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An investigation of low diastase activity in mānuka honey

Mānuka honey with high non-peroxide antibacterial (NPA) activity is a valuable export product in the multi-million-dollar New Zealand honey export industry, with monofloral mānuka honey accounting for 68% (NZ$290 million) of the total export value (NZ$425 million) for the 2019/20 season. Exported honey must meet strict export regulations. Diastase enzyme (alpha amylase) activity is a quality control indicator of detrimental storage conditions (excessive heat treatment or prolonged storage), decreasing over time. International export requires honey to have high diastase activity (Diastase Number, DN > 8). Medium to high NPA mānuka honeys often fail the diastase test, despite careful storage. It is important to determine the cause of low diastase activity in mānuka honey to prevent revenue loss for New Zealand honey exports. Bioactive compounds in the honey were hypothesised to inhibit alpha-amylase, therefore reducing diastase activity. A database of fresh mānuka and non-mānuka honeys were analysed for various chemical parameters (including moisture content, mānuka chemical marker compounds, dihydroxyacetone (DHA), methylglyoxal (MGO), 5-hydroxymethylfurfural (HMF) and diastase activity) to determine if any correlations were present with diastase activity. Parameters between and within floral honey types varied largely, however no correlations were observed to diastase activity. Storage experiments (20 °C, 161 days; 27 °C, 198 days) of selected database honeys showed the mānuka marker compounds were stable. However, the rates of change observed for DHA, MGO, HMF and diastase activity were reliant on intrinsic honey characteristics (not identified). While time and temperature had significant impact on loss of diastase activity in honey, correlation was observed between changes in diastase activity and DHA, MGO and mānuka marker 3-phenyllactic acid (3-PLA) concentrations. Additionally, a clover honey matrix was perturbed with single bioactive compounds (DHA (MGO-precursor), MGO, mānuka markers compounds, methyl syringate and tannic acid) to observe the effect on diastase activity over time (185 days) and at 20, 27 and 34 °C. MGO and 3 PLA caused diastase activity to decrease at a significantly faster rate than other compounds. The half-life of diastase at 27 °C for the control sample was 590 days compared to the much shorter half-life of diastase in the MGO (440 days) and 3 PLA (380 days) perturbed samples. The mechanism by which these two compounds decrease diastase activity is unknown, however MGO is a highly reactive molecule and has been reported to inhibit other enzymes. Time and temperature were also significant drivers of diastase activity loss; diastase half-life at 20 °C was more than eight times greater than it was at 34 °C (1340 vs. 160 days). The experiments indicate that diastase activity is highly variable between fresh honeys and the rate at which it decreases may be dependent not only on temperature and time honey is stored at, but on intrinsic honey compounds and physico-chemical parameters suggesting that diastase activity regulations may not be suitable for mānuka honey.
Type of thesis
The University of Waikato
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