Effect of Roasting Temperature on Production of Functional Wild-Rice Tea and Factors Influencing Purchase Intention Towards Wild-Rice Tea

Hyun Yi JUNG,Kyoung Rim HAN,Geon LEE,Joong Kyun KIM

(1Department of Biotechnology and Bioengineering,Pukyong National University,Nam-Gu,Busan 485137,Republic of Korea;2Department of Research &Development,Hyundae F&C,Busan 604-842,Republic of Korea;3Department of Environmental Engineering,Dong-A University,Busan 49315,Republic of Korea)

The properties of raw wild rice were investigated to produce a functional tea,and the value of this tea was evaluated.The highest antioxidant activity [80.5% 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity] was achieved when the raw wild-rice grass was roasted at 225 °C for 3 min.The contents of total phenolics in wild-rice grass and infused wild-rice grass were 24.38 and 22.57 mg/g,respectively.The wild-rice grass tea was free of caffeine and sugar,low in calories and sufficiently low in elements.The factors influencing tea purchase were the sensory quality and functionality of the tea.The tea containing only wild-rice grass was much preferred to the tea containing wild-rice grain.The results suggest that wild-rice grass tea with high antioxidant activity is a promising functional tea.

Nutrients contained in wild-rice tea (Zizania latifolia) include quality proteins,dietary fiber,and vitamins B1,B2 and E.The use ofZ.latifoliain drugs and food products has also been actively investigated for its potential antioxidant and immunomodulating effects (Yan et al,2018).The grains of wild rice have been demonstrated to possess a variety of biological activities including activities relating to prevention of obesity (Han et al,2012),hyperlipidemia and oxidative stress (Zhang et al,2009),and amelioration of high-fat/ cholesterol diet-induced insulin resistance in rats (Han et al,2013).Herbal tea containing amino acids,vitamins and minerals is considered to be a functional food due to its high content of polyphenols.Polyphenols in tea exhibit strong antimicrobial activity (Yeo et al,1995),and phenolic compounds exhibit diverse physiological properties including anti-allergenic,anti-artherogenic,anti-inflammatory,anti-microbial,antioxidant,anti-thrombotic,cardioprotective and vasodilatory effects (Hassan et al,2015).Since tea quality is closely related to aroma,roasting is a powerful processing technology that can be used to increase the amounts of aromatic compounds in whole-grain rice products (Youn and Chung,2012).Roasting also reduces concerns related to the presence of pathogens in the final product.Existing studies on wild rice mostly focus on its pretreatment for use as a feed stock,the contents of phenols and flavonoids (Chu et al,2020),and the contents of antioxidants,phenols and flavonoids in its extract (Zeng et al,2019).Although a material with good biological activity can be added to tea products,the value of wild rice has not been well evaluated yet.Therefore,in this study,the antioxidant activities of wild-rice tea prepared at various roasting temperatures were investigated,and the qualities of wild-rice tea prepared at various grass-to-grain ratios were estimated based on sensory evaluation.Moreover,the total contents of phenols and flavonoids were determined to assess the relationship between the antioxidant activity and the structural components of raw wild rice.The contents of vitamins,caffeine and minerals in the wild-rice tea samples were also analyzed to evaluate the value of wild rice as a healthy tea material.

In this study,effect of roasting temperature on the antioxidant activities of wild-rice tea was investigated to retain functionality during tea production.The roasting experiment was performed at temperatures between 175 °C and 250 °C,based on the result of the preliminary experiment (Fig.S1).The DPPH radical scavenging activity first increased as roasting temperature increased but then decreased when the temperature exceeded 225 °C (Fig.1-A).The increase in DPPH activity can be attributed to the conversion of cell wall-bound polyphenols to free-form polyphenols or the degradation of high-molecular-weight phenols to low-molecular-weight phenols by thermal treatment.These results are similar to those reported forSaururus chinensistea,which exhibits significantly higher DPPH activity after roasting at high temperatures (160 °C-200 °C) compared to after roasting at low temperatures (80 °C-120 °C) (Kang et al,2019).However,further increase of the roasting temperature results in rapid carbonization,destroying antioxidant substances (Kwon et al,2006).The highest DPPH radical scavenging activity (80.5%) was obtained at 225 °C and was significantly different (P＜ 0.05) from those obtained at other roasting temperatures.This trend in roasting temperature can be also found in green tea;the DPPH activity of green-tea leaves roasted at 200 °C exceeded that of tea leaves roasted at 300 °C,indicating the greater thermal degradation of the phenolic compounds at higher roasting temperature.The DPPH activity was also measured after the wild-rice grass tea-bag samples were infused with boiled mineral water for 5 min.Again,the highest activity (73.9%) was obtained for wild-rice grass roasted at 225 °C (Fig.1-B).This indicates that the antioxidant substances present in roasted wild-rice grass were adequately extracted by hot water.

