Flesh_foods_ii.pdf

In the second of a series of four articles Peter Bayliss examines the meat quality aspect of texture. A background to the general structure and the rigor process is provided in “Chemistry in the kitchen: the chemistry of flesh foods I” (NFS, No. 1, January/February 1995).

Chemistry in the kitchen: the chemistry of flesh foods II

Consumer surveys reveal that meat texture (i.e. the degree of toughness or tenderness) is the most important eating quality attributed to meat. Texture is therefore ranked by consumers above the other two most desirable characteristics, which are colour and flavour. This article examines the major factors which influence meat texture: pre-slaughter, postslaughter, and processing factors.

Peter Bayliss

The author Peter Bayliss is Senior Lecturer in Food Science and Technology at Manchester Metropolitan University, Manchester, UK.

Pre-slaughter factors Age of the meat animal Traditionally, meat traders have held the erroneous belief that meat from older animals was tough because they had exercised more than their younger counterparts and therefore would have developed more connective tissue. A more appropriate explanation is provided by the development of “mature cross links” between adjacent collagen fibres in connective tissue. These mature cross links are heat stable and are not easily broken down during heat processing. Hence, when meat is cooked, the collagen fibres do not hydrolyse into liquid gelatine but remain within the meat causing toughness. This increase in texture is referred to as “background toughness”, and the actual chemical nature of the mature cross link has not yet been characterized. This explanation is illustrated by quantification of collagen, the principal component of connective tissue. Veal (meat from young cattle, about 12 weeks of age) has a greater proportion of collagen compared with beef from an 18-month old bullock, and yet veal is never tough, whereas beef often can be. Moreover, pork has a greater amount of connective tissue than all the other meat species but is usually tender. This gives rise to the expression that it is the “quality of the collagen and not the quantity” that is responsible for the toughness of meat.

Abstract The meat quality attribute of texture is ranked as the most important by consumers. Details the mechanism of the major causes of meat toughness. Examines pre-slaughter, post-slaughter and processing factors that influence the texture of meat.

Variability in the connective tissue content of muscles Each muscle in an animal’s body has a particular physiological function (i.e. locomotion, support, etc.) and differs in the amount of

Nutrition & Food Science Number 2 · March/April 1995 · pp. 21–26 © MCB University Press · ISSN 0034-6659

21

The chemistry of flesh foods II

Nutrition & Food Science

Peter Bayliss

Number 2 · March/April 1995 · 21–26

connective tissue it contains. This gives rise to varying texture profiles for differing muscles. The amino acid hydroxyproline is unique to collagen and it can be ascertained from Table I that collagen is abundant in shin beef, making it tough (low-grade meat, normally moistly cooked). In comparison, tender fillet steak (high-grade meat, normally cooked by grilling) has a low collagen content. The silverside joint of beef (M. semitendinosus) is comparatively tough, and traditionally slow roasted or pickled. This muscle is unique in that up to 37 per cent of its connective tissue is made up of the other connective tissue protein elastin that does not hydrolyse on cooking.

Figure 1 Myofibrillar filament lattice illustrating sarcomere I band Z-line

Sarcomere

Figure 2 Influence of sarcomere length on toughness

Shear force (arbitrary units)

160

100 80 60 40 10

20 30 40 50 Percentage shortening

60

70

mere reaches 40 per cent of its resting length, meat toughness reaches a maximum. A possible explanation for this phenomenon is that the majority of myosin heads will be attached to the actin filaments. Increases and decreases of sarcomere length either side of 40 per cent result in less myosin cross bridge attachment and ensuing tenderness. Cold toughening Low temperatures during rigor mortis cause the sarcomere to shorten and produce tough meat. This is known as cold toughening (or cold shortening) and is predisposed by chilling the carcass after slaughter too early or too quickly. More precisely, if the carcass is chilled to below 10°C within ten hours of slaughter while ATP is still present in the muscle fibres, the sarcomeres will excessively shorten (see Figure 3). Sheep meat (particularly loin chops) is most commonly cold toughened owing to the comparatively small carcass mass that cools quickly, while in beef it tends to occur in superficial muscles, especially the Longissimus dorsi muscle of the sirloin. Cold toughening is not common in pigs and poultry (except turkeys) owing to the rapid post mortem glycolytic rate of their muscle. Although the actual physiology of cold toughening is not well understood, it is generally accepted that it is a result of low

Table I Connective tissue content of two retail cuts of beef

Collagen (hydroxyproline mg/g)

