196 Produce Degradation: Reaction Pathways and their Prevention
alkene chain of the carotenoid by a lipoxygenase-generated linoleolylperoxyl radical
followed by further chemical oxidation and rearrangements.
Other than enzymatic chemical changes of carotenoids during thermal process-
ing, irradiation, canning, drying, and storage are also very important. There is a
significant number of early publications concerning thermal degradation of caro-
tenoids. Most frequently, losses of beta-carotene and lycopene were reported earlier
without identification of the degradation products. Typically these losses were pro-
portional to the temperature and time of processing. Numerous authors reported
volatile thermal degradation products. Toluene and xylene were reported as thermal
degradation products of beta-carotene in more than 10 publications between 1963
and 1980. In addition, many researchers found 2,6-dimethylnaphthalene in thermally
treated carotenoids. Ionene and alpha- and beta-ionones, beta-cyclocitral, 5,6-epoxy-
beta-ionone, and dihydroactinidiolide have also been reported (Schreier et al., 1979).
A mechanism for the formation of toluene, xylene, and dimethylcyclodecapentaene
from beta-carotene was advanced by Schwieter et al. (1969). In this mechanism a
rearrangement of double bonds in the polyene chain is followed by the formation
of a four-ring intermediate, which undergoes cleavage to form toluene. Xylene and
dimethylcyclodecapentaene are formed in a similar way. The authors did not provide
information on the structure of the remaining part of the beta-carotene molecule
after formation of toluene, xylene, and related compounds. A mechanism for the
formation of ionene was postulated by Ohloff (1971). Only a limited number of
studies are available on the formation of nonvolatile thermal degradation products
of carotenoids. Halaby and Fagerson (1971) reported small amounts of polycyclic
aromatic hydrocarbons formed from beta-carotene at temperatures between 400 and
700°C, which is not relevant to food processing. During simulated commercial
deodorization of palm oil, Ouyang et al. (1980) identified beta-13-apo-carotenone,
beta-15-apo-carotenal, and beta-14-apo-carotenal. Onyewu et al. (1982) identified
two nonpolar compounds, dimethyl-(trimethylcyclohex-1-enyl) octatetraene and tri-
methyl-(trimethylcyclohex-1-enyl) dodecahexaene, in a model system simulating
time and temperature conditions of palm oil deodorization. In a subsequent publi-
cation Onyewu et al. (1986) described thermal degradation products of beta-carotene
formed under time and temperature conditions used in various methods of food
processing. Temperatures of 210, 155, and 100°C and times of 4 and 1 h and 15 and
5 min were investigated. At 210°C, 97.8, 97.2, 93.9, and 91.1% degradation of beta-
carotene occurred, respectively. At 155 and 100°C the losses were 30.6, 16.7, 8.3,
and 8.3%, and 8.3, 7.7, 0.9, and 0.9%, respectively. The presence of over 70 non-
volatile thermal decomposition products of beta-carotene was reported and several
compounds of relatively complicated structure were identified in samples heated for
4 h at 210°C.
Drying is frequently used in the processing of some produce. Perez-Galvez et
al. (2004) investigated changes of carotenoids during simulated traditional drying
of red pepper fruits. The temperature was increased from 25°C to approximately
45°C during the first 24 h. After 144 h the temperature was decreased slowly over
48 h. The content of eight carotenoids was analyzed by HPLC. Three different stages
were observed in relation to the determined amount of carotenoids. During the first
24 h there was a decrease of approximately 20%. After 5 d, recovery of the biosynthetic