To determine the concentrations of materials responsible for antioxidant activity,the contents of total phenols and total flavonoids in wild-rice grass roasted at various temperatures were investigated.The contents of total phenolics in wild-rice grass tea-bags and infused samples ranged from 10.63 to 24.38 and from 8.61 to 22.57 mg/g,respectively,while the contents of total flavonoids ranged from 10.76 to 32.62 and from 5.01 to 16.37 mg/g,respectively (Fig.1-C and -D).The contents of both total phenols and total flavonoids were the highest after roasting at 225 °C and decreased after wild-rice grass tea-bag was infused with hot water for making tea.In particular,the content of total flavonoids decreased significantly after infusion,indicating that the flavonoids present in wild-rice grass were destroyed to some extent by brief exposure to high temperature.This is because the glycosides in flavonoids are partly broken down to monomers by exposure to high temperature.Despite the significant decrease in the content of natural antioxidants after infusion,the antioxidant properties are maintained or even enhanced possibly due to chemical changes via Maillard reactions (Ho et al,2015).To assess the level of antioxidant activity of the wild-rice tea bag produced in this study,the total phenolic content was compared with previously reported values (Table S1).The highest antioxidant activity (209.00 mg/g) was found in lemon balm tea,while the lowest activity (11.50 mg/g) was obtained in bamboo leaf tea.Even within a single tea,the antioxidant activity differs considerably based on the extraction method for all cases of black,green,peppermint and white teas.In these cases,the extraction method was differently chosen on a part of raw material source,such as leaf,stem and fruit.Similarly,considerable variations in total phenols and antioxidant activity are also found among commercial products,which can be explained by differences in the amount of raw material used along with the extraction time and temperature.Considering the consumer preference with prevention from over-steeped extraction,wild-rice grass was extracted for a relatively short time interval,leading to its comparatively low antioxidant activity.

To establish a prototype functional wild-rice tea,the components of roasted wild-rice grass and grain were analyzed (Table S2).The calorie content of roasted wild-rice grass (3 420.0 kCal/kg) was less than that of roasted wild-rice grain (3 805.0 kCal/kg).Roasted wild-rice grass had higher contents of saturated fatty acids (7.0 g/kg),cholesterol (4.0 mg/kg) and sodium (4 821.0 mg/kg) than roasted wild-rice grain,while the grain had higher contents of carbohydrates (786.0 g/kg),sugars (9.0 g/kg),crude proteins (136.0 g/kg) and crude fats (13.0 g/kg).Since tea consumers diagnosed with diabetes are cautious about excess intake of daily calories,especially from sugar and fat,sugar-free tea manufactured from roasted wild-rice grass is recommended for these consumers.The content of trans fat was almost zero in both roasted wild-rice grass and grain,indicating that the tea will not contribute to the risk of heart disease.The contents of vitamins B1 (1.0 mg/kg),B2 (2.0 mg/kg) and B6 (2.0 mg/kg) were higher in the roasted grass than in the roasted grain,which are much lower than recommended daily allowances per day.Caffeine was not detected in either wild-rice grass or wild-rice grain,indicating that wild-rice tea is suitable for caffeine-sensitive or pregnant women with restricted caffeine intake.Therefore,consumers can enjoy wild-rice tea several times a day,unlike other caffeine-rich beverages including green tea and coffee.