Psoas major Superficial digital flexor

120

0

Sarcomere length As illustrated in Figure 1, the length of the sarcomere is the measurement between adjacent z-lines. At the end of rigor, the “resting length” of the sarcomere is 2.1µm. Reduction of this length causes meat to become tough. Although not well understood, the increase in toughness associated with shortened muscle fibres is thought to be due to the connective tissue sheath that surrounds the myofibril, the endomysium, which becomes more relaxed. On chewing there is an increased amount of endomysium to shear (due to bunching and thickening) providing the sensation of increased toughness. Sarcomeres shorten as a result of “cold toughening” or being unrestrained during rigor mortis , or a combination of both. It can be seen from Figure 2 that when the sarco-

Fillet steak Hind shin

Maximum toughness

140

20

Post-slaughtered factors

Muscle

I band Z-line Myosin Actin

Husbandry and breed Meat from rapidly grown animals tends to be comparatively tender. Late-maturing breeds of cattle (e.g. large continental beef breeds such as Charollais) that are fed on high protein diets are slaughtered at an earlier age, hence they have muscles with fewer mature cross links. In addition, the larger muscles provide a high muscle-fibre volume to a low endomysium surface area ratio (hence less connective tissue).

Cut of meat

A band

350 1,430 22

The chemistry of flesh foods II

Nutrition & Food Science

Peter Bayliss

Number 2 · March/April 1995 · 21–26

Figure 3 Shortening of muscle at various temperatures during rigor

Percentage shortening

50 Cold toughening

40

Hot toughening

30

20

10 0

10

20 30 Temperature ˚C

40

50

temperatures rendering Ca++ pumps ineffective, that in turn trigger sarcomere shortening. A method of overcoming cold toughening is by electrically stimulating the carcass. An electric current is passed through the animal’s carcass after it has been humanely slaughtered and prior to the commencement of rigor to use up ATP. The carcass then enters into rigor mortis more quickly and rapid chilling regimes will not produce toughened meat. Carcass hanging method If muscles are physically prevented from shortening during rigor, the meat will be more tender. Some muscles are prevented from shortening by being firmly attached by their insertion ligaments on to bones, e.g. fillet steak (psoas muscle). Normally, carcasses are suspended by a hook passed through the Achilles tendon at the distal part of the hind limb (see Figure 4). In this position muscles located on the dorsal aspects are not held taut and are free to shorten during rigor. These muscles include groups within the expensive retail cut of “topside of beef ”.

The UK meat industry has been slow to take up posture hanging of the carcass, but many of the major multiples are now including it (along with electrical stimulation) in their raw material specifications. One method developed in the USA (Tenderstretch) suspends the carcass from the pelvic bone, allowing the hind limb to bend into a standing position (see Figure 5). This has the effect of stretching the muscles categorized in the topside, silverside and down through part of the sirloin. This hanging method does not affect the muscles of the fore quarter. It also allows the psoas muscle to relax so that the fillet steak can, on occasions, be slightly tougher than normal. New Zealand researchers found that if the carcass was suspended from the pelvic bone and the front and hind legs were attached so that the carcass was in a more “natural standing position”, a net increase in tenderization in both hind and forequarter joints of meat occurred. Ultimate post mortem acidity levels The ultimate pH (pHu) of the carcass affects the water-holding capacity (WHC) of the meat. Muscle consists of about 85 per cent water that is held in the myofibril surrounding the thick and thin filaments. Losses or gains in water results in the myofibrils shrinking and swelling. Generally, meat with a high WHC will have less muscle fibre per unit area and therefore when chewed will be perceived as being more tender by the consumer (and more “juicy”). This is illustrated in Figure 6 where tenderness decreases until pH 6.0 and then increases again as the pH rises and falls. The pH of meat The isoelectric point of the myofibrillar proteins is pH c. 5.0. This is when the myofibrillar proteins will have a net zero charge.

