The changes of ethanol metabolism in the pulp of pineapples during the postharvest incidence of blackheart disease

Abstract:【Objective】Fermentation metabolism exists constantly in the pulp of pineapples, and the con⁃tents of metabolites (pyruvic acid, acetaldehyde and ethonol) and enzyme activities including pyruvate de⁃carboxylase (PDC) and alcohol dehydrogenase (ADH) are important indexes for evaluating the level of fer⁃mentation metabolism. Blackheart disease in pineapples [Ananas comosus (L.) Merr.‘Comte de Paris’] isa physiological disorder that may be induced by exposure to low temperature, either in the field or in post⁃harvest storage and results in severe internal browning of pineapple fruit, However, the biochemical path⁃way of blackheart disease has not been clearly documented. In order to study the variation of fermentationmetabolism in the process of pineapple blackheart disease, the pulp of pineapples from 6 mature (6 M)and 8 mature (8 M) fruits were investigated during storage at room temperature (25 ℃), respectively.【Methods】The severity of pineapple blackheart disease was evaluated by assessing the ratio of the brownarea in longitudinal transaction fruits. The content of pyruvic acid and the enzyme activities of PDC andADH were determined using conventional physical and chemical analysis methods. The content of acetaldehyde and ethanol were measured by using a gas chromatograph, and the changes of gene expression ofPDC and ADH were detected by real-time fluorescent quantitative RT-PCR.【Results】The blackheartindex increased gradually in the 6 M fruit and in the 8 M fruit during storage at 25 ℃, and the blackheartindex in the 6 M fruit was significantly higher in the 8 M at 4 d of storage. The pyruvic acid content de⁃creased continuously in the 6 M fruit, while it increased to 16.68 μg·g^-1 and then decreased gradually inthe 8 M fruit when stored at 25 ℃. During 0 to 4 d of storage, the pyruvic acid content in the 6 M fruit wassignificantly higher in the 8 M fruit. Contrasting to the falling of acetaldehyde content in the 8 M fruit dur⁃ing whole storage at 25 ℃, the variation trend of acetaldehyde content in the 6 M fruit grew steadily from 0to 6 d of storage and then declined rapidly, with the peak value reaching up to 23.31 μL·g^-1. And the acetaldehyde content in the 8 M fruit was notably lower than that in the 6 M fruit during the whole storage peri⁃od at 25 ℃ except for 0 day. After increasing smoothly both in the 6 M and 8 M fruit from 0 to 4 d of storage, the ethanol content was sharply raised in the 8 M and slowly in the 6 M at the later stages, respectively. The ethanol content in the 8 M fruit grew to 124.35 μL·g^-1 on the 10th day, which was significantlyhigher than that in the 6 M. The changes of PDC activities in the 6 M fruit and 8 M fruit stored at 25 ℃ ina similar manner, reached a plateau and then decreased, and grew slightly at the later stage of storage.The largest enzyme activities of PDC were 372.31 U·g^-1 in the 6 M fruit on the 4th d and 385.46 U·g^-1 inthe 8 M fruit on the 6th day, respectively. The ADH activity in the 6 M fruit storage at 25 ℃ increasedfrom 364.68 U·g^-1 to 3 618.51 U·g^-1 during the period from 0 to 6 d of storage, and then decreased to 270.82U·g-1 protein on the 10th day. Likewise, the change of ADH activity in the 8 M fruit storage at 25 ℃increased from 912.76 U·g^-1 to 5 713.30 U·g^-1 from 0 to 6 d of storage, and then decreased to 1 878.50 U·g-1after 8 d of storage. The ADH activity in the 8 M fruit was higher than that in the 6 M fruit during thewhole storage period. The relative gene expression of PDC in the 6 M fruit and in the 8 M fruit had anidentical growth mode that increased during the earlier storage but decreased at the later storage, whichwere both consistent with the change of ADH activity. And they reached a peak of 3.19 on the 4th day and8.90 on the 6th day, respectively. The relative gene expression of PDC in the 8 M fruit was significantlyhigher than in the 6 M fruit during the period from 6 to 10 d. As for the change of the expression level ofADH, it had a similarly changing pattern with the change of the PDC relative gene expression correspond⁃ing in the 6 M fruit and 8 M fruit, which grew sharply in the 6 M during 0 to 4 d and declined from 4 to 10 dof storage, and grew in the 8 M fruit during 0 to 6 d and declined later. ADH relative gene expression in the8 M fruit is significantly higher than that in the 6 M fruit at the stage during 6 to 10 d of storage, as well asthe maximum gene expression of ADH. As a result, the fermentation metabolism in pineapple fruit of the 8 M was more active than in that of 6 M.【Conclusion】The harvest maturity of pineapple pulp has a close cor⁃relation to blackheart in pineapple. And postharvest pineapple with a higher maturity has a more active fermentation metabolism but a lower blackheart incidence when stored at 25 ℃. The relationship between theethanol metabolism and the incidence of blackheart disease remains to be further explored and researched.