The triplet state energy level of the natural cis-carotenoid bixin (methyl hydrogen 9'Z-6,6-diapocarotene-6,6'-dioate) was determined using laser-induced photoacoustic calorimetry (PAC) in acetonitrile/methanol (1: 1) solution. An energy-transfer process with anthracene as sensitizer in O-2-saturated solution was used to populate the carotenoid triplet state. The energy balance of the system allowed the calculation of the triplet energy content of bixin as E-T = 18 +/- 2 kcal/mol. The result is discussed in relation with both its singlet molecular oxygen quenching and cis trans isomedzation abilities. The quantum yield, Phi = 0.69, for the formation of either the triplet state or the singlet molecular oxygen by the sensitizer anthracene was also determined. (c) 2006 Elsevier Ltd. All rights reserved.743561565