Effects of thermal processing on trans–cis-isomerization of β-carotene in carrot juices and carotene-containing preparations

The effects of thermal processing (blanching, pasteurization, sterilization) on trans–cis-isomerization of β-carotene in carrot juice produced on pilot plant scale and in β-carotene-containing preparations (bovine serum albumine and lecithine suspensions) were studied. While pasteurization and sterilization at 121 °C caused only minor isomerization, sterilization at 130 °C and blanching resulted in increased levels of cis-isomers. Dissolution of crystalline carotenes by cellular lipids during blanching of carrots was identified as the prerequisite for isomerization. Also addition of grape seed oil to the coarse mash enhanced isomerization both in unheated and heat-preserved juices. Model preparations containing crystalline β-carotene showed pronounced stability during heating, whereas thermal treatment of β-carotene dissolved in toluene resulted in temperature-dependent isomerization. UV/vis spectroscopy of carotene-containing particles obtained by isopycnic density-gradient centrifugation of carrot juice indicated the presence of crystalline carotene. The carotene-stabilizing principles present in carrot juice are assumed to consist of chromoplast fragments as the core coated by water-insoluble juice constituents such as pectin, cellulose and protein which prevent carotenes from being dissolved in neutral lipids upon moderate heating.