【作者】 尚海涛；

【导师】 郑永华；

【作者基本信息】 南京农业大学 ， 食品科学， 2011， 博士

【摘要】 水蜜桃色泽艳丽、香味浓郁、柔软多汁、营养丰富,深受广大消费者喜爱。但水蜜桃属于典型的呼吸跃变型果实,且采收季节多集中于盛夏高温多雨季节,采后呼吸旺盛,后熟软化迅速,极易发生腐烂变质。低温可有效延缓桃果实后熟衰老,但果实在低温下较长时间贮藏时易引起冷害,出现果肉絮败或木质化异常质构,同时伴随着出汁率的减少和果肉褐变,严重影响果实品质。目前虽有研究气调、热处理、1-甲基环丙烯(1-methylcyclopropene,1-MCP)或天然物质处理等减轻桃果实采后冷害的报道,但由于絮败和木质化形成的特点和调控机理还不十分清楚,这些处理的效果不尽一致甚至完全相反。因此在生产实际中仍缺乏有效的桃果实保鲜技术,研究桃果实采后冷害机理和及其控制技术,己成为解决桃果实保鲜问题的关键。本论文以“湖景蜜露”桃果实为试材,在研究果实正常后熟过程中硬度变化和冷害温度下絮败和木质化冷害症状发生规律的基础上,系统研究了絮败和木质化形成过程中细胞壁物质代谢和亚细胞水分分布的变化,同时研究了1-MCP、外源乙烯和热处理对桃果实絮败和木质化的影响及其机理,以阐明桃果实絮败和木质化两种冷害症状的形成特点和调控机理,为桃果实保鲜技术的开发和应用提供依据。研究结果分述如下：(1)研究了10-30℃温度下桃果实硬度的变化规律,以此为基础建立温度对桃果实硬度影响的动力学模型。此模型包括两个动力学方程,一个是跃变前期的零级动力学方程,活化能为68.8kJ/mol,另一个为跃变期的一级动力学方程,其活化能为76.9kJ/mol。验证实验中所得的预测值及实测值之间的决定系数为0.995,回归估计标准误为0.567N。所建立的模型在10-30℃范围内对果实硬度的预测具有较高准确性。(2)研究了0、3、5和7℃不同贮藏温度和不同成熟度对桃果实冷害的影响。桃果实在冷害低温下贮藏表现出絮败和木质化两种不同的冷害症状。温度越低,桃果实冷害的出现时间越迟。5℃贮藏时桃冷害以絮败为主,成熟度越高,絮败发生越早越严重,0℃贮藏时冷害以木质化为主,成熟度越高,木质化的发生越迟越轻。发生絮败或木质化的果实其出汁率,TSS和TA含量显著降低,从而降低食用品质。(3)研究了桃果实在正常后熟软化、絮败和木质化过程中细胞壁物质代谢的变化。正常成熟过程中,果胶甲酯酶(PME)活性下降,外切多聚半乳糖醛酸酶(exo-PG)和内切半乳糖醛酸酶(endo-PG)活性上升,导致大量的Na2CO3溶解性果胶(NSF1)和CDTA溶解性果胶(CSF)降解为水溶性果胶,致使果实后熟软化。5℃贮藏15天转入20℃3d后,桃果实PME活性显著高于正常后熟果实,exo-PG与正常后熟果实相似,endo-PG活性显著低于正常后熟果实。高活性的PME和exo-PG促进果胶降解,但由于endo-PG活性较低,致使果胶降解不充分,果实中CSF显著高于正常后熟果实,从而形成絮败冷害症状。0℃贮藏35天转入20℃3d后,果实PME活性缓慢下降,exo-PG和endo-PG活性仅略微升高,果胶物质不能正常降解；而果实中的苯丙氨酸解氨酶(PAL)和过氧化物酶(POD)活性升高并保持较高水平,木质素含量增加,从而形成木质化冷害症状。这些结果表明絮败产生是由于果胶降解不平衡,而木质化的形成是由于果实丧失后熟性能及木质素的合成。(4)采用核磁共振(NMR)技术研究桃果实正常后熟软化、絮败和木质化过程中亚细胞水分分布的变化。桃果实NMR图谱中有4个不同横向驰豫时间T2,分别对应液泡、细胞间、细胞质和细胞壁中的水分。果实正常后熟过程中液泡中水分含量下降,而细胞间水分含量上升,果实出汁率增加。发生絮败时果实液泡中水分向细胞间的水分转移量减少,果实出汁率略微上升,但显著低于正常后熟果实；而发生木质化的果实细胞间水分和出汁率都下降。这些结果表明絮败和木质化果肉干化少汁是由于液泡间水分向细胞间的转移受阻有关,通过NMR技术可以无损检测桃果实絮败或木质化冷害。(5)研究了5μl/L 1-MCP和5μl/L外源乙烯处理对桃果实在5℃和0℃贮藏时絮败和木质化的影响。0℃贮藏时桃果实贮藏后期ACC含量和ACC氧化酶(ACO)活性下降,抑制乙烯的释放速率,使果实丧失后熟性能,形成木质化冷害症状；采用1-MCP处理显著抑制了内源乙烯释放速率,加重了果实的木质化；外源乙烯处理提高内源乙烯的产生,减轻果实的木质化。5℃贮藏时ACC含量和ACO活性上升,乙烯释放速率增加,果实可以后熟软化,但形成絮败冷害症状；采用1-MCP处理显著抑制内源乙烯释放速率,减轻絮败的形成；而外源乙烯处理促进了内源乙烯的产生,加重果实的絮败。这些结果表明适量的乙烯是果实正常后熟软化所必需的,乙烯合成受抑会导致木质化发生,而过量的乙烯合成则促进絮败的形成。(6)研究了38℃12h热处理对0℃贮藏的桃果实冷害的影响。果实在0℃贮藏后期乙烯释放速率、exo-PG和endo-PG活性以及H2O2含量和O2-产生速率都下降,果实后熟性能丧失,形成木质化冷害症状。热处理促进果实乙烯释放,提高了PME、exo-PG、endo-PG、超氧化物歧化酶(SOD)、过氧化氢酶(CAT)和抗坏血酸过氧化物酶(APX)活性,果实能后熟软化,避免了木质化的形成,却出现了絮败的冷害症状。这些结果表明热处理可以延缓木质化的形成但会促进絮败的发生,这可能与热处理促进内源乙烯的产生和加速果实低温下后熟有关。更多还原

