On-line Assessment of Fat
The presence, absence, or quality of adipose tissue or fat makes vital contributions to meat yield and quality. Yellowness, softness and the smell of fat (boar taint) may detract from the overall quality of meat while, according to tradition, marbling may make a positive contribution. When fat is abundant in meat, either as marbling or intermuscular fat, it modifies the overall reflectance and appearance of the meat. There is a direct effect: as the proportion of fat is increased in relation to the muscle, reflectance increases at many wavelengths until reaching the spectrum of 100% adipose tissue. But also there are indirect effects of fat levels on meat color: for example, fat may retard oxygen penetration and metmyoglobin formatioadipose tissue is composed of
Mature adipose cells or adipocytes easily reach a diameter of about microns and are almost filled by a single large droplet of triglyceride. Thus, the nucleus and cytoplasm of an adipose cell are restricted to a thin layer under the plasma membrane, which accounts for the low water content of fat. Mature adipose cells with very little cytoplasm contain few organelles. The large triglyceride droplet that fills most of the cell is not directly bounded by a membrane, but is restrained by a cytoskeletal meshwork of 10-nm filaments, most conspicuous in the adipose cells of poultry. Adipose cells themselves are kept in place by a meshwork of fine reticular (Type III collagen) fibers outside the cell. Large adipose depots usually are subdivided into layers or lobules by partitions or septa of fibrous connective tissue. In the layered subcutaneous fat of a pork carcass, the septa may follow the body contours, creating a weak boundary layer echo in the ultrasonic estimation of fat depth. Adipose depots are well supplied by blood capillaries, normally emptied by proper exsanguination, but often retaining a trace of hemoglobin that may give beef fat an amber tinge.
The yellow coloration of fat by beta-carotene, because the yellowness of beef fat may be a cause for down-grading in some national grading schemes. Unlike lean meat, in which reflectance changes very little from 0 to 30 C, the reflectance of adipose tissue is far higher at low temperatures than it is at high temperatures. Fiber optic reflectance spectra of subcutaneous fat usually contain some trace of a Soret absorbance band and an oxyhemoglobin pattern at 542 and 578 nm which, presumably, may be attributed to incomplete exsanguination of the dense capillary bed of adipose tissue. The fiberoptic reflectance spectra shown below range from cold white fat (line 1) to warm yellow fat (line 2).
The Soret absorbance band (i.e., the dip around 420 nm) tends to be more conspicuous at a lower temperature (high reflectance) than at a high temperature (low reflectance). Fat with a high carotene content has an overall lower reflectance intensity than white fat, particularly from 440 to 500 nm where strong absorbance by carotene is separable from the Soret absorbance band of residual hemoglobin. In solution, carotene has a very low relative transmittance from 350 to nearly 500 nm. Thus, the on-line measurement of fat color is relatively simple, with a choice of technology ranging from fiber optics to video. The critical point is that the temperature must either be kept constant or measured, because temperature has a strong efect on the overall reflectance of fat.In this figure line 1 is for yellow fat ranging to white fat for line 3. The difference between lines 3 and 4 shows the effect of the arrangement of the optical fibers in the probe.
Video image analysis of marbling is a superior method for marbling because, in a ribbed side of beef, the whole cross-sectional area of the rib-eye is exposed. This may give a much better sample of the marbling present in the whole muscle than is possible with a probe. However, there are many situations when it is commercially disadvantageous to rib the carcass, particularly for pork. Thus, it is of interest that fat-depth probes also detect marbling fat within the muscle, as indicated in the name of the Danish MQM probe - meat quality and marbling. Another possibility for future on-line use is elastography as developed by Ophir, using ultrasonic pulses to detect small movements caused by an external stress applied to the meat. This technique enables marbling and connective tissues to be visualized but not readily separated. At present, however, samples are removed from the carcass and measured in a large temperature-regulated water tank. Optical Probe It is difficult to build a series of probes with different sized optical windows, but this effect may be simulated using a light guide with many optical fibers to form a window. Then the size of the effective window can be varied by altering the number of fibers used to make measurements. The effect of reducing the size of the optical window from approximately 2 to 0.5 mm diameter makes the signal more irregular along the whole transect, so that the boundary between back-fat and the longissimus dorsi is less distinct, but a larger response is produced by the seams of marbling. Thus, the smaller optical window is more sensitive than the larger window to small anatomical irregularities, favoring the detection of marbling but not the detection of the fat to muscle boundary.
