CHAP17

17 Cooking Meat

17.1 Why is meat cooking included in this course?

Cooking meat is extremely important - although not many animal scientists can see this. Animal scientists help farmers produce meat. But there is no point in producing meat unless it can be sold. Meat consumption depends on consumers knowing how to cook meat. Educating our customers so they do not ruin the quality of the fresh meat we sell them is extremely important for the future of the whole meat industry.
Restaurants and fast-food outlets already have a legal responsibility for meat cooking and, with the general trend to make institutions responsible for things that once were the responsibility of the individual, the burden may one day be placed on the meat industry to make a demonstrable attempt to ensure all meat is cooked appropriately by our customers.
A preventive approach to problems tends to be more effective than a retroactive salvage operation, and we should now be starting to educate more of our customers. What better place to start than with you students?
In the domestic kitchen, many meat eaters already are overcooking their meat. If the trend continues, driven by a growing concern with microbial contamination of meat, it will further increase customer dissatisfaction with prime cuts of meat, because they are expensive yet toughened by overcooking. Other pressures are pushing our customers in this direction as well, for example, PSE pork. An exudate from PSE pork coagulating on the meat surface during cooking encourages overcooking, until the meat surface looks dry, at which point the pork is overcooked and tough.

17.2 The basics

Cooking kills bacteria and parasites. Bacteria are everywhere. Parasites are rare. Most of the bacteria are on the surface of the meat and are soon killed by cooking unless the meat surface has been mixed deep into the meat - HAMBURGERS MUST BE COOKED ALL THE WAY THROUGH!
Cooking changes and strengthens the taste and aroma of meat. Raw meat has a rather subtle taste - consumers who like their steaks rare are really going for the texture. The strongest taste is found on the outsides of roasts.
Cooking changes the colour of meat - typically from red to brown. Consumers who like rare meat will want to see red, while those who like well-cooked meat will want to see brown. But we are all really using colour to guess what temperature the meat has reached during cooking.
Cooking changes the texture of meat. The key point many consumers are missing is - COOKING MAKES TENDER MEAT TOUGH whereas COOKING MAKES TOUGH MEAT TENDER. Why?

17.3 Measuring temperature

Hopefully there is no need to explain how ordinary thermometers work in the home, but we use some other methods industrially.

A thermocouple is composed of two dissimilar electrical conductors joined at their ends to generate a thermoelectric voltage directly proportional to the temperature difference between the two end junctions. Iron-constantan and chromel-alumel thermocouples are the most common examples. Junctions may be exposed or covered, and/or grounded.
A thermistor is a resistive circuit component, usually a two-terminal semiconductor, whose resistance decreases as temperature increases.
A Callendar's thermometer (platinum RTD probe) uses a thin film or wire of pure platinum, which has a relatively high resistance (for a metal), thus facilitating the measurement of the change in resistance with temperature. In metals, resistance increases as temperature increases.
Microwave cooking is monitored using a quartz optical fibre with a fluorescent tip. Typically, the fluorescence spectrum changes with the temperature and is determined from optical measurements at two wavelengths.
Everything emits infrared. The hotter the meat, the more active its molecules are, and the more infrared energy it emits. An infrared thermometer detects this radiant energy

In practice, things to watch out for are the time constant, the length of time the sensor takes to read the true temperature of the sample, and heat conduction along the sensor, its wire connections, and its protective shielding. For example, when monitoring the cooking of a roast, an unsuitable temperature probe may pick up heat from the air in the oven and conduct it into the roast faster than the heat can move through the meat. Thus, colour changes in the interior of the roast may take place first along the route of the temperature probe.

17.4 Colour changes

Meat with an appreciable myoglobin concentration changes from red to grey or greyish-brown when cooked. The brown pigments formed during cooking may include

denatured globin nicotinamide hemichromes,
denatured myoglobin,
Maillard reaction products,
metmyochromogen and
haematin di-imadazole complexes.

The spectrum above shows the change in reflectance as beef is cooked. Raw beef has a characteristic dimple around 560 nm which indicates myoglobin. Much of the green light is absorbed and the meat appears red. After cooking, there is a large increase in the reflectance of green light. This is the major cause of the meat appearing brown when cooked.

Something else occurs, as shown in the spectrum above for the cooking of lamb. The reflectance units are not directly comparable (they depend on the apparatus and how it is standardized against a 100% white object). Also, the myoglobin dimple is missing in the raw sample - because metmyoglobin formation has started. The major change between raw and cooked lamb is a massive increase in reflectance at all wavelengths - thus, the cooked lamb is scattering more light than the raw meat. In the CIE system this appears as a large increase in %Y. This increase in scattering is clearly visible because the myoglobin level is lower in lamb than in beef. What happens when egg albumen is cooked?

