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Green vegetables such as broccoli florets and asparagus spears are young, actively growing tissues and consequently have high metabolic and respiratory rates when harvested. These crops are prone to harvest-induced senescence, which is initiated within a few hours after removal from the parent plant (King and O’Donoghue, 1995; Downs and Somerfield, 1997). The senescence program is promoted by ethylene or dehydration and inhibited by cytokinin or sucrose, and is characterized by rapid depletion of sugars and organic acids, proteolysis, lipid peroxidation, increases in free amino acids and ammonia, and alteration of gene expression (King and Morris, 1994; Zhuang et al., 1995; Coupe et al., 2003; McKenzie et al., 2004; Gapper et al., 2005). The changes in gene expression include differentially regulated induction of at least four cysteine proteases (Coupe et al., 2003) and a marked up-regulation of asparagine synthase as a consequence of sucrose depletion (Downs and Somerfield, 1997; Winichayakul et al., 2004). Even in the early stages, these dramatic changes result in loss of fresh flavor and coincident increase in undesirable flavor and aroma. The flavor acceptability of asparagus spears decreased significantly after two days at 1 °C, with an additional decline during the subsequent five days of storage (King et al., 1988). This loss of sensory quality was associated with off flavor development, decreased soluble carbohydrates, and an increase in soluble protein. Benefits of low temperature, high humidity, and controlled atmosphere (CA) or modified atmosphere packaging (MAP) storage The best measures to retard harvest-induced senescence and maintain fresh-like sensory quality are rapid cooling with little delay after harvest and subsequent maintenance of low temperature and high humidity during storage (Toivonen and Forney, 2004; Jones et al., 2006). However, there also has been considerable success in devising suitable CA or MAP conditions for preservation of quality in broccoli florets up to twice as long as in air storage (Jacobsson et al., 2004; Toivonen and Forney, 2004; DeEll et al., 2006). Jacobsson et al. (2004) found that use of low-density polyethylene film, inclusion of an ethylene-absorbing sachet, and a package atmosphere of 5% O , 7% CO for MAP storage gave the best fresh- 2 2 like sensory quality of cooked broccoli. DeEll et al. (2006) showed that use of packets including sorbitol plus potassium permanganate markedly reduced accu- mulation of off odor fermentative volatiles in MAP broccoli during 29 days of storage at 0–1 °C. These good results notwithstanding, there is always the potential for production of malodorous volatile sulfur compounds, most notably methanethiol and dimethyl disulphide, when O falls below 1% or CO exceeds 10% (Forney et 2 2 al., 1991; Hansen et al., 1992; Di Pentima et al., 1995). Quite recently, as a first step toward elucidation of the molecular mechanisms whereby CA delays the rapid onset of harvest-induced senescence in broccoli florets, Eason et al. (2007) used differential display analysis to identify a set of stress response genes induced during the first 24 hours under 10% CO , 5% O CA. |
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| 这种衰老过程被乙烯或脱水所促进，为细胞分裂素或蔗糖抑制，并以糖类和有机酸迅速耗竭，蛋白质水解，脂质过氧化，游离氨基酸和氨增加以及基因表达变化为特点(King and Morris,1994; Zhuang et al., 1995; Coupe et al., 2003; McKenzie et al., 2004; Gapper et al.,2005)。基因表达的变化，包括至少四个半胱氨酸蛋白酶差异调节诱导（Coupe et al.，2003）和作为蔗糖消耗的结果的天冬酰胺合成酶向上调节（Downs and Somerfield, 1997; Winichayakul et al., 2004）。即使在早期阶段，这些显著的变化也会导致新鲜口味损失和不良口味和香味同步增加。芦笋口味的可接受度在1 ° C下两天后显著降低，并在其后的五天存储期间进一步下降（King et al., 1988）。这种感官质量损失是与气味丧失的发展、可溶性碳水化合物减少和可溶性蛋白增加相关。 |
3楼2011-03-04 13:51:12
2楼2011-03-04 12:18:20
4楼2011-03-04 13:53:49
| 延缓收获诱导的衰老以保持新鲜的感官质量的最好措施是收获后快速冷却，后续维护和储存期间保持低温高湿（Toivonen and Forney, 2004; Jones et al., 2006）。然而，在制订合适的保持花椰菜质量的CA或MAP条件方面也已取得相当的成功，在设计条件下存储时间可达到空气中的两倍长（Jacobsson et al., 2004; Toivonen and Forney, 2004; DeEll et al., 2006）。Jacobsson（2004年）发现，以低密度聚乙烯薄膜，包含吸收乙烯的小包和5％O、7％CO包裹气氛用于花椰菜的MAP仓储，烹调的花椰菜就像新鲜蔬菜一样具有最好的的感官品质。 DeEll等（2006年）表明，包装中使用山梨糖醇配以高锰酸钾显著减少在0-1 ° C下29天中 MAP贮藏花椰菜发酵过的气味挥发物的积累。尽管这些良好的效果，但当O含量低于1％或CO超过10％时，总有恶臭的挥发性含硫化合物的潜在生成，尤其是甲硫醇和二甲基二硫化物，（Forney et al., 1991; Hansen et al., 1992; Di Pentima et al., 1995）。最近，作为对澄清CA如何延缓快速启动的收获诱导的花椰菜衰老的分子机制的第一步， Eason灯（2007年）采用差异显示分析，确定了一系列在10％的CO，5％的O 的CA的第一个24小时诱导生成的压力响应基因。 |
5楼2011-03-04 15:36:24