Muscular steatosis resembles an excessive degree of marbling. It is found most frequently in beef and pork, but also may occur in mutton. There may be some indication of muscle abnormality before slaughter, such as an abnormal gait, but usually the condition is not found until the meat is cut. Excessive marbling fat and muscular steatosis sometimes overlap, and often it is only the restriction of muscular steatosis to a single muscle or muscle group in an otherwise poorly marbled carcass that makes it conspicuous. The causes of muscular steatosis are unknown, but strenuous muscle exertion may be involved, particularly in those muscles that are used when an animal rears up on its hindlegs as in mounting, so there might be a behavioural basis. The problem is notoriously sporadic. It may appear suddenly in carcasses from particular herds of animals, then disappear, thus making it very difficult to investigate scientifically.
Soft fat is a sporadic problem in many types of meat, but especially in pork, where it causes a variety of problems in meat cutting and marketing and tends to be worse in lean pigs with a rapid growth rate. Separation of the back-fat from the underlying loin muscles causes problems with automated equipment for removing the skin from pork loins whereas, at the retail level, soft fat has increased translucency and a gray appearance that is unattractive to customers. In packs of sliced bacon, fat softness may cause the rashers to clump into a solid mass after slicing. Soft fat also is more likely to develop rancidity than hard fat. The causes of soft fat are variable, ranging from nutritional increases in unsaturated fatty acids, notably linoleic acid, to inadequate carcass refrigeration. Thus, step one in an investigation is to check the deep temperature. If the cause has a biological basis in the fat, there are three on-line methods of measurement: ultrasonic, mechanical and optical. A new aspect of soft fat in pork is the deliberate feeding of fish oil to increase the content of eicosapentaenoic and docosahexaenoic acids, thought to have a role in human health by reducing the risk of atherosclerosis and heart disease. Effects on color are minimal, but softness and iodine number are increased.
On-line measurement is complicated by the fact that the back-fat on a pork carcass has three main layers. The middle layer shows the greatest development when the back-fat is very thick and has the greatest proportion of extractable fat. The deepest layer contains the highest water content and its thickness is correlated with marbling. Ultrasonic measurement of soft fat is based on the fact that the velocity of ultrasound is greater in solid fat (VS) than in liquid oil (VL). Miles proposed an acoustic parameter to determine the volume fraction of soft fat . The variation in ín the acoustic parameter explained 88% of the variance in penetrometer measurements of 18 samples of back-fat ranging from hard to soft. A hand-held fat hardness meter was developed by Dransfield at the Institute of Food Research at Bristol in the 1980s. A temperature sensor is used to correct a mechanical measurement of softness.
Optical properties of soft fat also have been investigated. The softness of the inner layer of pork back-fat was assessed subjectively in the meat cooler at 4 degrees C by palpation, using a four-point scale (1, soft; 2, slightly soft; 3, slightly hard; and 4, hard), as in the Japanese fat softness scores used for grading pork carcasses. Reflectance spectra obtained with a Colormet probe at 4 C for subjective fat softness scores 1 and 4 are shown below. Both spectra have low reflectance around 420, 550 and 580 nm, probably from hemoglobin of residual erythrocytes in the dense capillary bed of adipose tissue. Reflectance is proportional to firmness, but optical separation is greatest between the softer levels of fat. In other words, slightly hard and hard fat had spectra that were not much different. However, the correlation of reflectance with softness is significant (P < 0.01) at all visible wavelengths over the full range from soft to hard. Soft fat has a higher refractive index and a lower melting point than hard fat. The translucency of fat generally increases with temperature, with a corresponding decrease in reflectance. However, for pork back-fat there is little change from 4 to 22 C, and not until 40 C does a decrease occur at all wavelengths. At 22 C relative to 4 C, there was a slight increase in the discrimination of subjective fat softness scores 3 and 4. At 40 C, discrimination between subjective fat softness scores 3 and 4 was further enhanced, but with a loss of discrimination between subjective fat softness scores 2 and 3. One might expect fat with a high proportion of unsaturated fatty acids to be more liquid, more translucent, and with a lower fiber-optic reflectance than fat with a high proportion of saturated fatty acids at the same temperature. Although basically correct, there appear to be some other factors involved as well, and the relationships of temperature, softness and reflectance may be quite complex. Carcass temperature decreases from around 40 C shortly after slaughter to around 4 C in a commercial meat cooler the day after slaughter. Both times are convenient for making probe measurements, either when the pork carcass is graded for fat depth shortly after slaughter or, the next day, just before it is broken into wholesale cuts. At the earlier time, discrimination of subjective fat softness scores 3 versus 4 is difficult while, at the later time, discrimination of scores 2 versus 3 is difficult optically. Unfortunately, it appears that the combined effects of degree of saturation and temperature may not be linear. Thus, there may be maximum and minimum temperature plateaus in reflectance, beyond or below which intrinsic differences in saturation have a negligible effect. At the reflectance minimum, when triglyceride is molten because of high temperature or unsaturation, there may be an independent background level of reflectance from cell membranes and other elements of adipose tissue microstructure. At the reflectance maximum, triglyceride may become opaque and almost white, thus concealing differences associated with fatty acid saturation. Also, the potential effects of adipose cell diameter are unknown. In conclusion, optical methods might perhaps be suitable for grading the softness of pork fat, provided that the interactions of temperature and degree of saturation can be determined or cancelled. At present, the investment required to develop on-line methods for refractive index and melting point cannot be justified. Whether or not optical methods will prove superior to direct rheological measurement for on-line grading remains to be seen, but the advantage of being able to combine measurements of fat softness with other optical measurements of meat quality justifies further research on this topic.