Figure 4 Normal method of carcass suspension Figure 5 Methods of carcass posture hanging

Carcass suspended from Achilles tendon Muscle relaxation

USA method Muscle tension

New Zealand method

= Direction of muscle tension

23

The chemistry of flesh foods II

Nutrition & Food Science

Peter Bayliss

Number 2 · March/April 1995 · 21–26

cally, that little degradation of the connective tissue proteins occur, while in the myofibril there are gaps at the I-Z junctions and fracture sites around the Z-lines (see Figure 7). From Figure 8 it can be ascertained that conditioning has three phases, consisting of A pre-rigor, B rigor and C post-rigor. Prior to rigor commencing, meat is comparatively tender (position A). However, if meat is cooked prior to attainment of the ultimate pH, toughening occurs owing to “hot shortening”. Toughness increases as rigor ensues until it peaks at C, when the myosin heads have formed permanent cross bridges. During the time indicated by B there is a gradual increase in tenderness as proteolysis occurs. This period is generally referred to as the conditioning period. It should be noted that meat is tougher in the first eight days than it is in the pre-rigor period. Normally, meat is conditioned or tenderized in the carcass form or in vacuum packs, both under chilled storage conditions so as to maintain the cold chain.

Figure 6 Relationship between tenderness and pHu

Mean tenderness score

6

5

4

3 5.4

5.6

5.8

6.0 pH

6.2

6.4

6.6

Increases or decreases in pH either side of this value cause an increase in the meat’s WHC. Food animals with a low level of glycogen at the time of slaughter will produce meat that has a high pH and will have an increased WHC. Moreover, meat with a high water-holding capacity loses little exudate or “drip”. Influences affecting the water-holding capacity of meat The drop in pH accompanying rigor results in the filament lattice shrinking in a transverse direction as the isoelectric point of myosin is approached at pH 5.0. At this value, negative or positive charges on the thin and thick filaments are at a minimum. This shrinkage has the effect of putting pressure on the sarcoplasm and if the muscle fibres are cut the fluid is forced out under pressure as drip at the cut surface. High pH conditions result in an increased donation of negative ions to the filament lattice, producing increased electrostatic repulsion and swelling of the filament lattice in a transverse direction so that the compartment in which water is held is increased. The manufacturers of meat products take advantage of this effect by using polyphosphate in their products. Polyphosphate has a pH c. 9.0 and is often added to produce alkaline conditions to facilitate increased take-up of water, while in Germany some manufacturers now use pre-rigor meat that is high in pH.

Proteolytic enzymes involved in conditioning All organic matter undergoes “autolysis” after death (i.e. enzymic tissue disintegration). Game animals, for example, are traditionally

Figure 7 Tensile testing of conditioned muscle fibre Muscle fibre

Fracture line

Meat fibres with long axis parallel to the fibre direction are pulled apart until they break

Tension direction

Figure 8 Changes in meat texture during storage (<4°C) A

C

B

60 Shear force (or toughness)

50

Conditioning of meat The final conversion of muscle into meat occurs after rigor when the meat is conditioned or “hung”. Although the subject of intensive research, the actual physiology as to how meat becomes tender has not been elucidated. It has been found in conditioned muscle fibres which are pulled apart mechani-

40 30 20 10 0 1

24

2

3

4 5 Time (days)

6

7

8

The chemistry of flesh foods II

Nutrition & Food Science

Peter Bayliss

Number 2 · March/April 1995 · 21–26

held for an extended period in order to improve texture and develop a “gamey” flavour, e.g. pheasants with their viscera intact, can be “hung” for up to c. 28 days. Two enzymes have been identified as being involved in the conditioning process, i.e. proteolysis of the myofibrillar and cyto skeletal proteins. The basic proteolytic enzyme systems involved are the calcium-activated neutral proteinases (calpains I and II) and the lysosomal acidic proteinases (cathepsins B, D and L). For optimum proteolytic activity, calpain I and II require 1-2mM and 50-20µM Ca++ respectively, and degrade myofibrillar and cytoskeletal proteins. Cathepsins from the so-called “suicide bags” present in the muscle fibre have an optimum pH activity of 3-4, hydrolysing myofibril and isolated proteins.

• liquefaction of fat; • shrinkage due to loss of sarcoplasmic fluid (and fat); • flavour development; • reduction/elimination of micro-organisms; • development of cooked meat colour. It has now been established that there are two distinct phases during cooking that are common to both methods and a further third phase with a wet cooking method (see Figure 9). Each muscle protein has a particular temperature at which it denatures. Phase one: 40-50°C During this initial phase there is an increase in toughness due to the denaturation of myosin. This is a structural change involving the rearrangement of the myosin protein chains. The result is a net shrinkage of the muscle fibre in the transverse direction. Collagen is unaffected at this temperature. Illustrated in Figures 10 and 11 is the denaturation of the myosin in the muscle fibre that results in shrinkage and an increased widening of the annular channel first formed during rigor. The collagen fibre network in the endomysium exerts pressure on the sarcoplasmic fluid within the channel, forcing it out of the cut ends of the muscle fibres. This loss of drip results in: • shrinkage at the joint of meat in a transverse direction to its grain; and • toughening of the meat.

