Is it influenced by diet?

On average, only 11 percent of Irish beef is consumed on the home market. Of the 89% of Irish beef that was exported in 1998, 25% went to EU markets (not including UK), while 58% went to other international markets. According to An Bord Bia, "the future for Irish beef lies in developing sustainable, higher value markets within the EU while retaining a strong presence in international markets". The need to meet specific criteria in different markets has increased the awareness of the concept of "quality" in the beef industry. However, "quality" and in particular beef quality, is likely to be defined differently by exporters, retailers or purchasers and consumers. The processor may perceive beef quality as reflecting carcass weight, fatness/conformation grade, lean meat yield and the timeliness of carcass supply in relation to market requirements. Cut size, meat colour, fat colour, marbling, texture appearance and the healthiness and/or safety aspects of the meat influence the retailer/ purchaser. The view from those exporting Irish beef is that much of the beef lacks conformation, is over-fat, is from cattle that were too old at slaughter, has muscle that is too red, has fat that is too yellow and is too variable in the above characteristics and in pattern of supply (Horgan, 1999). In addition, for beef that is destined for the hotel, restaurant and institution trade, the consumer may not be involved with the purchase and only experiences the cooked product, when tenderness, juiciness and aroma/flavour become the most important quality criteria. Indeed, recent information from abroad indicates that while consumers highly rated, cooked, marbled steaks, they would not have chosen similar steaks in the supermarket because they were too fat!

Nutrition has a relatively minor influence on the conformation, age or pattern of supply of Irish beef but can have a large influence on its appearance, composition and eating quality. This paper firstly offers a brief description of compositional and quality attributes of beef that are relevant to EU markets/consumers before summarising research results pertaining to the effects of nutrition on these quality attributes.

Composition

The chemical composition of meat has a significant impact on quality attributes and can be measured in the laboratory. The intramuscular fat concentration or "marbling" is the term given to fat, which is deposited within the muscle bundles. This is an important issue in United States and Asian markets where marbling is highly valued. In contrast, beef for EU markets should have little visible marbling. While beef fat contains a higher proportion of saturated fatty acids than plant foods, it also contains fat molecules, which have beneficial effects on human health. The medical profession now recognises that some mono-unsaturated fatty acids, (i.e. fatty acids with one double bond in the chain of carbon atoms), are protective against heart disease; that longer chain polyunsaturated fatty acids, in particular those with the first double bond at the omega-3 position, such as those found in oily fish, are anti-thrombogenic (helps prevents clotting); and that conjugated linoleic acid, an isomer of linoleic acid, is protective against cancer, obesity and heart disease. These compounds are found in beef fat in varying amounts.

Meat and fat colour

The colour of meat is related to the level of pigmentation (myoglobin) present in the muscle. When beef is cut the myoglobin oxidises, giving rise to a bright red colour and a process known as "blooming". If beef is left exposed to air for a prolonged period, its colour changes slowly to brown due to conversion of myoglobin to metmyoglobin. Redness (myoglobin concentration) increases as an animal matures and with exercise. A high level of pre-slaughter stress can lead to a rise in pH, which can result in dark coloured beef.

Beef with yellow fat is not acceptable on most EU markets and may be perceived by the distributor/consumer as being from old or diseased animals. The major cause of yellow fat is the intake of the yellow carotenoid pigments, especially b -carotene, which can be metabolised to vitamin A (a vitamin essential to many body processes). Excess b -carotene is stored in fat, giving rise to a yellow-coloured fat. Grass is the major source of carotenoid pigments. Evidence from studies on fat colour carried out at Grange Research Centre suggest that carotenoid levels are also higher in grass silage than concentrates (see later).

Quality attributes such as conformation/fatness or colour can be subjectively estimated by trained assessors. Colour is objectively measured using instruments that give numerical values for "lightness", "redness" and "yellowness".