Young male pigs have a rapid rate of growth, high feed efficiency and produce lean carcasses yet, in the US and Canada, most of them are castrated, retarding their growth and encouraging fat deposition. Although there may be minor problems from soft fat, skin damage from fighting and low curing yields with intact males, the primary reason for castration is the risk of boar taint in the fat. Boar taint is an unpleasant odor that occasionally may become quite obnoxious when pork fat encounters a hot frying pan. Under typical commercial conditions in Europe, however, with pork from young males rather than old breeding boars, boar taint is only detectable by a small percentage of customers. Thus, the problem owes its notoriety to pork obtained from old boars that have been used for breeding, and it need not be a problem in intact young males slaughtered at a relatively light weight. Keeping males intact for pork production provides a much larger base for genetic selection and avoids distress to the castrated animal. The major cause of boar taint is the concentration of sex steroids in the fat identified the major factor as 5à-androst-16-ene-3-one, commonly called androstenone. Androstenone smells strongly of animal urine, but there are other testicular steroids in the 16-androstene family with a musk-like odor. Androstenone carried by the blood from the testes may accumulate in adipose tissue and parotid and submaxillary salivary glands. From the salivary glands, androstenone normally is transmitted as a pheromone in the boar's breath or saliva to the sow during mating. Boar taint is a heritable trait, although it may be supressed immunologically while retaining the greater leaness and growth efficiency of intact males. Other causes of boar taint include skatole and indole, with a fecal odor, produced from the amino acid tryptophan in the gut. Skatole accummulation may be controlled by a recessive gene. Off-line and On-line tests Suspect carcasses may be tested with a hot iron, but the detection of voltailes by the operator is completely subjective (at present). Two objective methods have been developed, one for skatole and one for androstenone. Both methods require a sample to be removed from the carcass for analysis, but the methods are sufficiently rapid to be considered commercially. A rapid, colorimetric test for skatole is used in Denmark and a comparable test (colorimetric and suitable for automation) was developed in Canada. The automated Danish skatole test may be run at 180 determinations per hour, using back-fat samples obtained 40 to 60 minutes after slaughter. Using an off-line test, data are available within 12 minutes, which is sufficient for sorting at the end of the chill tunnel. Data from 100,000 pigs are shown below. The rejection point is at a level of 0.25 ppm skatole. However, because indole also reacts in the test, the skatole levels are positvely biased and should really be termed skatole equivalents.
For androstenone, resorcylaldehyde and sulfuric acid in glacial acetic acid produces a purple color from androst-16-ene steroids, and a pink color from cholesterol. Cholesterol must be removed with a digitonin affinity column from fat samples, but this is not required for samples of salivary gland because androstenone levels are higher. Results are correlated with sensory assessments by taste panels, r 0.85 to 0.89. The skatole test is not highly regarded in some Eurpean countries, and there are plans to combine both methods to produce a test for both sources of boar taint (androstenone and skatole). However, there may be no advantage to testing for both androstenone and skatole. On-line testing for boar taint may be possible in future using the Alabaster-UV semiconductor system. The sensor is mounted in a stainless-steel chamber with a UV source and gases are exposed to a cycle of ozone and then flushed by air.