Summary of enzymic action involved with meat tenderization It is hypothesized that cathepsins are the most important enzymes involved in meat conditioning, even though they mainly degrade actin and myosin. Calpains start the degradation of proteins during the commencement of rigor, and at a pHu of c. 5.5 become inhibited by the acidic conditions. Cathepsins then become activated and continue to be so throughout the conditioning period. Traditional butchers hang their meat in carcass form for two or three weeks while most large multiples specify ten days (80 per cent of maximum tenderization) in wholesale cuts that are vacuum packed.

Figure 9 Effect of heat on meat texture 70

Processing factors Shear force (arbitrary units)

Phase 3

Enzyme treatment of meat using tenderizing enzymes Meat can be artificially tenderized by treatment with proteolytic enzymes such as papain (paw paw plant), bromelin (pineapples), ficin (figs) and bacterial and fungal enzymes. The enzyme can be dissolved or used in a powder form to dust, dip or spray cuts of meat such as steaks, etc. Larger primal joints of meat usually have the enzyme injected in.

60

Phase 2

50 40 Phase 1

30 20 10 0 0

20

40 60 80 Cooking temperature (ºC)

100

120

Figure 10 Muscle fibre illustrating shrinkage effects of myosin (longitudinal section)

Heat processing of meat Certain cooking methods and cooking regimes can result in an increase in meat toughness. The effects of heat on meat are: • degradation of proteins, e.g. solubilization of collagen and coagulation of myosin; • changes in toughness and eventual tenderization;

Drip loss Endomysium Muscle fibre

Longitudinal section

25

The chemistry of flesh foods II

Nutrition & Food Science

Peter Bayliss

Number 2 · March/April 1995 · 21–26

In the collagen of meat derived from older animals the heat-stable “mature cross links” will have developed. The collagen in meat from younger animals containing heat labile cross links is solubilized to gelatine during cooking, making the meat tender. In collagen fibres containing a high proportion of heatstable cross links, heating results in derangement of the fibres, but tenderness does not ensue owing to the formation of covalent cross links that result in greater shrinkage, drip loss and toughness (see Figure 14).

Figure 11 Muscle fibre illustrating shrinkage effects of myosin (transverse section)

Heat-induced shrinkage of myosin increases diameter of annular channel

Transverse section

Phase three: > 80°C (wet cooking) Wet cooking procedures result in slow tenderization that increases with time. This slow hydrolysis is thought to be a result of breaking heat-stable cross links and does not involve any shrinkage. During the solubilization, collagen fragments are released into the surrounding cooking solution. The wet cooking method facilitates hydrolysis of the collagen and because of its high heat coefficient is more thorough in heat transfer. Hence poor quality cuts of meat (high collagen content, e.g. shin) are wet cooked or “stewed”.

Phase two: 60-70°C During this temperature increment a second toughening occurs with a net shrinkage in the longitudinal direction. This is due to: • a further denaturation of myosin resulting in a longitudinal shrinkage; and • denaturation of collagen in the endomysium with the shrinkage in the tranverse and longitudinal direction. Illustrated in Figures 12 and 13 is the endomysial collagen arranged in a laminated cross ply, hence the shrinkage direction. The net overall shrinkage direction with cooking phase-one and two is in a longitudinal direction, but in order, physically, to reduce further in size, the fluid or drip has to be squeezed out.

Mechanical processing methods All size reduction operations will have the effect of increasing the tenderness of the meat, e.g. bowl chopping, mincing, flaking, etc. Some chefs advocate the “hammering” of grilling steak or braising steak using a culinary bat. Tenderization machines are available that lacerate the surface of poorer quality steaklike slices of meat to upgrade them. Normally meat is cut or carved at right angles to the longitudinal grain of the muscle so that it fragments more easily when chewed.

Figure 12 Shrinkage of endomysial collagen fibres (longitudinal section)

Direction of shrinkage

Longitudinal section Figure 14 Heat-induced rearrangement of mature cross links in collagen Figure 13 Shrinkage of endomysial collagen fibres (transverse section) Heat-stable cross links maintain continuity of the collagen, even when dearranged by heat

Direction of shrinkage

Linear structure

Transverse section

26

Heat

Random structure which shrinks to 14 of its original length