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 t he degree of isomerization of retinol (vitamin A) in various dairy pro ducts. In ram milks, which are not subjected to thermal processing, th ere was no conversion of the predominant all-trans-isomers to cis-isom ers in samples from various species. Pasteurized milks with mild heat treatment thigh quality milk) had an average 13-cis:all-trans ratio of 2.6 %, while pasteurized milk treated for 15 s at temperatures rangin g from 72 to 76 degrees C had an average ratio of 6.4 %. Milk subjecte d 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 cre am, the increase in 13-cis-isomer nias also a consequence of heat trea tment (pasteurized cream, 3.0 %; UHT cream, 14.4 %). The presence of c is isomers in fermented milli suggests that fermentation processes, di rectly or indirectly, can induce cis-trans isomerization. In the chees es analysed, the extent of retinol isomerization ranged from 7.6 to 35 .0 %. Our results confirm title importance of individual isomers in ev aluating the vitamin A status of dairy products.