Variations Of Key Enzymes Activities In Garlic Greening Process

Garlic （Allium Sativum L.） bulb has unique flavor, high nutritional and medicinal value.However, garlic always becomes green in further processing which has bad effect on itsquality. At present, heat treatment is available to reduce alliinase activity to prevent garlicgreening, but it can affect the generation of flavor substances resulting in shorten edibleness.Therefore, variations of key enzymes activities in garlic greening are of great significance toexplain the garlic greening mechanism and prevent the phenomenon of garlic greening infurther processing. The research used garlic bulb as the raw material to regulate the keyenzymes activities in tricarboxylic acid cycle and bypass system in order to investigate therelationship between key enzymes and garlic greening. The main results were as follows:1. Effect of tricarboxylic acid cycle on garlic greeningThe present study determined the organic acid contents and related enzymes activitieschanges in tricarboxylic acid cycle of the garlic at different storage temperature （4℃androom temperature respectively）. The results showed that with the storage time prolonged, thecontents of citric acid and α-ketoglutarate all increased gradually at different storagetemperature, there was significantly positive correlation with garlic greening strength （P＜0.01）. Citrate synthase and isocitrate dehydrogenase activities, fumarate content firstincreased and then decreased. The activity of succinate dehydrogenase of garlic stored atroom temperature was the same variation tendency, while succinate dehydrogenase activity ofgarlic stored at4℃first increased and then reduced repeatedly. Treatment with malonic acidreduced the activity of succinate dehydrogenase and the content of fumarate significantly, butfirst increased the strength of garlic greening pigments and content of porphobilinogen, thendecreased slightly. So there was obvious relationship between tricarboxylic acid cycle andbiosynthesis of garlic greening pigments. The intermediates in tricarboxylic acid cycle couldenter into bypass system to generate pyrrole compounds to provide precursors for theformation of garlic greening pigments.2. Effect of aminolevulinate dehydratase on garlic greeningThe activity of aminolevulinate dehydratase and porphobilinogen content of garlic weredetermined under different storage temperature （4℃and room temperature respectively）. Theresults showed that with the storage time prolonged, the activity of aminolevulinatedehydratase and porphobilinogen content increased significantly, and there was significantlypositive correlation with garlic greening strength （P＜0.01）. When being incubated atdifferent temperature and being treated with activator Ca²⁺and inhibitor Pb²⁺, the activity of aminolevulinate dehydratase was the same variation tendency with the content ofporphobilinogen and strength of garlic greening. So aminolevulinate dehydratase wasimportant in the biosynthesis of garlic greening pigments, and it could affect the formation ofthe prescuors of garlic greening pigments.3. Effect of different storage temperature on garlic greeningThe objective of this research was to evaluate the effect of different storage temperature（-2℃,4℃,20℃and35℃respectively） on the variations of organic acid contents andenzyme activities in tricarboxylic acid cycle, the activity of aminolevulinate dehydratase, thecontent of porphobilinogen and garlic greening strength after being stored for20days. Theresults showed that the contents of citric acid, α-ketoglutarate and porphobilinogen, theactivities of citrate synthase, isocitrate dehydrogenase and aminolevulinate dehydratase, andgarlic greening strength showed the highest value at4℃, while the activity of succinatedehydrogenase and fumarate content achieved the highest value at35℃. So low temperaturecould promote the phenomenon of garlic greening. The reason was that low temperature couldbreak garlic dormancy quickly, accelerate the process of citric acid to succinyl-CoA intricarboxylic acid cycle, and result in the accumulation of pyrrole compounds. The pyrrolecompounds could provide the prescuors of garlic greening pigments.