ナツミカン果実の凍結による苦味の増大について

Abstract

Natsudaidai fruits, which account for respectable fraction of Japanese citrus fruit production, have not been frozen commercially because severe bitterness develops in the pulp due to freezing. This paper describes the general aspects and the mechanism of bitterness increase due to freezing.1. The organoleptic test showed that pulp which had been frozen at -20°C tasted much more bitter than the unfrozen raw pulp. 2. When segments were frozen with or without segment cover (membrane), bitterness of pulp (juice sacs without segment cover) taken from the former was greater than that from the latter. 3. When the segments were frozen by immersion in liquid nitrogen and subsequently stored at -20°C, the bitterness was not so severe as when they were frozen and stored at -20°C. No substantial bitterness change was found for either material during storage at -20°C. 4. The contents of soluble and total naringin in the pulp did not show any change due to freezing and during storage in the frozen state. 5. To determine the leakage of naringin, pulp or segment cover was immersed in water for a short time and filtered, then naringin contents in the filtrate were measured; this may suggest the ease of exudation of naringin. The leakage of naringin increased obviously due to freezing at -20°C as compared with leakage from the raw materials. The leakage from pulp frozen with liquid nitrogen, on the contrary, did not show any increase coinciding with the organoleptic evaluation. 6. No change of bitterness was found when extracted juice was frozen.It is concluded that the increase of bitterness must be fundamentally due to the increase of readiness of exudation of naringin associated with tissue damage by the freezing. But the total amount of naringin is not the cause. Since most of the naringin is located in the outer part of the pulp and segment cover, it would leak out at thawing and bring about a high concentration on the pulp surface. The high local concentration of naringin causes the bitterness increase.