Colour changes during cooking are important for a variety of reasons. Cooking protect us against trichinosis and potentially deadly bacteria, but overcooking beyond the safe point may render meat unpalatable. In most practical situations the safe point is judged not by temperature, but by colour, which is why failure to attain a cooked meat colour even after reaching a safe temperature is a problem - partcularly in poultry meat.

There are numerous reasons proper cooking of meat may fail to produce a brown colour. Nitrites act on myoglobin to form a heat-stable pink pigment (often call dinitrosylhaemochrome). Nitrites are sometimes found in high levels in well-water and spices (for example, in dried celery powder). Carbon monoxide forms a heat-stable pink pigment from myoglobin. Animal exposed to vehicle exhaust gases during transport may produce meat which will not brown properly when cooked. When cooking, gas ovens may produce nitrous oxides leading to surface formation of dinitrosylhaemochrome. The surface of meat cooked with bacon (residual nitrites present) may fail to brown properly.

17.5 Effect of cooking on collagen

Cooking has major effects on the connective tissue of meat. Firstly, cooking causes collagen fibres to contract (this decrease in length has NO connection to muscle fibre contraction!).

The diagram below explains why fluid is released from meat as it is cooked.

Secondly, further cooking causes the gelatinization of collagen fibres. In other words, collagen fibres are converted from structures like miniature steel cables to jelly.

The diagram below explains the weakest point through the connective tissue of cooked meat.

The key point? Cuts of meat with a high connective tissue content must be cooked with moist heat for an extended time - for example, stewed or casseroled. This takes time and effort - so the meat is less expensive than cuts with a low connective tissue content which can be barbecued. However, cuts of meat with a high connective tissue content usually have an excellent taste. If they are put through a meat grinder (technical term - ), the connective tissue toughness is cancelled and, as you all know, hamburgers can be barbecued! So, in the summer, tough beef gets used for hamburger while, in the winter, more of the tough beef is sold intact for stewing.

The bright, enquiring mind of a good student might ask at what temperatures do collagen contraction and gelatinization occur? Are there breed effects? Which breed has the lowest gelatinization temperature? Are there nutritional effects? What feeding causes the lowest collagen gelatinization temperatures? How does animal age affect all this? Sadly, because animal scientists tend to ignore cooking, and food scientists tend to ignore animal science - we do not know. Observations on the Swatland family Sunday roast show considerable variability. Contraction starts before gelatinization, and both are somewhere between 60 and 70º C. Despite the millions spent on agricultural research across Canada, it is no wonder we all get annoyed by tough meat now and again!

17.6 Effect of cooking on myofibrils

Cooking makes myofibrils stronger. The longer we cook myofibrils, the tougher they get. Cooking cross-links the proteins. If sarcomere length is short, there is more cross-linking and cooked sracomeres are very strong - TOUGH MEAT. So, take a steak like beef filet with a very low connective tissue content. The longer you cook it, the tougher it gets. Take a steak like a cross-cut arm roast of beef with quite a high connective tissue content. Cooking makes all the myofibrils tougher, but reduces the strength of connective tissue. Thus, the overall effect is - the longer the cooking, the more tender the meat.

17.7 Objective measurement of meat toughness

The best method of quantify meat toughness is sensory evaluation. Cooked meat is tested under standardized conditions by typical consumers or a trained group of experts. The results are obtained by asking questions - rate the meat tenderness on a scale of, say, from -5 (very tough), through 0 (undecided), to +5 (very tender). But this is a lengthy and expensive process. Objective methods are used more commonly in the meat industry.

Method A. The cooked meat sample is compressed between two blades. Blunt blades are preferred because sharp blades cannot be kept in a constant state of sharpness.

Method B. The is the most commonly used method. It is called the Warner-Bratzler test after the two scientists who first invented it many years ago. The meat sample is restrained in the triangular opening (with squared edges) of a metal blade.

Method C. Needles are pushed into the meat.

Method D. The force needed to push the meat through a meat grinder is found electrically.

Methods E and F. The tensile strength of the meat is measured - but it is difficult to get hold of the sample without altering its properties. Glue is used in method F.

Meat tenderness is studied by rheology (the study of flowing systems, whether they be meat or concrete). The forces used to rupture the meat are measured in Newtons. Although often called shear tests - this is incorrect. Shearing only occurs in rigid materials. When meat fails, either between your teeth or in rheological apparatus, the ultimate failure is the tensile strength of either connective tissues or myofibrils.

Further information

Structure and Development of Meat Animals and Poultry. Pages 255-269.