Scientists at ARS Market Quality and Handling Research Unit, Raleigh, North Carolina, studied changes in antioxidant levels of roasted peanuts and their blanched skins across an industrially relevant range of roast treatments. Peanuts were incrementally roasted at 362 deg F from zero to 77 minutes before examining the water- and oil-soluble antioxidant activity levels of the roasted samples.
“Dark-roasting consistently increased water- and oil-soluble antioxidant capacities for both commercially available peanut flours and blanched peanuts,” according to a report on the ARS website. “Peanut skins, currently considered a waste product of industrial peanut processing, had remarkably high antioxidant capacities across all roast conditions,” it added.
The increase in antioxidant levels after roasting were attributed to greater concentrations of phenolic compounds and browning reaction products. Browning occurs during thousands of complex chemical reactions in which proteins and sugars interact to create brown pigmentation. These reactions, known as Maillard browning, are thought to contribute to the characteristic flavor of roasted peanuts.
The scientists also measured vitamin E in the roasted peanuts. “Vitamin E degradation was most rapid in oil from lightly roasted peanuts; however, oil from darker roasted peanuts had better vitamin E retention than that of lightly roasted or even raw peanuts,” said the report. “This preservation of vitamin E could be due to the increased concentration of oil-soluble Maillard reaction products, which seem to protect vitamin E from oxidation,” it continued.
Although darker roasted peanuts are inappropriate for some uses, they can be used to prepare darker roasted peanut flours and flavor extracts.
The authors said the report expands the fundamental knowledge of roasting as it relates to the antioxidant capacity of peanuts and peanut ingredients.
The study was conducted by food technologist Jack P. Davis and his colleagues in the ARS is the principal intramural scientific research agency of the USDA. It was first published in the journal Food Chemistry and a report appears on the USDA website.