Abstract

Corn (Zea mays L.) is one of the most diverse grain crops and most widely planted cereal in the world. As one of the corn products, popcorn has an increasing popularity which brings necessity to investigate bioactive constituents depending on the corn types and popping methods. In this study, different corn grains with different colors (white, yellow, red, purple, black) were investigated in detail, before and after popping, to find out their physical and chemical properties and sensory evaluations of popped corns were recorded. Colored corns exhibited having a higher amount of phenolic content and antioxidant activity when compared to yellow and white corns. Besides, microwave popping was exhibited to protect healty constituents more than other methods, however, sensory evaluations exhibited significantly (P <0.05) machine popping as a better way in terms of apperance and taste. Aim of this study is increasing the awareness of the consumer on the colored corns with healthy constituents and give information for choosing the best corn popping method.

1. Introduction

The antioxidant capacity of whole grain, colored grain, leguminous grain, red-violet fruit and vegetables is quite high. Vitamin E, folates, iron, zinc, selenium, copper, manganese, lignin, betaine, carotenoids, phytic acid, choline, sulfur containing amino acids and alkyl resorcinols are antioxidant compounds. ^{1, 2, 3} Due to the increasing interest of people in healty food, the scientists have focused more on red, purple, blue, yellow pigmented fruits, vegetables and grains. ⁴

Due to changing food habits and industrial requirements, corn has recorded the highest growth rate among all grains. The decrease in wheat and rice consumption in the daily diet has led to an increased interest in corn. In recent years, there has been an increase in the demand for bioactive components for a higher quality product. Phytochemicals, especially phenolics which are predominantly concentrated in the bran part of the grains, exhibit significant antioxidant activity with a number of other bioactivities such as cell differentiation, deactivation of pro-carcinogens, DNA repair, inhibition of N-nitrosamine formation and estrogen replacement. ^{1 5, 6, 7, 8}.

A corn grain consists of 70% - 90% endosperm, 10 - 12% embryo and 5 - 6% bran fractions. The minerals present in corn are found in the embryo, which is 78% of the nucleus. Corn has the phosphorus, magnesium, and potassium minerals mostly besides calcium, copper, iron, manganese, selenium and zinc minerals and also vitamins A, C, D, E, K, B6, B12, tiamin, niacin and folate ⁹. When corn grains are heated quickly and evenly, the inner water turns into steam increasing the internal pressure to cause expansion of the pericarp and it changes the nutrients such as fat, starch and fiber. Popped corn, with almost 20% of the fiber content and low calorie, is an enjoyable snack for many people. ¹⁰ Popcorn is consumed as a nutritious snack with increasing popularity. This consumption contributes approximately one third of the nutritional needs in Central America and South African. ⁹ In 2017, corn production was more than one billion metric tonnes and corn products were consumed worldwide by approximately 1.2 billion people.

Corn and ground corn meals, besides flour and bran fractions are an integral part of feeding all socio-economic classes. Due to their colorful appearance, red, purple, blue and black corn seeds are produced for some special dishes or decorations. Colored corn grains are rich for phenolic compounds and flavonoids which are the most common form of phenolic acids found in corn starch and found in the form of bound and soluble fractions. ¹¹ Flavonoids are responsible for colors (yellow, ivory, cream, orange, red, purple, blue) of the plants. Anthocyanins, from the flavanoid class, are water-soluble polyhydroxy glycosides and polymethoxy derivatives of 2-phenylbenzopyrylium or flavinium salts which are soluble in water and their color is blue, purple or red depending on the pH and the presence of chelating metal ions. Contrary to other flavonoids, their acidic solution carries a positive charge. Anthocyanin-rich foods inhibit the proliferation of colon cancer cells and reduce the risk of colon cancer. The glycosylated or aglycone forms of anthocyanins are called anthocyanidins such as cyanidin, delphinidin, pelargonidin, malvidin, petunidin and peonidin as the most common ones. ¹²

Colored corn kernel is in the functional food category because it is rich in phenolic compounds and carotenoids, provides biochemical support to the diet and reduces the risk of colon cancer. ¹² In this study, it is aimed to determine the effect of different popping methods (machine, pan, microwave) on different functional constituents of colored (white, yellow, red, black, purple) corns. Since there are limited studies on the colored corns, it is expected that this study will make significant contributions to the literature and raise the condciousness of the consumer.

