Science of Slow Cooking
--Of all the attributes of eating quality, tenderness is rated the most important factor affecting beef palatability--
Slow cooked meals are generally easier to make and very cost effective using cuts of meat that improve in texture and flavor when cooked for long periods of time at low temperatures. These tough cuts of meat contain large amounts of collagen which require long cooking times to break down into a rich gelatin.
HOW DOES SLOW COOKING WORK?
When you cook, collagen begins to melt at about 160F and turns to a rich liquid, gelatin. This gives meat a lot of flavor and a wonderful silky texture. When cooking it is important to liquify collagen.
Denaturation of the collagen molecule is a kinetic process, and hence a function of both temperature and duration of heating. Cooking at low temperatures require long periods of time to liquify collagen.
COOKING MEAT TEMPERATURES
105F/40C - 122F/50C --Calpains begin to denature and lose activity till around 105F, cathepsains at 122F. Since enzyme activity increases up to those temperatures, slow cooking can provide a significant aging effect during cooking. Meat should however be quickly seared or blanched first to kill surface microbes.
120°F/50°C -- Meat develops a white opacity as heat sensitive myosin denatures. Coagulation produces large enough clumps to scatter light. Red meat turns pink.
Rare Meats: 120°F/50°C is the early stages of juiciness in meats as the the protein myosin, begins to coagulate . This lends each cell some solidity and the meat some firmness. As the myosin molecules bond to each other they begin to squeeze out water molecules that separated them. Water then collects around the solidifyed protein core and is squeezed out of the cell by connective tissue. At this temperature meat is considered rare and when sliced juices will break through weak spots in the connective tissue
140°F/60°C -- Red myoglobin begins to denature into tan colored hemichrome. Meat turns from pink to brown-grey color.
140°F/60°C -- Meat suddely releases lots of juice, shrinks noticebly, and becomes chewy as a result of collagen denaturing which squeezes out liquids.
Medium -- Well Meats: Collagen shrinks as the meat tmeperature rises to 140/60 more of the protein coagulates and cells become more seggregated into a solid core and surrounding liquid as the meat gets progressively firmer and moister. At 140-150 the meat suddenly releases lots of juices, shrinks noticeably and becomes chewier as a result of collagen shrinkage. Meat served at this temperature is considered medium and begins to change from juicy to dry.
160°F/70°C -- Connective tissue collagen begins to dissolve to gelatin. Melting of collagen starts to accelerate at 160F and continues rapidly up to 180F.
Well Done Slow Cooked Meats: Falling apart tenderness collagen turns to gelatin at 160/70. The meat gets dryer, but at 160F the connective tissues containing collagen begins to dissolve into gelatin. With time muscle fibers that had been held tightly together begin to easily spread apart. Although the fibers are still very stiff and dry the meat appears more tender since the gelatins provide succulence.
NOTES: At 140°F changes are caused by the denaturing of collagen in the cells. Meat served at this temperature med-rare is changing from juicy to dry. At 160°F/ 70°C connective tissue collagen begins to dissolve to gelatin. This however is a very lengthy process. The fibers are still stiff and dry but meat seems more tender. Source: Harold McGee -- On Food and Cooking
Anatomy of muscle fiber
|A muscle is completely enclosed by a thick sheath of connective tissue (the epimysium) and is divided into bundles of fibres by a connective tissue network (perimysium). Individual muscle fibres are bounded by a plasma membrene surrounded by connective tissue (endomysium) which consists of a basement membrane surrounded by a reticular later in which a meshwork of fine collage fibrils is embedded in a matrix. Tendons are elastic collagenous tissues.|
Scattered among the muscle fibers are fat cells which store energy for the muscles. Fat is crucial to meat texture. Waxy when it is cold, fat does not evaporate when you are cooking as does water. It melts and lubricates the fibers as they are getting tougher under the heat. Fat is also the source of much of the flavor in meat. As the animal ages the flavor compounds build up and get stronger. After the animal is slaughtered, the fat can turn rancid if stored improperly or too long.
THE CHALLENGE IN COOKING MEAT
We like our meat tender and juicy at the same time...
We therefore want our meat to be cooked tender where tough collagen is converted to gelatin but with a minimum loss of moisture. The reality is that these methods are contracdictory and hence the challenge or dilemma to cooking meats. To minimize moisture loss requires temperatures less than 130F, however .turning collagen into gelatin requires temperatures above 160F and for extended time periods. As moisture evaporates, the meat begins to shrink. A slab can lose 20% or more of its weight in cooking due to shrinkage. Even meat cooked in liquid will dry out although not as quickly. So we are faced with a dilemma. To liquefy the collagen we need to cook the meat to 180F and hold it there for for long periods of time. But by then it is well past well-done and the muscle fibers can be dryed out. As a result, we need to add moisture.
How to slow loss of moisture
Brining. Brining adds a significant amount of moisture, it helps retain moisture during cooking, contributes noticeable flavor enhancements.
Steaming. Another method of adding moisture is to cook the meat in very high humidity by wrapping it in foil with a little water or juice. This keeps moisture from escaping and some vapors penetrate the meat.
Braising or poaching (--low temperatures--). Braising is a method of cooking by submerging the meat in hot liquid, but not hot enough to boil. Braising can give you juicy, tender, and flavorful meat, especially if you use a flavorful braising liquid. But it tends to pull all the collagen out and rob the meat of its natural flavor. Flavor the liquid (water with pickling spices is a nice simple start), completely submerge the slab, keep the lid off, keep the temp down to about 160-180F for about 30 minutes, and let the meat cool in the liquid for 20-30 minutes so it will absorb some of the water before putting it on the grill.
Breakage of collagen covalent links using Acids -- (Tenderizing meats with acid) -- It is well known that adding a little vinegar to a stock will help tenderize meat while cooking. It is also useful to marinate meat for a few hours using vinegar to tenderize meat. Offer and Knight (1988) suggested that one of the mechanisms of pH induced tenderisation of meat could be a breakage of covalent collagen cross-links and of some specific peptide bonds.
Here are tips to keep in mind when slow-low roasting:
- Develop a caramelized crust before slow cooking -- by searing the meat either in a dry pan or with a small amount of oil or fat.
- Place the meat or roast fat side up in the pan so it self-bastes.
- Tenderize your cuts of meat --e..g, pounding meat, buying aged meats (Note: meats cooked longer a 120F will age and be more tender), marinading meats with acids with tenderize the meat.
- Tent the resting meat with foil and allow 10 to 15 minutes before cutting it so the meat's juices will return to the center; slice the meat against the grain.
Review: Collagen contribution to meat toughness: Theoretical aspects Jacques Lepetit ..Meat Science 80 (2008) 960–967
Offer, G., & Knight, P. (1988). The structural basis of water-holding in meat. Part 1: General principles and water uptake in meat processing. Developments in Meat Science, 4, 63–171.