4.1. Quantity of Antioxidants
Among the different grains (oats, wheat and rice), corn has been shown to contain the greatest amount of antioxidant activity due to phenolic acids [11
]. However, most of the phenolic acids are bound in insoluble fiber. Dewanto et al. reported the free total phenolic content of raw sweet corn to be 0.250 ± 2.0 mg/g while a total phenolic content of 2.64 ± 0.10 mg gallic acid equiv/g was reported for corn samples extracted by basic hydrolysis (based on dry weight) [11
]. This compares favorably with our average value of 5.93 ± 0.92 mg catechin equiv/g for basic hydrolysis and 2.66 ± 0.15 mg catechin equiv/g for in vitro digestion and supports our low value for free phenolic content. The varieties of corn used for popcorn seem to have an equal, if not slightly greater total dry phenolic content compared to other studied corn varieties. Popcorn can be consumed with little processing or preparation. According to a recent review, thermal treatments such as cooking or drying can have mixed results on the phenolic content of food depending on the cooking method and species [20
]. With regard to thermal treatments, the phenolic content of grains seems to be largely unaffected by baking and the phenolic content of cooked sweet corn increases by 54% after baking [19
]. In this work, the antioxidant capacity and total phenolic content were shown to not be significantly affected by popping in a microwave oven when the phenolics were extracted with chemical hydrolysis (p
> 0.05) Thus, the popping process does not appear to degrade the phenolic compounds originally present in the kernel. However, when extracted through in vitro digestion, the popped kernels had significantly more antioxidant capacity compared to raw kernels.
In vitro digestion has been shown to increase total phenolic content in cooked foods. For example, recently Ti et al. showed an increase in total phenolic content and antioxidant capacity of cooked brown rice after in vitro digestion[22
]. Generally it is thought that cooking or thermal processing breaks down the cell wall, thus freeing any bound phenolic compounds [19
]. This would lead to increased extraction, especially for enzymatic digestion as we observed.
4.2. Popcorn as a Source of Dietary Antioxidants
Since popcorn is a popular whole grain snack food, it may be a significant potential source of phytochemicals in the American diet [6
]. In fact, of the total amount of whole grains Americans consume, 17% comes from popcorn [23
]. Popcorn consumers also have 250% higher whole grain intake and 22% higher intake of fiber versus non-popcorn consumers [6
]. The per capita intake of total polyphenols in the average US diet is about 2000 mg by our data for foods and beverages in the US (Vinson, unpublished results). If we consider that the average popcorn consumer consumes 39 g of popcorn per day [6
], then this consumer is taking in an average of approximately 240 mg (12%) of their total dietary polyphenols from popcorn based upon our values for total polyphenols as measured by the Folin–Ciocalteu assay.
In this study popcorn has been found to contain a substantial quantity of polyphenols. Furthermore, our in vitro digestion study indicates up to 50% of the total polyphenols in popcorn that is ingested has the potential to be bioavailable. Fiber bound ferulic acid is already known to be bioavailable in mammals via esterases with activity towards ester bound hydroxycinnamates
In addition, it has been shown in humans that ferulic acid is liberated from whole grain during the digestion process and makes its way into the body [25
]. Consumption of ferulic acid in animals has been shown to act to lower oxidative stress, increase insulin, decrease lipids and atherosclerosis and reduce blood pressure [26
]. Thus, the reduction of risk of diabetes, heart disease and hypertension may accrue from consuming ferulate in whole grains such as popcorn. However, it is crucial that the entire kernel be consumed. Approximately 98% of the phenolic content and antioxidant content is in the pericarp, despite it being only 15–20% of the total weight of the popcorn kernel. Sweet corn has a similar weight distribution, with 20% of the kernel weight being the pericarp [11
]. This agrees with previous studies on maizes and their antioxidant capacity [19
]. Corn bran is among the highest antioxidant food in existence.
The popping process does not have any substantial effect on the phenolic content in popcorn, making it an ideal source of unprocessed grains and polyphenols. In fact, popcorn eaten plain and air popped is the only food that is 100% whole grain by weight. However it is often consumed with added fat and salt, ingredients which when eaten excessively could negatively affect diet and health [28
]. The popularity and availability of popcorn makes it a possible source of a large portion of the daily antioxidant intake along with other sources such as fruits and beverages. Popcorn is also the most satiating food among the snacks and desserts [30
]. For example, it is 1.6 times more satiating than potato chips, a snack high in acrylamide which is shown to increase oxidative stress and inflammation in humans [31
]. Thus, whole grain popcorn is a prudent choice for those wanting to reduce feelings of hunger while managing energy intake and ultimately, body weight. For reference, a cup of air popped popcorn only contains about 30 Calories. Furthermore coupled with seasonings such as spices and nuts, popcorn can become an even more ideal snack since nuts and spices have large quantities of antioxidants themselves [33
]. Therefore, of the available snack foods, popcorn certainly has the potential to provide the most health benefits as part of a balanced diet.