2. Materials and methods

Yellow, red, black, purple and white colored 10 different corn grains (A: yellow, B: black, C yellow, D: black, E: yellow, F: red, G: purple, H: purple, I: purple, J: white) were obtained from Bafra and Carsamba provinces of Samsun city, Turkey (Figure 1). The samples were collected as corncobs and stored as corn grain in refrigator at +4°C.

All reagents and chemicals are analytical grade. Hydrochloric acid, methanol, sodium carbonate buffer, 2,2 diphenyl 1-picrylhydrazy (DPPH), 2,2-azino-bis 3-ethylbenzothiazoline-6-sulphonic acid (ABTS), potasium persulfate, sodium chloride tripyridyl triazine (TPTZ), glacial acetic acid, sodium acetate buffer, acetonitrile and o-phosphoric acid were purchased from Sigma. Folin- Ciocalteu reagent and monobasic sodium phosphate were purchased from Merck. Iron III chloride and potassium chloride were purchased from Carlo Erba.

Corn samples were prepared for chemical analysis by milling in laboratory type miller (Yazıcılar, Turkey) and sieving through 1 mm diameter sieves. Milled samples were stored at +4°C in 3 fold polyethylene bags after removing the inside air. Before the analysis, samples were rested for 1 hour at room temperature.

Popping method of Ceylan and Karababa was adapted. ¹³ For popping, microwave oven of Arçelik MD 581/ Türkiye model which has power of 750W was used for 3 minutes at 120-130°C. Into a 800 ml beaker, 15 g of corn sample was weighed and a 20 ml beaker was placed upside down in order to prevent the burning of the grains. Large beaker was covered with parafilm, and 5-6 small holes were drilled not to allow the vapor in the beaker to damage the samples such as burning. The beaker was placed in the center of the microwave oven and operated for 3 minutes, and the popping process was terminated. The same procedure was applied for all corn grains.

Popping method of Quinn et al. was adapted and Scarlett TPM-320 (Turkey) corn popping machine was used for 2 minutes around 200°C for 15 g of corn grain and 5 g of sunflower oil was used for each sample. ¹⁴

Popping method of Ceylan and Karababa was adapted. ¹³ Before popping 1 g of oil and 15 g of corn grain were placed in a teflon pan with a diameter of 30 cm and a height of 2.30 cm, the mouth of the teflon pan was closed and the content of the teflon pan was stirred at certain intervals. Popping process took 3 minutes at around 150°C.

After weighing approximately 3-5 g of samples into aluminum containers, they were dried in an oven at 105°C. The drying process was continued until the samples reached at a constant weight, then they were cooled in a dessicator for 15-20 min and after weighing again % moisture content was calculated from the weight differences.

L (brightness, 100: white, 0: black), a [(+: red (+100), -: green (-80), 0: grey] and b [+: yellow (+70), - : blue (– 80), 0: grey] values of corn samples were measured by using Minolta CR-400 colorimeter. 10 measurements were made for one sample to determine each value.

The water activity was measured at 25°C using Aqua Lab Dew Point Water Activity Meter 4TE, USA prior to the calibration procedure. Each sample was measured after being adjusted to fill the measurement vessels completely.

The porcelain crucibles were left in the washing solution for 24 hours, washed and rinsed with distilled water and brought to constant weight. Approximately 3 g of corn sample was weighed into porcelain crucibles at constant weight and 1-2 ml of ethanol was added and burned slightly to form a crust. After this process, the samples placed in the furnace to be burned at 900°C until no black spots were found. Then, crucibles were weighed after cooling in a dessicator and the % ash contents were determination of water content on a mass basis. ¹⁵

Method of Singleton and Rossi was adapted for the extraction of phenolic substances from the samples. ¹⁶ After adding 10 ml of 80% methanol with 1% of HCl onto the sample, the mixture was vortexed at 1800 rpm for 15 min and left for extraction for 18 hours under refrigeration conditions. Thermo Spectronic Helios UV-visible spectrophotometer (Gamma, USA) was used for the analysis. Onto a 20 µL of sample extract which was taken into microcuvette, 1.58 mL of distilled water and 100 μL of Folin-Ciocalteu reagent were added and after waiting for 5 minutes, 300 μL Na2CO3 was added and incubated for 2 hours in the dark. Two parallel studies were conducted and the absorbances at 760 nm were measured. Control analysis was performed with distilled water, then continued to sample measurement. The results were expressed in mg GAE/g as gallic acid equivalents.

