Introduction
Deep frying is one of the oldest and most common cooking methods worldwide. Donuts are one of the most popular fried foods, consumed as a snack or breakfast in many countries. Donuts are usually made from flour, sugar, eggs, milk, salt, baking powder, and sometimes butter. Using whole wheat flour instead of refined white flour significantly increases the nutritional value of donuts, as the bran and germ of wheat contain high amounts of fiber, vitamins, minerals, and antioxidant phenolic compounds.
Materials and Methods
Raw materials Three types of wheat flour were used:
Soft white flour (SWFD)
Hard white flour (SWMD)
Whole hard red spring wheat flour (HWMD)
Method of preparing dough for donuts The dough was prepared with a fixed formula including 400 g flour, 140 ml water, 100 ml whole milk, 60 g sugar, 30 g butter, 10 g salt, 8 g yeast and 1 egg. After kneading, the dough was rested for 1 hour, then shaped into donuts and secondary fermentation was performed at 37°C for 45 minutes.
Frying The donuts were fried in canola oil at four temperatures (120, 140, 160 and 180°C) and for different times until the temperature of the center of the dough reached 99°C.
Measurement of oil absorption, moisture and volume
Oil absorption by Soxhlet method
Moisture by oven drying at 105°C
Volume by rapeseed displacement method
Total phenolic content (TPC) measurement by Folin-Ciocalteu method and expressed as gallic acid equivalent (mg GAE g⁻¹)
Antioxidant activity measurement
DPPH radical scavenging (%)
Lipid peroxidation scavenging (%)
Identification of phenolic compounds by HPLC
Results and discussion
Oil absorption and moisture As the frying temperature increased from 120 to 180°C, oil absorption increased in all three types of donuts and moisture decreased. Donuts prepared with whole wheat flour (HWMD) had the lowest oil absorption and the highest moisture.
Total phenolic content (TPC) Donuts prepared with whole wheat flour had the highest TPC (up to 1.32 mg GAE g⁻¹). As the temperature increased from 120 to 180°C, TPC decreased by an average of 48.3%.
DPPH radical scavenging activity At 120 to 160°C, DPPH scavenging activity increased (up to 80% in HWMD), but decreased again at 180°C.
Phenolic compounds identified by HPLC were the most important compounds: ferulic acid, vanillic acid, caffeic acid, sinapic acid, and catechin. Ferulic acid was the most dominant compound, which showed a significant decrease with increasing temperature.
Conclusion
The use of whole wheat flour significantly increased the phenolic content and antioxidant activity of donuts.
The best temperature to maintain maximum antioxidant activity was between 140 and 160°C.
Temperatures above 180°C caused severe degradation of phenolic compounds and decreased antioxidant activity, although it increased oil absorption.
