Effects Of Processing On Naturally Occurring Carotenoids And Carotenoid Colorants In Foods

As discussed earlier, carotenoids tend to be sensitive to oxygen, light, and heat. However, cooking-associated losses generally tend to be less than 30% (38), depending on circumstances and product. Heat-intensive processing of fruits and vegetables, such as commercial sterilization (canning), have in some cases been reported to result in significant carotenoid losses (38). However, such data have often not accounted for changes in moisture content, which generally increase with canning and other cooking procedures involving addition of water. Accounting for alteration in moisture content, carotenoid retention during canning is generally high. The major reactions which occur during canning are those involving isomerization rather than oxidation. Carotene losses during storage of canned fruits or vegetables at ambient temperatures are minimal, probably because of low oxygen tension and high water content. In most foods, an acid pH tends to stabilize carotenoid pigments by inhibiting oxidation and stereomuta-tions.

Carotenoid retention during common moist cooking procedures is generally high. Because of their low water solubility, leaching losses are low, and moist heat is insufficient to induce significant isomerization. Thus, carotenoid retention in vegetables during boiling is usually 85 to 100% after taking into account changes in moisture content (38). There are relatively little data on the effect of home or commercial frying operations on carotenoid stability, but losses could be expected to be higher than during other unit thermal processes because of the combination of high temperature and oxygen exposure. Losses of carotenoids with freeze drying are generally low to moderate, ranging from 4 to 13% for freeze-dried orange juice and carrots, respectively, whereas tray-drying followed by explosion puffing can result in losses of up to 25 percent (39). Carotenoids commonly exhibit differential stabilities. During both processing and dark storage of paprika, /¿-carotene is lost more rapidly than capsanthin, with losses variable across variety of pepper (40).

Carotenoid Isomerization During Thermal Processing

Thermal processing conditions can result in isomerization of one or more trans double bonds to the cis configuration, and carotenes appear to undergo isomerization more readily than xanthophylls (5). Newer analytical methods now permit more precise quantification of cis carotenoid iso-

mers in foods. Typical canning procedures can result in isomerization of 10 to 40% of carotenes in some products, including carrot, sweet potato, broccoli, and green leafy vegetables (41). Isomerization results in slight decreases in extinction coefficient and absorption maxima, but these generally have less impact on hue than reactions involving oxidation. Nutritionally, the vitamin A value of eis isomers of the provitamin A carotenoids is substantially less than that of their aW-trans counterparts.

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