Yolk colour

Yolk colour plays a role in consumer acceptance, but the preferred colour varies in different countries. Yolk colour has no relation to nutrient content, flavour or freshness, but is often enhanced in conventional production systems by addition of synthetic pigments to the animal feed. In organic production, synthetic yolk pigments are prohibited and this normally results in paler yolks, but may also lead to greater variability in yolk colour intensity. In many European countries paler yolk colour is perceived by consumers as being associated with less natural production systems, an issue that clearly needs to be addressed by improved consumer information. 

Two aspects of forage are of interest how much do free-range poultry consume and how well is it utilized The topic of forage intake and its significance in relation to nutrient needs is of practical importance. Another consideration is that one of the most important egg-quality parameters for the consumer is the yolk colour, which can be affected by forage intake and quality (Fig. 7.3). 

The same also holds for the carotenoid content ofpoultry food (Fig. 1.6). In this connection it is important to attain the yolk colours expected by the consumer pale, almost colourless yolks in Norway, strong yellow yolks in Germany, and orange-red duck egg yolks in Thailand (cf. Section 7.1). 

To convert 4- 5 toimes of fresh grass to 1 tonne of dried material requires about 300 1 of oil, and so although dried forages could be regarded as excellent feeds for ruminants, the high cost of preparing them restricts their use to speciality feeds for non-ruminants. In Britain, much of the dried forage is consumed by horses. Elsewhere, and especially in the USA, dried lucerne is used for poultry as a source of vitamins and also to provide xanthophyU as an egg-yolk colourant. 

Machlin, L.J. and Gabriel, E. (1982) Kinetics of tissue a-tocopherol uptake and depletion following administration of high levels of vitamin E. Ann. N.Y. Acad. Sci. 393, 48-60. Madiedo, G. and Sunde, M.L. (1964) The effect of algae, dried lake weed, alfalfa and ethoxyquin on yolk colour. Poult. Sci. 43, 1056-1061. 

Egg yolk and white were used in paintings either separated or, more often, together. Egg or egg yolk temperas served as the most common protein binders in colour layers of artworks. Egg white alone was used in book illustrations, but it was usually not involved in other artworks because colour layers containing it were too fragile. Egg yolk in temperas has always been popular because of the higher content of lipids, which make the layers elastic and increases their adhesion to the surface. 

Artificial butter is commonly obtained by emulsifying oleomargarine with skim milk, usually with the addition of varying proportion of vegetable oils (sesamd, arachis, cottonseed), animal fats (beef fat, neutral hog s fat), aromatic substances (ethyl butyrate, coumarin), and sometimes also casein, lactose and egg-yolk in some cases it is artificially coloured (annatto, coal-tar colours). 

Lucerne is one of the most widely grown forage crops on a worldwide basis and is used commonly in livestock diets. It is a good source of many nutrients and in the past the inclusion of lucerne meal was considered to be essential in diets for a wide range of animals as a source of unidentified factors. Currently lucerne meal is used in poultry diets as a source of pigments to provide a yellow colour in egg yolks, shanks and skin (e.g. Karadas et al, 2006). 

Francis and Clydesdale (1972) urged that egg yolks should be homogenised before colour measurement. This is because yolks are formed in a layered... 

---