Aroma Compound In Food

warfarin, inhibit the y-carboxylation reaction, so preventing clotting.

Role in bone metabolism

It was only in 1975 that it was discovered that bone tissue contains a y-carboxyglutamyl protein, osteocalcin, which accounts for up to 15% of non-collagenous proteins in bone. Fully carboxylated osteocalcin is able to form a strong complex with hydroxyapatite, the calcium phosphate mineral component of bone. Much circumstantial evidence suggests a key role for vitamin K in bone health. For example, treatment of pregnant mothers with vitamin K anticoagulants, such as warfarin, can lead to bone defects in their infants. Measurement of 'undercarboxylated' species of osteocalcin has been explored as an index of vitamin K deficiency. Yet in practice, there is no direct evidence that vitamin K deficiency has long-term adverse effects on bone health. This apparent paradox is probably due to insufficient detailed information and this area promises to be an important growth point in bone research.

Nutritional aspects

In vitamin K deficiency, the time taken for blood to clot is prolonged and the activities of factors II, VII, IX and X are reduced. It is rare to see vitamin K deficiency in an adult except when fat absorption is impaired. Deficiency signs occur more frequently in infants. This is because they have an immature liver that synthesizes prothrombin only slowly. They also have an undeveloped intestinal flora, that might otherwise provide some menaquinones and, finally, human milk contains only small amounts of vitamin K. Dietary amounts needed to satisfy most individuals' requirements are thought to be about 10 p,g day"1 for infants and between 40 and 80 p,g day"1 for adults. In some new-born infants, insufficient intakes of vitamin K give rise to bleeding into tissues, known as haemorrhagic disease of the new-born. In some countries (e.g. the UK) vitamin K is given routinely at birth, but this is not universally the practice.

There is little evidence of harm from high intakes of natural vitamin K, although high intakes of menadione can lead to haemolysis and liver damage in infants.

4.2.5 Lipids play an important role in enhancing the flavour and texture and therefore the palatability of foods

Whatever its nutrient composition might be, food has no nutritive value if it is not eaten. In countries where choice of food is abundant, palatability is a major factor in determining food choice and, therefore, nutrient intakes.

Fats contribute to palatability mainly in two ways. Firstly by responses to their texture in the mouth (sometimes called mouthfeel) and secondly by olfactory responses, namely taste in the mouth and aroma or odour in the nose; together, these are called flavour. Fat is, on the one hand, a source of taste and aroma compounds and, on the other, a medium that regulates the distribution of these compounds between water, fat and vapour phases, which influence their perception by the sense organs. Some flavour compounds result from the decomposition of lipids by lipolysis, oxidation (Section 2.3.4), and microbial or thermal degradation. These processes may produce free fatty acids, aldehydes, ketones, lactones and other volatile compounds (Fig. 4.10). PUFA play a dominant role because of their susceptibility to oxidation. Figure 4.10 illustrates the generation of a specific odour compound, whereas Fig. 4.11 summarizes the different aspects of food quality that may be influenced by oxidative metabolites of PUFA.

When low molecular weight volatile compounds interact with receptors in the nose, they give rise to the sensation of odour or aroma. This sensation depends on features of the molecular structure of the odour compounds and on their partitioning between the fat and the vapour phases. Thus, short-chain length fatty acids (e.g. in butter) have a more intense odour than longer chain acids; those with chain lengths greater than about ten carbon atoms

13 12 10 9

02 lipoxygenase

CH3(CH2)4-CH = CH-CH = CH-CH/-(CH2)7C00H cis trans I / V OOhK

Cleavage

Protein Test Food Ppt

Fig. 4.10 The formation of an odour compound. 2-trans, 4-cis-Decadienal is responsible for some of the flavour of oxidized soybeans.

have little odour because they are not sufficiently volatile. The fat content of the food has an important influence on aroma. A high fat content favours partition into the fat phase and may slow down the loss of volatiles, which are generally hydrophobic. Sometimes an advantage of a high fat content may be in lowering the intensity of the sensation of an unpleasant odour. Frequently the perception of a particular odour is quite different (and often more unpleasant) at high compared to low concentrations and the effective concentrations will be much influenced by the fat content of the foods. If food products do not contain much lipid, then there will be an early release of aroma compounds as the food is eaten. The nature and time-scale of the perceived sensation will be quite different, and possibly less pleasant, than when a fatty food is eaten.

As food is eaten, taste compounds, which may elicit acid, sweet, salty or bitter tastes, must pass into the aqueous phase before they are sensed by the taste receptors in the mouth. The aqueous phase is

Unsaturated lipids

Unsaturated lipids

Polyunsaturated Fatty Acids Foods

Fig. 4.11 Reactions of polyunsaturated fatty acids leading to changes in food quality.

Changed nutrients

Changed nutrients

Fig. 4.11 Reactions of polyunsaturated fatty acids leading to changes in food quality.

composed of the water from the food mixed with saliva in the mouth. Thus, factors such as water solubility, pH and salt formation, which influence the partitioning of taste compounds between aqueous and fat phases, influence flavour perception. For example, the presence of fat may retard their passage into saliva and thus limit the perception of taste. The way in which fat and water are distributed in a food, and the proportions of each, influence flavour perception. Many foods have a poor flavour if they contain too little fat (cheese is a good example). The effect, while very obvious to the consumer, is not fully understood scientifically but may be related to the state of dispersion of the fat.

4.2.5.3 Texture

Fat may also have an important effect on the texture of food as perceived during eating. The physical state of the fat is important. Pure oil is unpleasant to swallow for most people, while an emulsion may be perceived as pleasantly creamy. An oil-in-water emulsion gives quite a different sensation from a water-in-oil emulsion of the same chemical composition. Thus, full fat milk, cream and butter each has its individual sensory characteristics. A high content of solid fat (e.g. in chocolate) gives a cooling effect on the tongue during eating because of the heat needed for melting. If the fat does not melt completely during eating, however, an impression of stickiness results. Firmness and graininess are other attributes affected by solid fat content. People vary greatly in their appreciation of fat in relation to palatability. In this respect, composition and state of dispersion of the fat may be crucial.

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