One Reason Sun-Dried Raisins Brown So Much

Thompson Seedless grapes dried in direct sunlight produce the typical dark reddish-brown California raisin. Dried without any bruising, heat or light, the berries can raisin without appreciable color change or caftaric acid loss, and once fully dried this condition was retained. In the sun, caftaric acid (the major phenol of the pulp) rapidly disappeared, but S-glutathionyl caftaric acid (a major oxidation product in musts) was not detected. If grapes were allowed to dry in subdued light at room temperature, caftaric acid persisted with little change for some time and then disappeared rather suddenly, again without production of S-glutathionyl caftaric acid. The sudden disappearance of caftaric acid coincided with the onset of browning and occurred spontaneously at different times in different berries or was brought about by bruising. The resultant color was a less intense gray-brown, but extended rapidly through the pulp of the berry. Active enzyme and potential glutathione were demonstrable by oxidative crushing of partly dehydrated berries still containing caftaric acid. During raisining, it appears caftaric acid and polyphenoloxidase are separately compartmentalized and browning is delayed until and unless this separation is broken. Glutathione appears to remain compartmentalized away from the other two reactants or in an inactive form during raisining or gentle bruising, but not if berry tissue is disrupted as in crushing. Caftaric acid remains free during raisining to progress to brown pigments and is not tied up, as in musts, into S-glutathionyl caftaric acid which is not brown and does not brown enzymically.