Physiological, biochemical and sensory characterization of the response to waxing and storage of two mandarin varieties differing in postharvest ethanol accumulation

Authors: UMMARAT, NITTAYA - University Of California; ARPAIA, MARY LU - University Of California; Obenland, David - Dave

Submitted to: Postharvest Biology and Technology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/8/2015
Publication Date: 6/20/2015
Citation: Ummarat, N., Arpaia, M., Obenland, D.M. 2015. Physiological, biochemical and sensory characterization of the response to waxing and storage of two mandarin varieties differing in postharvest ethanol accumulation. Postharvest Biology and Technology. 109:82-96.

Interpretive Summary: Mandarins are very prone to developing off-flavor during storage that harms consumer acceptability. In order to enhance understanding of the problem, two mandarin varieties were compared that were known to differ in the amount of ethanol, a key component of off-flavor, accumulated during storage. High ethanol accumulation during storage did not always lead to enhanced concentrations of ethyl esters, compounds that can be produced using ethanol and that impact off-flavor. Flavor following storage was, however, worse in the variety that produced more ethanol. Differences in ethanol production between the two varieties were closely related to oxygen concentration within the fruit after waxing, with lower oxygen leading to more ethanol being produced. Harvest date and location impacted the response of the mandarins to waxing and storage. Measured biochemical parameters, such as the activities of enzymes key to the production of ethanol and ethyl esters, differed between the two varieties, but the impact of these differences on off-flavor production was uncertain. This research has identified varietal differences that impact the production of off-flavor compounds in mandarins and made progress toward finding ways to eliminate or at least minimize the loss in flavor quality during storage that commonly occurs in mandarins.

Technical Abstract: Accumulation of ethyl esters, whose synthesis is stimulated as a result of the buildup of ethanol in the fruit following the induction of anaerobic metabolism, has been proposed as being an especially important causal agent of the loss in flavor quality in mandarins that often occurs following waxing and during storage. The goal of this study was to determine the importance of postharvest ethyl ester accumulation to mandarin flavor as well as to ascertain potential causes for varietal differences in ethanol accumulation and enhance knowledge regarding the importance of ethanol accumulation to ethyl ester synthesis and overall flavor. In order to do this comparisons were made between ‘Pixie’ (P) and ‘Gold Nugget’ (GN), mandarins that were previously identified as accumulating high and low amounts of ethanol, respectively, after waxing. In three of the four harvests (H) that were conducted at two different locations P accumulated much higher concentrations of ethanol than GN after waxing and storage for 3 weeks at 5°C and 1 week at 20°C. Sensory panel analysis indicated that off-flavor development during storage was more pronounced in P than GN as were declines in overall acceptability. Flavor in fruit harvested from Ojai, California (H4) was less negatively impacted by storage than fruit from the San Joaquin Valley of California (H1, H2, H3), this being true of both varieties. Consistent with prior research, alcohols, esters and aldehydes were greatly altered in amount as a result of waxing and storage, with ethanol and the ethyl esters being the most prominent compounds. Ethyl ester concentration did not consistently relate to the amount of ethanol present. In H1 P had higher ethanol following storage than GN and correspondingly higher ethyl esters, while in H2 and H3 GN had significantly higher ethyl ester concentrations than P but did not have higher ethanol. Fruit from H4 had similar volatile concentrations between the varieties but ethanol was higher in P than GN. Pixie internal oxygen concentrations after waxing were lower than those in GN and likely were largely responsible for the greater ethanol accumulation observed in P. Following storage P tended to have higher pyruvate concentration and alcohol dehydrogenase (ADH) activity, while differences in pyruvate decarboxylase (PDC) activity were not consistent across the harvests examined. Although the impact of waxing and storage on flavor was more negative for P than GN, the cause of this could not be simply ascribed to the greater tendency of P than GN to produce ethanol that in turn leads to greater ethyl ester accumulation.