Onto a 1 g of sample, 80% of methanol without acid was added. The obtained mixture was vortexed at 1800 rpm for 15 min and left for extraction under refrigeration conditions for 18 hours.

DPPH (1,1- diphenyl-2-picryllhydrazyl) Assay: Freshly prepared DPPH solution was added onto 50 μl sample extract. After waiting 30 minutes in the dark, the absorbances were recorded at 517 nm with UV-visible spectrophotometer. Another measurement was performed after 1 hour in order to check if the reduction occurred. The absorbance of the control sample which was prepared with the solvent and DPPH was recorded before the actual sample to calculate the % reduction. Results were expressed as %DPPH radical scavenger activity [(Absorbance of control sample – Absorbance of sample) / Absorbance of control sample x 100]. ¹⁷

FRAP (ferric reducing and antioxidant power) Method: Analysis solution was prepared by mixing 300 mmol/L of acetate buffer, 10mmol/L of tripyridyl triazine (TPTZ) solution and 20 mmol/L of FeCl3.6H2O solution with the ratio of 10:1:1 respectively. After adding 700 µL of analysis solution onto 50 µL of sample extract and waiting for 5 min in dark, the absorbances were measured at 593 nm with a UV-Visible spectrophotometer. The results were expressed as trolox equivalent. ¹⁸

10 ml of 80% methanol was added onto 1g of sample. The mixture vortexed at 1800 rpm for 15 min and left for extraction under refrigeration conditions for 18 hours. The solution of pH 1.0 was prepared by weighing 1.86 g of KCl into 980 ml of distilled water and adjusted with concentrated HCl. For the solution of pH 4.5, 54.43 g of sodium acetate was weighed into 960 ml distilled water and adjusted with concentrated HCl. The absorbances were recorded at 520 nm and 700 nm. ¹⁹

Extraction solution for the corn samples were prepared by mixing 1.5 N HCl solution and 95% ethanol with the ratio of 15:85. Onto 1 g of corn samples, 25ml of the extraction solution was added and left for extraction at 4 C° for 18 hours. Shimadzu high performance liquid chromatography (HPLC) instrument with Shimadzu LC-20AT pressure pomp, Shimadzu SIL-10A auto-sampler, Shimadzu CTO-10AS VP thermostated column compartment, photo dioderay dedector (PDA) with reverse phase C18 column (250 x 4.6 mm, 5 cm, Phenomenex) using a mixture of o-phosphoric acid/acetic acid/acetonitrile/water with the ratio of 1:10:5:84 (v/v/v/v) as mobile phase at flow rate of 0.7 mL dak-1, injection volume of 20 μL and wavelength of 520 nm was used to determine cyanidine 3-glucoside and calculate its amount. The results were expressed as mg/kg. ^{20, 21}

Specific volume: The washed, dried and sieved sand was taken up to a 400 ml beaker. After weighing 5 g of popcorn and placing into a 400 ml beaker, sand was discharged into the beaker containing the popped corn and the volume (cm3) of overflowing sand was measured with a graduated cylinder.

Volume increase: Volumes of 5 g unpopped and 5 g popped corns were measured by using displacement method and the results were compared to calculate the ratio of volume increase.

Popping ratio: 200 pieces of corn grains were counted and popped with popping machine, microwave and pan. After the popping operation, the unpopped ones were counted to calculate the ratio. ¹³

To evaluate differences in appearance, color, volume, taste, texture, mouth-feel perception and overall acceptability, 10 trained panelists from the Food Engineering Department of Ondokuz Mayıs University tasted the popped corn samples from a white container. The evaluation was conducted in a quiet and odor-free room, water was provided for rinsing in-between samples and each criterion was graded from 0 to 10 on a sensory analysis form adapted from Pyler. ²¹

The data of the study were subjected to the variance analysis in 10x3x2 factorial order by SPSS Statistics 21 program and the statistical significance limits of the differences were determined. Variable sources of variation were compared using the Duncan multiple comparison test. The results with standard deviations are summarized in tables and same letters in the Duncan test indicates the results with significant difference (P) less than 0.05.

3. Results and Discussion

Raw corn samples were analyzed before popping to identify their dry matter, color (L: luminance, two color channels: a, b), water activity (a_w), ash, total phenolic content (TPC), antioxidant capacity (DPPH: 2,2-diphenyl-1-picrylhydrazyl and FRAP: ferric reducing antioxidant power), total anthocyanin content (TAC) and individual anthocyanin (cyanidin-3-glucoside) contents (IAC).

