Oak flour as a replacement of wheat and corn flour to improve biscuit antioxidant activity

Abstract Due to the high antioxidant activity of oak fruits, the partial substitution effects of wheat flour (WF) or corn flour (CF) with oak flour (OF) have been investigated. WF or CF was replaced by OF at levels of 0%, 15%, 30%, and 45% in the biscuit formulations to prevent the spontaneous oxidation of lipids, and the characteristics, including peroxide value, antioxidant activity, and sensory properties, were evaluated during 28 days storage. According to the results obtained, biscuit samples with higher OF percentage had higher antioxidant activity and less peroxide value. In terms of sensory evaluation, no adverse effect was observed in the samples containing OF. Therefore, it could be introduced as a good natural source of antioxidant compounds for use in food formulations.

The loss of color and nutrient value, and more importantly, the accumulation of deleterious compounds are the other results of this chemical reaction (Mildner-Szkudlarz et al., 2009). Therefore, maintaining the high quality of biscuits is of great nutritional and economic importance, since this product is widely used and often stored for extended periods before consumption (Mildner-Szkudlarz et al., 2009;Reddy et al., 2005). Lipid oxidation may be prevented by the use of oxidation inhibitors or antioxidants. Synthetic antioxidants have been used in food since the beginning of the last century; however, their use has begun to be restricted because of their toxicity (Byrd, 2001;Reddy et al., 2005). Recently, natural plants have received much attention as sources of biologically active substances, including antioxidants, antimutagens, and anticarcinogens (Dillard & German, 2000). Acorns, the fruit of oak trees, have been an important part of traditional diets of people throughout the world and are reported to have potential health benefits (Polimac, Komlenic, & Lukinac, 2015). Acorn flour is desirable from a nutritional point of view, because of the content of fat (of which over 80% is unsaturated), proteins, and considerable amount of electrolytes (calcium, magnesium, potassium, and phosphorus), but little or no sodium, and is rich in iron, copper, and zinc. Acorn meal could be a nutritionally functional ingredient in foods that use wheat flour such as cookies, muffins, breads, noodles, pastries, and deserts with a growing presence in the food industry to improve the eating habits of individual clients and the general population for health benefits and disease prevention . Aside from nutritional properties, oak fruits, acorn, contain phenolic compounds such as the derivatives of benzoic acid, cinnamic acid, proanthocyanidins, quinones, flavons, chalkons, and amino phenolic compounds with high antioxidant activity, which can restrain free radicals, break the chain of radical reactions, and entrap the metals. Increasing the consumption of these compounds leads to a decrease in the risk of digestive diseases and dangerous cancers (Bonoli, Marconi, & Caboni, 2004;Cantos et al., 2003;Muchuweti, Ndhlala, & Kasiamhuru, 2006).
The present study is an attempt to evaluate the effect of oak flour (OF) utilization as a source of natural antioxidants on the stability of fat in biscuits. OF was used in replacement of wheat (WF) or corn flour (CF), and its effects were investigated.

| Materials
The oak fruits (Quercus brantii) were collected from the forests of Fars province in Iran. WF and shortening were purchased from the Tarkhineh Karaj Co. and the Kamoor Co., respectively, in Iran. Sugar, sodium carbonate, eggs, and invert syrup were bought from the local market.

| Oak flour preparation
The wooden shell of oak fruits was removed and dried in an ambient condition for 28 days. The inner shell (placenta) of the fruit was separated and pounded by a hammer mill. Finally, OF was produced by sieving (mesh size = 20).

| Biscuit preparation
The biscuit dough was produced according to Table 1. In order to evaluate the effect of OF on biscuits properties, this flour was used as a replacement for WF or CF, according to the formulations given in Table 2. All ingredients were mixed for 5 min in a mixer (Major Classic KM636, Kenwood, UK), at the second speed stage. Then, the biscuit dough was rolled out to a thickness of 5.5 mm and cut into a diameter of 5 cm. The biscuits were baked for 15 min at a temperature of 250°C of the oven (Imen Gas, Iran). Biscuits were baked for 15 min, cooled in ambient temperature (20-25°C), and stored in polyethylene packages until the testing time.

