THE TENDERNESS OF FRESH MEAT

The tenderness of meat ranks as the second most important criterion to ensure repeat customer purchase of meat, the first being attractive colour and appearance. A lack of tenderness in meat is caused by a combination of toughness in the fibre structure of meat and toughness in the connective tissue. The degree of toughness of meat can predominantly be linked to the age of the animal and to a small degree to the species of animal. Muscle tissue of older animals shows signs of an increased number of cross-links between actin and myosin as well as increased numbers of cross-links within collagen.

To obtain tender meat, a certain temperature – pH relationship should be adhered to post-slaughter; the pH value must be at or below 5.7 once a temperature of 7°C is present within meat. If the meat or carcass cools too quickly and the temperature is below 7°C, once a pH value of 5.7 is reached, there is a risk of cold shortening. To avoid cold shortening, another temperature – pH relationship to be observed is that the internal temperature of meat must not be below 14°C if the pH is still at 6.2 or above. Of course, a carcass must also not be cooled too slowly for microbiological reasons; microbial growth must be kept under control.

Ageing of meat for enhancing tenderness

Tenderness of meat is related to a combination of breakdown within muscle fibres, predominantly due to the activity of enzymes, and loosening of connective tissue, specifically collagen. Collagen in raw meat is usually loosened by the enzyme collagenase over a prolonged period of time but this process takes place only to a small degree as the action of collagenase is very slow and meat would be microbiologically spoiled before collagen would be significantly softened. Another method of softening collagen in raw meat is to place meat in a sour (acidic) soaking solution containing wine and/or vinegar under chilled conditions for 24–48 h as practised during the preparation of meat dishes in countries such as Germany (Sauerbraten). Collagen exposed to a sour environment starts to swell, thus taking up moisture and loosening the collagen structure is the result. As discussed, the toughness in meat is the combination of toughness in muscle and the toughness in connective tissue.

Naturally occurring enzymes in meat, predominantly cathepsins (which are cysteine proteases) tenderize meat by slowly breaking down the muscle fibres. Cathepsins are found in the lysosomes of a muscle cell, protected by a wall of fat, and over 30 different enzymes are involved. During rigor mortis, as a result of the formation of lactic acid and therefore the decrease in pH value, the walls of fat are destroyed by the impact of lactic acid, and cathepsins are released. Cathepsins work on the bonds between actin and myosin and contribute greatly to the tenderness of meat.

Calpains (calcium-activated proteases), which are present in the sarcoplasm is another type of enzyme naturally present in meat, contribute to the tenderization of meat in a different way; these enzymes cut along the Z lines and long fibres are ‘cut’ into smaller units. The level of enzyme activity is largely determined by the temperature that meat is stored under as well as the level of enzymes naturally present in meat. Calpains are known to work more effectively at higher pH values in meat such as 6.2–7.0, while cathepsins prefer a lower pH value around 5.4–5.9. This leads to the assumption that calpains are more important during the early stage of post-mortem glycolysis than are cathepsins, which seem to act later in the process. It is not fully understood yet the degree to which each enzyme ultimately contributes to overall tenderness, but it is thought that both types of enzyme, namely calpains and cathepsins, act synergistically towards enhancing the tenderness of meat.

Another contributing factor to the tenderness of meat is the loosening of the actomyosin complex. During maturing of meat, actin and myosin remain to a large degree cross-linked together as the actomyosin complex and are therefore responsible only to a small degree for the tenderness of meat. The impact of enzymes such as cathepsins and calpains on the toughness of collagen is insignificant.

During ageing, the visual appearance of meat does not change, given that breakdown of muscle fibres takes place on a microscopic level. Meat can age without being packed for up to 14 days depending on the microbiological status of the meat and the storage temperature. Vacuum-packed meat can age for up to 3 months as aerobic spoilage bacteria are kept away effectively.

The speed of tenderization varies dramatically between different species and ageing of beef requires a significantly longer time than pork and chicken in order to achieve a comparable degree of tenderness. The reason for this is that pork and chicken contain a significantly higher level of proteolytic enzymes such as cathepsins and tenderization occurs in pork and chicken at a significantly faster rate as a result. In order to obtain a tender piece of beef, stored between 0 and 3°C, at least 2 weeks are needed. A similar tenderizing effect can be seen in pork within 2–3 days at the same storage temperature. On the other hand, a comparable effect on tenderness in poultry can be seen within 1–2 days. Lamb lies somewhat in between beef and pork and generally requires around 7–10 days in order to become tender. The ‘speed of tenderization’ between 4 and 45°C is 85 times faster; meat exposed for 1 h to a temperature of 45°C would need to tenderize for 85 h at a temperature of 4°C in order to demonstrate similar tenderizing enzyme activity. This fact is often utilized in restaurants where chefs place raw meat, which is shortly to be prepared, in the oven at the above-mentioned temperatures to speed up tenderization.

