A current model for carotenoid transport and absorption in the gut pro
poses an initial solubilization in the oil phase of dietary emulsions
followed by incorporation of the carotenoids in mixed bile salt micell
es. To assess the relevance of the first stage of this model to what i
s observed in vivo we have examined the transfer of carotene from carr
ot juice to olive oil. Increased acidity enhanced the transfer from bo
th whole juice and carotene crystals isolated from carrot chromoplasts
. The transfer was significantly slower from whole juice. By using exo
genous P-carotene and measuring its transfer to oil in the presence an
d absence of carrot juice we have demonstrated that the inhibition of
the transfer in juice arises, at least in part, from soluble juice fac
tors. The inhibition is relieved by a fall in pH, which leads to lower
ing of the electric potential at the oil/aqueous phase interface and a
ggregation of carrot tissue including crystalline carotene. Under cond
itions of low pH, oil droplets adhere to the tissue aggregates, allowi
ng carotene to pass into the oil. Our results provide an explanation f
or why carotene absorption in vivo is depressed by conditions of low g
astric acidity.