The eating quality of meat is assessed ultimately by the consumer. The texture of meat can be defined as the sensory manifestation of the structure of meat and the manner in which this structure reacts to the force applied during biting and the specific senses involved in eating. It is how meat feels in the mouth during manipulation and mastication. Attributes of meat which are part of the eating sensation do not lend themselves easily to objective measurement and taste panels, either a trained panel, common in scientific studies, or a panel of "typical consumers" are used. Meat is prepared under standardised cooking conditions and the members of the panel are typically asked to score arithmetically the sensations of texture, tenderness, juiciness, firmness, chewiness, flavour and overall acceptability. The expectations of members of a taste panel, in particular a consumer taste panel, are conditioned by their experience/culture. An example of this was when a Spanish taste panel rated Spanish lamb more highly for flavour and overall acceptability than British lamb. When the same samples were offered to a British panel, the British lamb was rated more highly.

Tenderness

Tenderness is considered by American consumers to be the most important component of meat quality. There are no similar data for the perceptions of Irish consumers on the relative importance of the different aspects of beef quality. Tenderness can be measured using an instrument in which a blade is forced through (shears) a piece of meat of fixed dimensions that has been cooked following a standard procedure. The force required is measured and this is taken as a measurement of the toughness of the meat. This measurement is sometimes referred to as the Warner Bratzler shear force after its developers. There are two main components to meat tenderness, a myofibrillar (muscle) component and a connective tissue (collagen) component. The size of the muscle fibres increase with increasing age and may be tougher. Muscle fibre toughness can be minimised by good animal handling prior to slaughter, the use of electrical stimulation, the handling procedures used in the factory, appropriate carcass chilling practices, and by ageing post slaughter.

Factors that influence the connective tissue component of toughness have been less well defined. Fibres of collagen are the major elements of connective tissue. As an animal ages, there is an increase in heat stable collagen cross-links and a resulting reduction in the amount of soluble collagen, rather than an increase in the amount of connective tissue present. Connective tissue content varies from muscle to muscle depending upon its position and role in the body. The longissimus dorsi muscle has a relatively low connective tissue content and animal age will have relatively little effect on its tenderness. The semitendinosus muscle has a higher content of collagen and animal age has a more marked effect on tenderness. Low pH is essential for tender beef. Although there is an anomaly of high pH beef also being tender, such beef is dark in colour and will have poorer keeping qualities.

Juiciness

Juiciness is an important component of meat texture and palatability and it has two major components. The first is the impression of wetness produced by the release of fluids from the meat during the first few chews. The second is the more sustained juiciness that apparently results from the stimulating effect of fat on the production of saliva and the coating of fat that builds up on the tongue, teeth and other parts of the mouth. Juiciness tends to be associated with marbling, hence heavier, fatter animals produce beef which seems juicier. Juiciness tends to decline as an animal ages. The meat of young animals gives an initial impression of juiciness, but because of the relative absence of fat, ultimately a dry sensation in the mouth. While this difference is measurable it is not large.

Aroma and Flavour

The flavours of meats can be associated with either the water in the meat or the fat components of the tissue. The chemical components responsible for "meat flavour" per se are found in the water-soluble fraction and this flavour is essentially the same for all species. As the fat content of meat increases, so does flavour. Thus, beef from older animals is more intense in flavour than meat from younger animals.

Flavour is influenced by the deposition of compounds from the feed in the fat of the animal. This is characteristic of some but not all pasture/grass diets. Some plants, particularly legumes, contain specific flavour inducing components. The concentrations of these components are higher when plants are younger and leafy and there are thus seasonal effects on flavour.

The tenderness and acceptability of beef is frequently controlled/modified by events that occur beyond the farm gate, through pre-slaughter stress, e.g. during transport and lairage. Post-slaughter handling and processing can also have large effects and perhaps the greatest of all is the variation in palatability that can occur during cooking. This is a point on the pathway from "pasture to plate" that is frequently overlooked.

Fatness

The amount of fat present in the carcass is determined by sex, the weight of the carcass, how close the animal is to its ultimate mature size when slaughtered, breed (e.g. a Charolais cross animal can be taken to heavier weights than a Hereford cross animal and still have the same level of fat cover) and nutrition. An increase in fat score, which largely reflects the amount of subcutaneous fat on the carcass, is generally accompanied by an increase in marbling fat. This in turn can influence the composition of meat and many of its palatability or eating characteristics.

