Carotenoids are a group of relatively heat stable fat-soluble pigments. These lipid soluble pigments range in color from orange to yellow to red. They are very widespread and occur naturally in large quantities. The carotenoids can exist in the free state in plant tissue as crystals or amorphous solids, or in solution in lipid media. They also occur in esters or in combination with sugars and proteins. The occurrence of carotenoids as esters of fatty acids has been know for a long time.

The carotenoids include a class of hydrocarbons, called carotenes, and their oxygenated derivatives, call xanthophylls. Their basic structure consists of a series of isoprene units with ring structures on each end. An open or closed ring structure, the presence and absence of hydroxyl structures, and the degree of double bond conjugation all influence the color of this 40 carbon chain hydrocarbon. The greater the degree of unsaturation the darker the color. Additionally, if the double bond is in the cis form it will likely be paler, whereas, when in the trans form it will be a vivid orange.

Because of the structural variation, the hues can vary from a pale yellow to a dark red. The "typical" carrot color is most frequently that of in reference to this pigment; however, watermelon, grapefruit, and red tomato colors are due to carotenoids. The latter three vegetables are due to the fat-soluble carotenoid, lycopene.

Carotenoids are considered to be heat, acid and base stable with little effect, although acid and heat stress may cause a slight paling. Carotenoids do oxidize and become paler if dehydrated and exposed to oxygen.

CAROTENOID COMPOSITION OF SELECTED FRUITS AND VEGETABLES.

PRODUCT	REFERENCE	CAROTENOIDS
Apricot	2	beta-carotene, gamma-carotene, lycopene
Asparagus	1	alpha-carotene none detected, beta-carotene 317 ug/100 g
Broccoli	1	alpha-carotene none detected, beta-carotene 8668 ug/100g
Carrots	1 2	alpha-carotene 3924 ug/100 g, beta-carotene 8668 ug/100 g alpha-carotene, beta-carotene, gamma-carotene, xanthophyll
Corn	2	zeaxanthin, cryptoxanthin, xanthophyll, alpha-carotene, beta- carotene, gamma-carotene, kappa- carotene, neo-cryptoxanthin, hydroxy-alpha-carotene
Cowpea	2	beta-carotene, xanthophyll
Grapefruit Pink	1	alpha-carotene none detected, beta-carotene 279 ug/100 g
Grapefruit, White	2	alpha-carotene 8 ug/100 g beta-carotene 13 ug/100g beta-carotene, lycopene
Orange	2	beta-carotene, lycopene, cryptoxanthin, xanthophyll, violaxanthin, zeaxanthin, beta- citraurin, citraxanthin
Pepper, Green	1	alpha-carotene none detected, beta-carotene 81 ug/100 g
Pepper, Red or Chili	2	capsanthin, alpha-carotene, beta- carotene
Peach	2	beta-carotene, cryptoxanthin, xanthophyll, zeaxanthin
Soya Bean	2	alpha-carotene, beta-carotene
Spinach	1	alpha-carotene none detected, beta-carotene 4538 ug/100 g
Squash	1, 2	alpha-carotene none detected, beta-carotene 4422 ug/100 g alpha-carotene, beta-carotene, xanthophyll, violaxanthin
Sweet Potato	1	alpha-carotene none detected, beta-carotene 9492 ug/100 g
Wheat Germ	2	xanthophyll, carotene

1- Bushway. 1986. Determination of alpha- and beta-carotene in some raw fruits and vegetables by high-performance liquid chromatography. Journal of Agricultural and Food Chemistry 34:409.

2- Meyer, L. 19 . Food Chemistry. Reinhold. p. 219.