Analysis of Pigments at the Surface of Fresh Beef With Reflectance Spectrophotometry

SUMMARY There is need for a nondestructive method of measuring the proportions of myoglobin derivatives in meat samples. Such a method was developed in which reflectance spectra are recorded on the absorbancy scale for samples of fresh beef containing predominantly myoglobin (Mb), oxymyoglobin (MbO2), or met-myoglobin (Mb+) at the surface. The spectra are adjusted so that reflectance measured on the absorbancy scale (RA), is 1.0 at 525 mμ, an isobestic point for the three derivatives. With this adjustment, the isobestic point for MbO2 and Mb+ at 474 μ and the isobestic point for Mb and MbO2 at 571 mμ are reproducible, and the RA values at 474 mμ and 571 mμ are less variable. In that way, 100% reflectance values were established, but a model system was needed to determine intermediate reflectance values due to mixtures of myoglobin derivatives. The model system, containing known amounts of MbO2 and Mb+, indicated that Beer's law was not followed, but the deviation was small. Some of the difficulties inherent in this type of study are discussed, and a hypothesis is proposed to explain the peculiarities of reflectance spectra.