As it can be seen from Table 1, type A (yellow) corn has the highest (89.43 %) dry matter where type E (yellow) corn has the lowest (85.89 %). The moisture content of all samples were condition to 13±0.1% either by evaporating or adding water in order to determine the best popping method. It was controlled by dry matter analysis and mass balance equations for each sample.

According to the raw material analysis results (Table 1), the highest and lowest L values were J (white corn) and H (purple corn), respectively, the highest and lowest a values were F (red corn) and B (black corn), respectively and the highest and lowest b values were found to be A (yellow corn) and B (black corn), respectively. The results of the study are similar to Munoz et al. ²²

As expected, TPC was found highest (235.42 mg GAE/kg) in the type H (purple) corn and lowest (117.63 mg GAE/kg) in the type J (white) corn. These results are not quite compatible with previous studies. For instance, in a recent study with purple corn, Harakotr et al. ²³ found TPC in between 4.8 – 12.3 mg GAE/kg which is fairly lower than our findings and according to the studies of Zilic et al. ²⁵ on colored corns, TPC varies between 4494.1 – 10528.8 mg GAE/kg which is extremely higher. Also, Zhao et al. ^{11, 26} and Montilla et al. found acceptable higher results (114.7 – 489.8 mg GAE/kg and 311.0 – 817.6 mg GAE/kg respectively) when compared to our study. These differences might be due to soil type and growing conditions.

Total antioxidant capacity results (Table 1) were determined by the FRAP method, with the highest value in I (purple corn) corn and the lowest value in J (white corn) corn. The FRAP value results for corn are in the range of 8.58-14.64 µmol/g. Yang and Zhai determined the FRAP results of purple corn between 16.2 µmol/g and 20.7 µmol/g, which is not similar to the results of this study. ²⁶ In addition, Harakotr et al. determined the FRAP value in the range of 4.3 µmol/g -69.8 µmol/g and 18.2 µmol/g - 159.2 µmol/g in milk and ripening corn, respectively, and it was determined that this was not in line with our results. ²⁴

Obviously, colored corns, especially purple ones have higher antioxidant capacity as well as total and individual anthocyanin contents which is quite understandable since phenolic compounds, carotenoids and some flavanoids (anthocyanins) are responsible for the colors of a plant such as orange, red, purple and blue ²⁵.

The corn grains were popped with three different methods: machine, microwave oven and pan, then physical, chemical and sensory tests were performed for the popped corns. The results were statistically tested and compared in terms of corn type and popping method. Both corn type and popping method were found significantly (P<0.05) effective on all analysis results.

Physical analysis

Table 2 summarizes the results according to corn type and the results according to popping method are given in Table 3.

According to these results (Table 2), the highest dry matter ratio among corn was observed in D (black) corn (97.00%) and the least in H (purple) corn (95.84%) and microwave popping gives the highest dry matter ratio (96.98%). This result is expected because of the fact that the heat treatment in microwave oven is faster than other methods since the heat production starts at the molecular level and distributes the heat uniformly. ¹⁴ According to these results (Table 3), it was determined that the highest amount of dry matter among the popcorn methods was obtained as a result of blasting with microwave. It has been determined that the reason for this is that the resistances of the popping machine are at the bottom, so one surface of the corn is in contact and mixing process such as popping in the teflon pan cannot be done.

When the effect of the popping method on the specific volume was examined, it was determined that the highest results were obtained on C (yellow) corn (15.66 cm3/g) and by microwave popping (13.97 cm3/g). However, volume increase was observed highest in J (white) corn (12.99%) and by corn popping machine (9.57%) slightly more than microwave oven (9.51%). Among all the methods, microwave oven resulted the least (78.20%) popping ratio while traditional pan resulted the highest (81.35%) (In Duncan statistics test, the same letters indicate the same level analysis results). The volume of popped corn is related with the water content of the grains and the popping temperature. For optimal popping, sufficient pressure must be obtained inside the grain by reaching high temperature. ²⁷ As a result, microwave and popcorn machine spread the heat in the grain in a shorter time, which is important for the popping mechanism.

According to the results of the popping ratio (Table 2 and Table 3), it was determined that the highest rate was found in G corn in teflon pan and the lowest in E corn with microwave method. Erbaş et al. determined in their study that the lowest blasting rate was the microwave blasting method, which is in line with this study. ³

In order to understand the effect of popping method on each measured physical property of the corn samples, interaction graphics are given in Figure 2.

