Meat color and iridescence: Origin, analysis, and approaches to modulation.

Meat color is an important aspect for the meat industry since it strongly determines the consumers' perception of product quality and thereby significantly influences the purchase decision. Emergence of new vegan meat analogs has renewed interest in the fundamental aspects of meat color in order to replicate it. The appearance of meat is based on a complex interplay between the pigment-based meat color from myoglobin and its chemical forms and light scattering from the muscle's microstructure. While myoglobin biochemistry and pigment-based meat color have been extensively studied, research on the physicochemical contribution of light scattering to meat color and the special case of structural colors causing meat iridescence has received only little attention. Former review articles focused mostly on the biochemical or physical mechanisms rather than the interplay between them, in particular the role that structural colors play. While from an economic point of view, meat iridescence might be considered negligible, an enhanced understanding of the underlying mechanisms and the interactions of light with meat microstructures can improve our overall understanding of meat color. Therefore, this review discusses both biochemical and physicochemical aspects of meat color including the origin of structural colors, highlights new color measurement methodologies suitable to investigate color phenomena such as meat iridescence, and finally presents approaches to modulate meat color in terms of base composition, additives, and processing.

A research note: Effect of Hofmeister salts on meat iridescence in cooked pork.

The effect of Hofmeister salts (NaCl, NaSCN, Na2SO4, KCl, LiCl, CaCl2) on surface iridescence in cooked pork was investigated. Strongest iridescence occurred in samples treated with NaSCN, NaCl and KCl. Control samples and LiCl, CaCl2 and Na2SO4 treatments showed weaker iridescence. However, differences between KCl and LiCl, CaCl2 and Na2SO4 were not statistically significant (p > 0.05). Nevertheless, a tendency of chaotropic salts (NaSCN, NaCl, KCl) to cause stronger iridescence was noted that might be explained with a more effective solubilization of myofibrillar proteins (MPs), reducing incoherent scattering from the myofibrils and thus enhancing multilayer interference.

A research note: effect of pH on meat iridescence in precooked cured pork.

OBJECTIVE: The objective of this study was to investigate the effect of pH change of cooked cured pork M. longissimus thoracis et lumborum on iridescence intensity and extent (= percentage of iridescent area) since interaction with light may be related to pH-induced alterations in microstructure. Muscles were injected with brines of different pH values, cooked, sliced perpendicular to muscle fiber direction, and visually evaluated by a panel of 20 experienced panelists. RESULTS: Muscles with lowest pH (5.38) showed the lowest iridescence score of 4.63 (p < 0.05). Iridescence was greatest in muscles with normal (5.78) and high pH (6.03, respectively 6.59), but did not differ significantly (p > 0.05). Iridescence was positively correlated (p < 0.01) with pH and water content, and negatively correlated (p < 0.01) with cooking loss. Hence, hydration state and light scattering from microstructure may be important factors that determine the degree of iridescence in cooked meat products.

Influence of muscle type and microstructure on iridescence in cooked, cured pork meat products.

Microstructural factors associated with surface iridescence in cooked, cured pork products were investigated. Meat iridescence is a commonly observed physical phenomenon in raw meat and meat products that consist of intact muscle tissue. Since the purchase decision of consumers is mainly driven by the first impression of meat color and appearance, products showing colorful iridescence may be rejected. Four different muscles (RF: M. rectus femoris, BF: M. biceps femoris, ST: M. semitendinosus, and LD: M. longissimus thoracis et lumborum) were brine-injected, cooked, sliced, and iridescence was evaluated by digital image analysis and sensory analysis. Sarcomere lengths, fiber diameters, and surface microstructure were analyzed in iridescent and noniridescent sections. Highest iridescence extent by image analysis was found in LD (37.3 ± 16.4%), and highest overall iridescence score (extent and intensity, 6.11 ± 1.78) was observed in BF. Sarcomere lengths did not differ significantly between iridescent (1.05 ± 0.09 µm LD) and noniridescent areas (1.08 ± 0.94 µm LD) within muscles (p > 0.05). Iridescent sections showed smooth and ordered surface structures with cross-sectioned myofibers, whereas in noniridescent sections, surfaces were more unstructured and myofibers obliquely cut. The results of the study indicate that the sarcomere length and fiber diameters may thus be only of minor importance for the explanation of meat iridescence in cooked meat products and are rather related to multiple scattering and absorption effects on smaller structural entities such as the myofilament lattice or larger entities such as fiber bundles. PRACTICAL APPLICATION: Iridescence can be a problem for the meat industry due to consumers concerns about green-iridescent colors in meat. The underlying mechanisms and structures have not yet been fully clarified, and thus no practical solutions to eliminate iridescence have been found so far. This research presents new insights into the structural attributes that are interrelated with meat iridescence and shows that iridescence is rather influenced by cutting angle of muscle fibers and surface homogeneity than by muscle fiber diameters or sarcomere lengths. This should be considered by the industry when seeking for ways to reduce the potential problem of iridescence.

Quantification of surface iridescence in meat products by digital image analysis.

Iridescence extent is commonly evaluated by sensory analysis but it is a time-consuming and cost-intensive method. A low-cost, rapid and objective alternative is digital image analysis. Here we report the development of an image analysis method for quantification of iridescence in meat products. Two segmentation techniques (global thresholding and k-means clustering algorithm) were tested for their capability to divide images into segments of iridescent and non-iridescent areas. Images segmented using k-means clustering algorithm resulted in slightly higher iridescent areas than images segmented with global thresholding (mean difference of 1.24%) but no significant difference (P > .05) between the iridescent areas calculated by both methods was observed. Almost perfect agreement (κ = 0.800, p = .001) was observed between the image analysis and the visual evaluation. The results from this study showed that digital image analysis is an effective tool for evaluating surface iridescence in meat and meat products.

In vitro release of grape-seed polyphenols encapsulated from uncoated and chitosan-coated liposomes.

Grape-seed extract (GSE), a rich source for polyphenols, was incorporated into liposomes (1.1% w/w soy lecithin) using high-pressure homogenization (22,500psi). A chitosan coating (1% w/w) was used to obtain more stable liposomes. Physiochemical properties (ζ-potential, mean particle size) of all liposomes were analyzed. In vitro release of GSE-polyphenols from various liposomes was investigated by measuring the total phenolic content of the dialysate (acetate buffer, pH3.8±0.1, 25mM) over time. Diverse kinetic models were used to describe the release of the polyphenols incorporated from liposomes. Z-average particle diameters increased with the incorporation of GSE and chitosan coating. Chitosan-coated liposomes containing GSE had larger particle sizes than coated liposomes without GSE. The ζ-potential changed from -38mV in uncoated liposomes to +65mV in coated liposomes. Entrapment efficiency for uncoated and coated liposomes was 88.2±4.7% and 99.5±2.3%, respectively. The release rate increased gradually by increasing time. In vitro release of GSE polyphenols from both uncoated and coated liposomes followed an exponential equation (first order Q(t)=a·(1-exp(-k·t))). The release from coated liposomes was much lower than uncoated liposomes. The release rate after 24h from uncoated liposomes was 0.55 and from coated liposomes was 0.24. This study indicates that the release of bioactive compounds from liposomes can be reduced by coating with chitosan, allowing an application of coated liposomes with a controlled release of GSE polyphenols in water-based foods.