For any particular ration, an increase in intake will promote a higher growth rate and a fatter carcass (at a similar carcass weight), i.e. growth rate per se will increase fat deposition relative to protein deposition. The challenge for the nutritionist is to design rations that allow maximum protein deposition while restricting fat deposition. For this, protein supply must be adequate to allow the animal express its protein growth potential at the energy supplied.

• Grass silage: there is a body of evidence that unwilted, extensively fermented grass silage can increase fatness relative to wilted silage/hay or non-silage-based diets.
• Concentrate type: there is evidence that starchy ingredients promote greater fatness than digestible fibre-based ingredients and that maize gluten and maize distillers may be more "fattening" ingredients than barley.
• Protein supply: In a grass silage-based ration protein supplied in excess of that predicted by the UK Metabolisable Protein system increased carcass fatness (Steen and Robson, 1995).
• Pattern of concentrate supply: In a grass silage and concentrate finishing system, feeding silage only, initially, and then concentrates ad libitum resulted in less fat deposited than when the same amount of concentrates were offered continuously during the 135-day study (Keane, 1999).

When examining the effects of diet on beef quality it is important to separate the direct effects of dietary ingredients from indirect effects of higher energy intake on carcass weight and fatness. Direct and indirect effects of diet are summarised in Table 1.

TABLE 1. Direct and indirect effects of diet on beef quality attributes*.

*Indirect = attributes that can be influenced by the level of feeding,

i.e. by increasing carcass weight and age, rather than by the type of ration being offered.

Direct = attributes that can be influenced by type of ration

? = Inconsistent findings in the literature

Effects of energy intake

In Irish beef production systems maximum growth rate is generally a pre-requisite for profitability. If attainment of a target growth rate on a forage-based diet is not possible, supplementary energy must be offered, usually in the form of energy dense concentrates. When compared to unsupplemented cattle, cattle fed concentrates will have a heavier and fatter carcass at a similar age or will be younger at any common carcass weight. An increase in the level of energy supply during the finishing period generally has a positive effect on tenderness, juiciness and flavour which is most likely due to an increase in intramuscular fat deposition and a decrease in the heat stability of the muscle connective tissue. Much of the data in support of this observation comes from the United States where steers that have been fed a forage-based pre-feedlot diet are implanted with steroid hormones and fed high energy diets ad libitum in a feedlot for various lengths of time. Responses in meat quality therefore reflect the high growth rates achieved rather than diet per se. This production system could allow comparison of the effects of individual feed ingredients on meat quality but there is a paucity of such comparisons in the literature.

Within conventional Irish production systems, there are opportunities to alter the pattern of concentrate and silage supply, as described earlier, to maximise growth rate close to slaughter. Whether high growth rate can be economically maintained long enough to effect the improvements in meat quality seen in the United States remains to be examined.

New Zealand studies have shown that short-term grain feeding (12 weeks after 2-4 week adjustment to a 70% cereal ration) of cattle previously at pasture resulted in lighter coloured muscle when compared to cattle that remained at pasture. While the carcasses of the concentrate-fed animals were heavier, intramuscular fat concentration was not different and the authors ascribed the difference in meat colour to the increased opportunity for exercise by the grazing animals, thereby increasing the concentration of myoglobin in muscle.

Changes in a beef production system may change some aspects of meat quality. In a study by Keane and Allen (1998), the composition and meat quality attributes of muscle from steers produced in a conventional, 2 year-old system in which cattle are finished on silage and concentrates were compared with muscle from cattle produced in an extensive system (i.e. cattle were grazed for a third season and finished on grazed grass). The findings are summarised in Table 2.

The animals from the conventional system had less moisture and more fat than those from the extensive system. Production system affected all meat colour parameters with the extensive system animals being darker (lower L value) and less red (lower a value) than those in the conventional system. Neither shear force, nor any taste panel trait differed between system. In this comparison, differences in meat quality reflect the combined effect of changes in diet, season, exercise, age and carcass weight. 

TABLE 2. Chemical composition and meat quality traits of muscle from cattle in two production systems.

(after Keane and Allen, 1998)