EFFECT OF 1-METHYLCYCLOPROPENE (1-MCP) ON THE FLAVOUR METABOLITES OF APPLE JUICE
This study determined the effect of 1-methylcyclopropene (1-MCP), storage atmosphere storage time and harvest maturity on the content of sugars, titratable acidity (TA), malic acid, volatile compounds, total phenolic content (TPC), total antioxidant activity (TAA), and physiochemical juice quality parameters of cloudy, clear and fresh juices prepared from ‘McIntosh’ and ‘Honeycrisp’ apples. The effectiveness of 1-MCP treatment was confirmed by the suppressed ethylene production of intact fruits. 1-MCP treatment had a significant (p<0.05) effect on TA, malic acid content, volatile aroma concentration, TPC and TAA of juice samples from both apple cultivars. Generally, juice samples from 1-MCP treated apples had significantly (p<0.05) higher TA, malic acid content, TPC and TAA, but with reduced content of esters, aldehydes and total volatile aroma compounds. Improved acidity retention in 1-MCP treated ‘McIntosh’ was attributed to the downregulated MdcyME and upregulated MdcyPEPC genes, which regulate malic acid degradation and biosynthesis, respectively. Similarly, acidity retention in ‘Honeycrisp’ was associated with the upregulated V-ATPase and MdcyMDH genes that regulate the vacuolar transport and cytosolic malic acid biosynthesis, respectively. Fructose, glucose and sucrose were the major sugars in all juice samples tested. In contrast to acidity, the content and composition of these sugars were not influenced by 1-MCP treatment; instead, it was substantially influenced by storage time. During long-term fruit storage, the content of sucrose decreased with a corresponding accumulation of glucose, fructose and total sugar. Juice processing techniques had a significant (p<0.05) effect on the concentration of volatile aroma, TPC, TAA and juice colour. As compared to clear juices, cloudy juice samples had significantly (p<0.05) higher content of most volatiles, TPC and TAA as compared to clear juice counterpart. The cloudy juice prepared from both cultivars could able to meet the expected yellowish colour. Turbidity and cloud stability values obtained from late harvested ‘McIntosh’ fruit fulfilled the requirement of stable cloudy juice. However, ‘Honeycrisp’ apples were not suitable for the production of stable cloudy juice as the turbidity (<250 NTU) and cloud stability (<50%) values were much lower than the minimum requirement.