Comparison of phenolic acids and flavonoids in black garlic at different thermal processing steps

The phenolic acid and the flavonoid constituents of garlic subjected to different thermal processing steps were examined. Black garlic was produced in a ripening chamber by using a programmed stepwise heating schedule, as follows. Step 1: 90 °C and 100% RH for 34 h; Step 2: 60 °C and 60% RH for 6 h; Step 3: 75 °C and 70% RH for 48 h; Step 4: 70 °C and 60% RH for 60 h; Step 5: 65 °C and 50% RH for 192 h. The results of the present investigations showed that thermal processing affected quantities of each phenolic acid and flavonoid component. The total phenolic content (TPC) and total flavonoid content (TFC) of the garlic subjected to different thermal processing steps were higher than those of fresh garlic. In particular, the black garlic cloves ripened using Step 1 (BG1), black garlic cloves ripened using Step 2 (BG2), black garlic cloves ripened using Step 3 (BG3), and black garlic cloves ripened using Step 5 (BG5) samples exhibited levels of TPC that were higher than the TFC, while the TFC in the fresh garlic (FG) and black garlic cloves ripened using Step 4 (BG4) samples were higher than the TPCs. Hydroxycinnamic acid derivatives were found to be the major phenolic acids of garlic at different processing steps. Among the four major flavonoid subgroups in garlic, flavanols were found at the highest concentrations followed by flavanones and flavones, except in the FG sample.