Chemical analysis

Functional constituents (TPC, antioxidants, TAC and IAC) of corn samples were analyzed and the results according corn type are given in Table 4.

Type F (red) corn gave the highest (538.08 mg/kg) value of TPC and type E (yellow) corn gave the lowest (133.56 mg/kg). Although it is expected to see the highest TPC in black or purple corns, this result might be due to the corn variety. When compared to raw materials, popped corns resulted higher TPC values. This result might be due to conversion of bound form to free form as it was stated in a study of Dewanto. ²⁸ Because, the distribution of phenolic substances in the plants is not equal, the insoluble phenolic substances are present in the cell wall of the plant, and the soluble phenolics are in the interior of the plant cell. Thus, the outer layer of the plant contains a higher amount of phenolic compounds (ferulic and p-coumaric acids) and the heat treatment results their release. ²⁹ Among the methods, pan popping gave the highest TPC value, most probably due to the corn is exposed to heat for a longer time with the pan popping method and the phenolic components in teh bound form pass into the free form.

The antioxidant capacity of the samples were analyed via DPPH, radical-scavenging acitivity and FRAP method, ability to reduce Fe3+ to Fe2+ in the presence of TPTZ. Purple corns (G and I) resulted considerably higher (37.02% and 37.21% respectively) radical-scavenging activity while B (black), C (yellow), D (black) corn resulted the lowest (respectively 17.75 %, 16.90%, 17.01 %). Besides, popping with corn popping microwave gives the best antioxidant activity (27.58%). On the other hand, the highest (19.86 µmolTE/g) FRAP value was obtained in the type C (yellow) corn and type H (purple) corn gave the lowest (13.50 µmolTE/g). Also, pan popping resulted the highest anti-oxidant activity via FRAP method. DPPH is a stable free radical which turns into a stable diamagnetic molecule after accepting an electron or hydrogen radical and its color changes from violet to pale yellow to result a decrease of the absorbance after scavenging antioxidants. ³⁰ In the FRAP assay, Fe (III) is reduced to Fe (II) with single electron transfer and color change in the reaction is recorded. Since the antioxidant compounds are electron donors, they also reduce the oxidized intermediate of the lipid peroxidation process, so that they can act as both primary and secondary antioxidants. [31] Although both DPPH and FRAP assays serve for the same purpose, different mechanisms under the experimental conditions may alter the results.

In correlation with DPPH method, type G (purple) and H (purple) corn resulted the highest TAC (208.53 mgCGE/kg) and type G (purple) corn resulted the highest IAC (74.75 mgCGE/kg). The lowest TAC (128.96 mgCGE/kg) was observed in another yellow corn (type E) and the lowest IAC (24.13 mgCGE/kg) in type J (white). As it was found via DPPH method, popping machine resulted highest TAC and IAC while microwave popping resulted the lowest in agreement with literature. ⁴

In order to understand the effect of popping method on each measured chemical property of the corn samples, interaction graphics are given in Figure 3.

Sensory evaluation

Sensory evaluation results of popped corn samples according to corn type and popping method are presented in Table 6 and Table 7 respectively. Both corn type and popping method were found significantly (P<0.05) effective on all analysis.

As it can be seen from Table 6, type G (purple) corn gives better results overall acceptability. It is followed by type A (yellow) corn gives better results apperance and color. On the other hand, type E (yellow) corn has lower results from all evaluations.

In order to understand the effect of popping method on each measured sensory property of the corn samples, interaction graphics are given in Figure 4.

Machine popping gives by far the best results in all evaluations and it is followed by popping with pan. Compatible with the results of this study, Paraginski reported sensory evaluations with the lowest results after microwave popping. ¹⁰ The reason of this fact is thought to be the easier loss of volatile tasty flavors during microwave popping.

4. Conclusion

Yellow corn is the most consumed corn variety in our country and in the world. However, it is observed that both the production and the consumption of colored corn are less due to the less commercialization. It is demonstrated that colored corns contain a higher amount of phenolic content and antioxidant activity when compared with yellow and white corns. Also, the popped colored corns have higher functional constituents than their raw material.

When the popping method is evaluated, it is determined that the phenolic content is obtained the highest after popping with pan and highest with corn popping machine. According to overall acceptibility of sensory evaluations, the best result was obtained with corn popping machine, while microwave popping was evaluated with the lowest score.

It is expected that this study will be fruitful in terms of increasing the awareness of the colored corn which is thought to be healthy for the consumer and give information to choose the best method for corn popping.

References