Influence of thermal and other manufacturing stresses on retinol isomerization in milk and dairy products.

An analytical procedure, based upon HPLC, has been used to determine the degree of isomerization of retinol (vitamin A) in various dairy products. In raw milks, which are not subjected to thermal processing, there was no conversion of the predominant all-trans-isomers to cis-isomers in samples from various species. Pasteurized milks with mild heat treatment (high quality milk) had an average 13-cis: all-trans ratio of 2.6%, while pasteurized milk treated for 15 s at temperatures ranging from 72 to 76 degrees C had an average ratio of 6.4%. Milk subjected to more severe heat treatments had a higher degree of isomerization (UHT milk, 15.7%; sterilized milk, 33.5%), consistent with increased thermal conversion of the retinol isomers. In pasteurized and UHT cream, the increase in 13-cis-isomer was also a consequence of heat treatment (pasteurized cream, 3.0%; UHT cream, 14.4%). The presence of cis isomers in fermented milk suggests that fermentation processes, directly or indirectly, can induce cis-trans isomerization. In the cheeses analysed, the extent of retinol isomerization ranged from 7.6 to 35.0%. Our results confirm the importance of individual isomers in evaluating the vitamin A status of dairy products.