The pH value of meat rises significantly during ageing owing to the formation of alkaline metabolic by-products of enzyme activity, which eventually lead to spoilage. Once the pH value exceeds 6.4–6.5, meat is spoiled from a sensorial point of view as high levels of ammonia (NH³) and other metabolic by-products are produced. Formation of slime is observed commonly at this stage and discolouration also takes place at this point.

If deboned meat is aged whilst vacuum packed, this has the benefit that anaerobic bacteria, such as Lactobacillus spp., have an advantage over aerobic spoilage bacteria. Because of the degree of acidity obtained within the vacuum-packed meat, the collagen swells, resulting in softening of the connective tissue. When meat is vacuum packed, contamination via handling is also avoided, extending the shelf life of the product.

Research has been carried out to investigate whether injecting a calcium chloride or enzyme solution into living animals (e.g. cattle) shortly before slaughter increases the tenderness of meat after slaughter. The results, however, were not convincing and the meat even exhibited a slight bitter taste after this treatment.

Beef can be tenderized by the application of the tender-stretch method: hanging the halves of carcasses straight after slaughter by the hip or pelvic bone. Gravity pulls down on both ends of the carcass and therefore the muscles cannot contract as severely as they would if the carcass were hung the conventional way, vertically from the Achilles tendon. Because of this counter-force, a reduced number of cross-links between actin are myosin are formed and the degree of toughness is reduced. However, the technique of hanging the carcass on the pelvic bone is not used as often as the technique of hanging on the Achilles tendon, given that the space needed per half of a carcass is significantly more than if the carcass is hung on the Achilles tendon. Cutting up a carcass hanging on its pelvic bone into a forequarter and a hindquarter is also more difficult compared with cutting up a carcass hanging on its Achilles tendon. Another occasionally practised method is to hang half of a carcass (cattle) on the pelvic bone first for several hours for the above-mentioned reasons and then to hang it on its Achilles tendon during further cooling and maturing.

Another means of tenderizing meat is steaking or scoring. Fine needles introduced into the muscle meat literally ‘cut’ fibres into smaller units. Such smaller units of muscle fibres require less shearing or chewing forces and the meat appears tender. Yet another means of tenderizing meat is to thaw frozen meat, as ice crystals are formed during freezing of meat. Ice crystals have sharp edges and also ‘cut’ fibres into smaller units, which enhances tenderness slightly.

Enzymes used for enhancing the tenderness of meat

Enzymes have been used to tenderize meat for hundreds of years. One of the most common enzymes applied is papain and the origin of this enzyme is papaya, a fruit grown in tropical countries. Papain is usually produced as a crude dried material by collecting the latex from the fruit of the papaya tree. This latex is further dried and purified and papain is sold in liquid and powdered form. The enzyme is totally inactivated at a temperature of 82°C and the optimum pH of papain is similar to that of meat itself. The application of papain has to be tightly controlled. If papain is left to tenderize meat for a long period of time, the result will be a structureless piece of meat with no texture and bite at all.

Other enzymes occasionally applied are ficin, from the milky juice of the fig tree, and bromelain. Bromelain originates from the stump or root of the pineapple plant after harvest of the fruit. The stump is peeled and crushed to extract the juice, which contains the enzyme. Bromelain is sold in powdered form. Contrary to papain, bromelain and ficin also enhance the tenderness of connective tissue, while papain focuses almost solely on the tenderness of meat. However, bromelain or ficin are hardly used. Ginger root also tenderizes meat but is not used commercially.

The major disadvantage of all these ‘natural’ tenderizers is that, if they are applied in a slight overdose, or if the tenderization process is too lengthy, the tenderizing effect becomes very strong and the entire fibre structure of the meat is destroyed. Meat exhibiting a mushy soft texture, with no or little bite, is the result of such excessive enzyme activity.

By Gerhard Feiner in "Meat Products Handbook - Practical Science and Technology", Woodhead Publishing Limited, Cambridge, England, 2005, excerpts p. 42-45. Adapted and illustrated to be posted by Leopoldo Costa.