, while 100% MetMb was obtained after oxidising a sample in a 1% (w/v) solution of potassium ferricyanide
Each value was the mean of 30 determinations.
2.5. Lipid oxidation
Lipid oxidation was measured by the 2-thiobarbituric acid method and results were expressed as 2-thiobarbituric acid reactive substances (TBARS) in mg malondialdehyde/kg sample.
2.6. Microbial sampling and analysis
Twenty-five grams of the meat mixture were taken from the patties, and diluted in 225 ml 0.1% peptone water.
Each sample was homogenised in a Stomacher Lab Blender for 1 min.
Serial 10-fold dilutions were prepared by diluting 1 ml in 9 ml of 0.1% peptone water.
Two plates were prepared from each dilution by pouring 1 ml in fluid agar.
Counts of aerobic psychrotrophic flora were determined from plates bearing 20–200 colonies in plate count agar incubated at 7°C for 10 days.
Counts were expressed as the log10 of colony forming units/g.