1.1 Water
1.2 Dry matter and its components
1.3 Analysis and characterisation of foods
Food is material that, after ingestion by animals, is capable of being digested, absorbed
and utilised. In a more general sense we use the term ‘food’ to describe edible material.
Grass and hay, for example, are described as foods, but not all their components are
digestible. Where the term ‘food’ is used in the general sense, as in this book, those
components capable of being utilised by animals are described as nutrients.
The animals associated with humans cover the spectrum from herbivores, the plant
eaters (ruminants, horses and small animals such as rabbits and guinea pigs); through omnivores,
which eat all types of food (pigs and poultry); to carnivores, which eat chiefly meat
(dogs and cats). Under the control of humans these major classes of animal still pertain, but
the range of foods that animals are now offered is far greater than they might normally
consume in the wild (for example, ruminants are given plant by-products of various human
food industries and some dog foods contain appreciable amounts of cereals). Nevertheless,
plants and plant products form the major source of nutrients in animal nutrition.
The diet of farm animals in particular consists of plants and plant products, although
some foods of animal origin such as fishmeal and milk are used in limited amounts.
Animals depend upon plants for their existence and consequently a study of animal
nutrition must necessarily begin with the plant itself.
Plants are able to synthesise complex materials from simple substances such as carbon
dioxide from the air, and water and inorganic elements from the soil. By means of photosynthesis,
energy from sunlight is trapped and used in these synthetic processes. The
greater part of the energy, however, is stored as chemical energy within the plant itself and
it is this energy that is used by the animal for the maintenance of life and synthesis of its
own body tissues. Plants and animals contain similar types of chemical substances, and we
can group these into classes according to constitution, properties and function.
.2 WATER
The water content of the animal body varies with age.The newborn animal contains
750–800 g/kg water but this falls to about 500 g/kg in the mature fat animal. It is
vital to the life of the organism that the water content of the body be maintained: an
animal will die more rapidly if deprived of water than if deprived of food.Water
functions in the body as a solvent in which nutrients are transported about the body
and in which waste products are excreted. Many of the chemical reactions brought
about by enzymes take place in solution and involve hydrolysis. Because of the high
specific heat of water, large changes in heat production can take place within the
animal with very little alteration in body temperature.Water also has a high latent
heat of evaporation, and its evaporation from the lungs and skin gives it a further
role in the regulation of body temperature.
The animal obtains its water from three sources: drinking water, water present in its
food, and metabolic water, this last being formed during metabolism by the oxidation of
hydrogen-containing organic nutrients.The water content of foods is variable and can
range from as little as 60 g/kg in concentrates to over 900 g/kg in some root crops. Because
of this great variation in water content, the composition of foods is often expressed
on a dry matter basis,which allows a more valid comparison of nutrient content.
The water content of growing plants is related to the stage of growth, being greater
in younger plants than in older plants. In temperate climates the acquisition of drinking
water is not usually a problem and animals are provided with a continuous supply.
There is no evidence that under normal conditions an excess of drinking water is harmful, and animals normally drink what they require.
1.2 DRY MATTER AND ITS COMPONENTS
The dry matter (DM) of foods is conveniently divided into organic and inorganic material,
although in living organisms there is no such sharp distinction. Many organic
compounds contain mineral elements as structural components. Proteins, for example,
contain sulphur, and many lipids and carbohydrates contain phosphorus.
The oilseeds, such as
groundnuts, are exceptional in containing large amounts of protein and lipid material.
In contrast, the carbohydrate content of the animal body is very low. One of the
main reasons for the difference between plants and animals is that, whereas the cell
walls of plants consist of carbohydrate material, mainly cellulose, the walls of animal
cells are composed almost entirely of lipid and protein. Furthermore, plants store
energy largely in the form of carbohydrates such as starch and fructans, whereas an
animal’s main energy store is in the form of lipid.
The lipid content of the animal body is variable and is related to age, the older animal
containing a much greater proportion than the young animal.The lipid content of
living plants is relatively low, that of pasture grass, for example, being 40–50 g/kg DM.
In both plants and animals, proteins are the major nitrogen-containing compounds.
In plants, in which most of the protein is present as enzymes, the concentration
is high in the young growing plant and falls as the plant matures. In animals,
muscle, skin, hair, feathers, wool and nails consist mainly of protein.
Like proteins, nucleic acids are also nitrogen-containing compounds and they
play a basic role in the synthesis of proteins in all living organisms.They also carry
the genetic information of the living cell.
The organic acids that occur in plants and animals include citric, malic, fumaric,
succinic and pyruvic acids. Although these are normally present in small quantities,
they nevertheless play an important role as intermediates in the general metabolism
of the cell. Other organic acids occur as fermentation products in the rumen, or in
silage, and these include acetic, propionic, butyric and lactic acids.
Vitamins are present in plants and animals in minute amounts, and many of them
are important as components of enzyme systems. An important difference between
plants and animals is that, whereas the former can synthesise all the vitamins they
require for metabolism, animals cannot, or have very limited powers of synthesis,
and are dependent upon an external supply.
The inorganic matter contains all those elements present in plants and animals
other than carbon, hydrogen, oxygen and nitrogen. Calcium and phosphorus are the
major inorganic components of animals, whereas potassium and silicon are the main
inorganic elements in plants.
1.3 ANALYSIS AND CHARACTERISATION OF FOODS
Originally the most extensive information about the composition of foods was based
on a system of analysis described as the proximate analysis of foods, which was
devised over 100 years ago by two German scientists, Henneberg and Stohmann.
More recently, new analytical techniques have been introduced, and the information
about food composition is rapidly expanding . However, the system of proximate analysis still forms the basis for the statutory declaration of the composition
of foods in Europe