| Analysis of flour types
Moisture, fat, protein, ash, fiber, and pH of flour types were measured according to AACC (2000) methods.

| Determination of peroxide value
The lipids of grinded biscuits (50 g) were extracted according to the method given by Mildner-Szkudlarz et al. (2009) with some modifications. The extraction was done by using hexane solvent in a laboratory shaker in ambient conditions. After filtration and separation of lipid fraction, solvent was removed by evaporation under vacuum condition onto the rotary evaporator (Bokhi, Switzerland). The extracted oil was immediately used to calculate the peroxide value (PV) according to AOAC (1990) methods.

| Determination of antioxidant activity
The antioxidant activity of biscuit samples was measured in terms of radical scavenging ability using the stable radical 2,2-diphenyl-1-pykrylhydrazyl (DPPH). One gram of the samples was mixed with 9 ml of ethanol solution (50%) and stirred for a minute. The samples were centrifuged for 10 min at 25°C and 203 ×g. One ml of 1 mmol/L DPPH methanolic solution was added to 3 ml of the supernatant phase and the resulting mixture was vigorously stirred. The mixture was left for 30 min in the dark at room temperature. The absorbance was measured at 517 nm. The radical scavenging activity was expressed as the inhibition percentage of free radical by the sample and was calculated using the following formula (Salmanian, Sadeghi Mahoonak, Alami, & Ghorbani, 2014): where A 0 is the absorbance of the control (blank, without sample) and A t is the absorbance of sample. Step

Raw materials
Amount (%, on the basis of the weight of flour) Method

| Statistical analysis
All tests were conducted in a completely randomized design with three replications using the SAS 9.0 software (SAS Institute Inc., Cary, NC). All statistical comparisons were performed using the LSD test at the level of 5%.

| Chemical analysis of flour types
The flour types used were analyzed for pH, moisture, protein, fat, ash, and fiber contents. The results are shown in Table 3. An important factor influencing the overall quality (color, flavor, texture) is the pH (Sabrin, 2009). A comparison between the pH of these three flour types showed that CF had significantly (p < .05) higher pH (6.26) than WF or OF. The pH of WF and OF was the same (5.77), that is, they are more acidic than CF. OF had the lowest moisture (6.46%) and protein (4.14%) contents in comparison to CF and WF, while the highest fat (7.70%) and ash (1.83%) contents were also related to this flour type. There was a significant difference (p < .05) between flour types for these factors. Also, OF fiber content (2.51%) was significantly (p < .05) lower than that of WF (5.06%). In general, the differences between chemical components of the different flour types may be due to geographical location, the culture, and climatic conditions. Sabrin (2009)  Also, according to Silva et al. (2016), OFs exhibit relevant amounts of lipids (8%-14%), low protein content (4%-5%), and 1.8%-2.1% ash content, which are in close agreement with our results.

| Peroxide value
Peroxide value of biscuit samples was measured during storage; the results are shown in lower PV than the W/100 sample, which showed higher phenolic compound amount in CF. This is in agreement with the observed higher phenolic compounds in corn muffin in comparison to wheat muffin (Korus, Gumul, Krystyjan, Juszczak, & Korus, 2017;Soong, Tan, Leong, & Henry, 2014). Cereal grains are a good source of antioxidants, including vitamin E, folates, minerals (iron, zinc), trace elements (copper, selenium, and manganese), carotenoids, and phytic acid. The most abundant free phenolic acids in wheat are ferulic, vanillic, and ρ-coumaric acids, while corn is rich in ferulic acid and carotenoid (Fardet, Rock, & Rémésy, 2008). According to the results presented in Table 5, PV has increased in all biscuits during storage and there is a significant difference (p < .05) between Day 1 and Day 28.

| Sensory evaluation
The panelists evaluated the organoleptic properties of biscuit samples; the results are presented in T A B L E 4 Peroxide value (O 2 meq/kg oil) of biscuit samples during the 28 days storage