【Abstract】 Peach fruit (Prunus persica) are very popular for their attractive color, appealing flavor, soft and juicy texture, and rich in nutrient. But peach fruit are very perishable after harvest due to their rapid ripening and softening for high temearature in mid summer and climacteric characteristics. Low temperature can effectively delay ripening and senescence processes, but fruit are susceptible to chilling injury with prolonged storage time. It menifested itself as mealiness, leatheriness, dry or fresh browning. At present, although many technologies such as controlled atmosphere, heat treatment、low temperature conditioning or natural volatile compounds were used to alleviate chilling injury. But for the characteristics and mechanisms of mealiness or leatheriness are unclear, the effects of these technologies are controversial. It is therefore crucial to study the mechanisms and control technologies of chilling injury. Basing on the kinetic of firmness of peach fruit in normal ripening and the effect of chilling temperatures and maturities on chilling injury. The present study was to investigate the differences between mealiness and leatheriness of cell wall metabolisms, subcellular water distribution. And effects of hot air treatment, 1-MCP, exogenous ethylene treatments on mealiness or leatheriness were also studied. The results were as follows:(1) Effects of various storage temperatures from 10 to 30℃(283.15 to 303.15 K) on firmness were investigated, and a model for predicting the effects of storage temperature on fruit firmness was developed. The model contained two kinetics, a zero-order kinetic in pre-climacteric stage with the energy activation of 68.8 kJ/mol, and a first-order kinetic in climacteric stage with the energy activation of 76.9 kJ/mol. In validate, the coefficient of determination (R2) and root mean squared error (RMSE) between the predicted and measured firmness were 0.995 and 0.567 N, respectively. The model gave satisfactory predictions of fruit firmness in postharvest peach fruit. (2) The chilling injury intensity and symptoms in peach fruit at 0,3,5 and 7℃and effect of maturity on chilling injury at 0,5℃were investigated. Leatheriness and mealiness were two different types of disorders. The lower the temperature, the lower susceptibility. At 0℃, leathereness was the main CI symptom, while at 5℃, mealiness was the main CI symptom. The higher maturity, the earlier mealiness, but the later leatheriness. Juice rate, TSS and TA signinficantly decreased after mealiness or letheriness, and the effect of leathiness was higher than that of mealiness.(3) The changes in cell wall pectins and lignin in normally ripening (juicy) and in mealy or leathery peach fruit was investigated. Fruit at 5℃15d plus 3d at 20℃, a higher level of PME, exo-PG and a lower level of endo-PG, and a higher level of CSF and NSF1 and a lower proportion of WSF and NSF2, inducing mealiness symptom. Fruit at 0℃35d plus 3d at 20℃, low exo-PG and endo-PG and higher level of PAL and POD, losing fruit ripening and inducing leatheriness symptoms. These results suggested that mealiness was due to the imbalance of pectin degradation, while leatheriness was due to lack of ripening ability and lignin synthesis.(4) Proton NMR relaxation was used to evaluate the changes of water compartments in normal ripening, chilling-injured peach fruit. Four different Proton NMR relaxation times can identify four populations of water with different relaxation characteristics. The observed relaxation times originated from particular water compartments:the vacuole, the extra cellular space, the cytoplasm and the cell wall. The relaxation time distributions during ripening showed that relative area in extra cellular space increase and in vacuole decrease during ripening. Both mealiness and leatheriness decreased water content in extracelluar space. These results suggested that the low water content in extracelluar space cause dry fresh of peach frui, NMR could be used to detect the CI.(5) Peach fruit were treated with 5 uL/L 1-MCP or 5 ul/L ethylene at 20℃for 12h, and then stored at 5℃or 0℃to study the effects of ethylene metabolisms on the development of mealiness or leatheriness, respectively. In the later stage at 0℃, ACC content, ACO activity and ethylene production decreased, fruit lost ripening ability and manifested as leatheriness. Exogenous ethylene alleviated leatheriness, while 1-MCP advanced it. Fruit at 0℃, ripen with increase of ethylene production, and manifested as mealiness.1-MCP alleviated mealiness, while exogenous ethylene advance it. These results suggested that endogenous was necessary to fruit ripening, but excess ethylene accelerate the development of mealiness. (6) Effect of hot air threatment (HAT) on leateriness of peach fruit stored at 0℃was investigated. The higher PME but low level of exo-PG and endo-PG in the later stage at 0℃, fruit lost ripening ability and manifested as leatheriness. A higher ethylene production, PME, exo-PG and endo-PG activities increased in heat-treated fruit, restored the ripening ability. Exo-PG activities increased but endo-PG decreased, inducing the developmnet of mealiness. These results suggested it may be related with the increase of ethylene after HAT, which have different effects on the development of mealiness or leatheriness.更多还原