Since tea materials contain trace amounts of elements that play important roles in human metabolism,elemental composition of raw wild-rice was investigated (Table S3).Calcium (Ca) (46 449 mg/kg) and iron (Fe) (35 862 mg/kg) were the most abundant elements in roots,while magnesium (Mg) (2 957 mg/kg) and zinc (Zn) (307 mg/kg) were the most abundant in grain.Manganese (Mn) (645 mg/kg) was the most abundant in the stem,while germanium (Ge) and selenium (Se) (≥ 0.4 mg/kg) were equally distributed among the parts.The root of wild rice contains high contents of elements including Ca and Fe,contributing to the bitter taste of wild-rice tea.On the other hand,the contents of elements in both roasted grass and roasted grain were sufficiently low.The contents of Ca,Fe and Mn were higher in wild-rice grass than in wild-rice grain,whereas the contents of Mg and Zn were higher in wild-rice grain.Ca,Fe and Zn deficiency is associated with obesity,and Ca may play a role in the regulation of abdominal fat masses in obese people (Suliburska et al,2012),while Mn deficiency can result in poor reproduction,growth retardation,congenital malformations in offspring,and abnormal function in bone and cartilage (Goldhaber,2003).Moderate deficits of Ge can contribute to infertility in men,nephropathy,prostate cancer,neurological diseases,ischemic heart disease and impaired immune system functioning (Bian et al,2017).Moreover,supranutritional dietary levels of Se are needed for proper antioxidant and anticancer defense (Kuršvietienė et al,2020).The content of K in roasted wild-rice grass was 14 488.7 mg/kg.The maximum K amount infused from a tea bag in hot water was approximately 11.6 mg,much lower than the recommended dietary allowance per day.

In the sensory evaluation (Table S4),the panel preferred Samples A,B and C over Samples D and E in terms of color.However,there was no significant difference in preference among the five samples in terms of the other evaluation factors (flavor,taste and overall preference;P＞ 0.05).The sensory properties (brownness,astringency,savoriness and cut-grass smell) of Sample A were also significantly higher than those of the other tea samples.This indicates that the content of roasted wild-rice grass in the wild-rice tea bag should be high to suit consumer preference for sensory attributes.Correlation analysis revealed a high correlation (r=0.925) between brownness and color preference,indicating that the degree of brownness of wild-rice tea can affect color preference.As the content of wild-rice grain increased,the degree of brownness decreased,resulting in lower preference from the test panel.This explains the greater color preference for Samples A,B and C compared to Samples D and E.In conclusion,wild-rice tea with sole grass was preferred in terms of savoriness and brownness,complementing the strong astringency and cut-grass smell.

To identify correlations between the panel preference and sensory attributes of wild-rice tea,principal component analysis was performed for the five tea-bag samples (Fig.1-E).Principal components 1 and 2 explained 83.99% and 10.27% of the total variances,respectively.Samples A,B and C were plotted in the positive direction of this dimension,while Samples D and E located in the negative direction.In particular,among samples,Sample A was the farthest from the origin in the positive direction due to its significantly higher scores (P＜ 0.05) for all factors.Furthermore,Sample E was located farthest from the origin in the negative direction.Samples B and C located in the positive direction due to their significantly higher scores (P＜ 0.05) for overall preference,color preference,taste preference and flavor preference compared to the other samples.In terms of overall preference for wild-rice tea,Sample B was the most highly rated.

A preliminary survey of the test panel indicated that the panel was composed of 27 males (34.2%) and 52 females (65.8%) with most between the ages of 30 and 39 (58.2%) (Table S5).Sixty-seven panel members (84.8%) reported enjoying tea,while 62 (78.5%) enjoyed tea more than 1-2 times per week.All panel members had no experience tasting wild-rice tea.The highest reported purchase intention (54.3%) was obtained for Samples A and B,while Samples D and E received the lowest purchase intentions.This indicates that the panel preferred wild-rice tea containing mostly roasted grass with little roasted grain was preferred.To explain the panel’s preference for Samples A and B,the deciding factors in the panel member’s purchase intention were surveyed through a questionnaire (Table S6).The most influential decision factor was sensory quality,including taste,flavor and aroma.The importance scores for factors affecting the intention to purchase

tea showed that the test panel most highly regarded the functionality of tea followed by the sensory quality and the price.The highest antioxidant activity was obtained from the infused tea bag after roasting wild-rice grass at 225 °C.The wild-rice tea was free of caffeine and sugar,and was low in calories,and the element contents were far below the daily recommended dietary allowances,indicating that the tea can be enjoyed several times a day for health.The influential factors affecting the test panel’s likelihood of purchasing the tea were sensory quality and functionality.Wild-rice tea containing only grass was preferred because of its strong savory quality and brownish color.As a result,wild rice is worth developing as a functional tea.

ACKNOWLEGEMENT

This study was supported by the Korea Technology and Information Promotion Agency for Small and Medium Enterprises (SMEs) grant funded by the Korea government (Ministry of SMEs and Startups) (Grant No.CD558230).

SUPPLEMENTAL DATA

The following materials are available in the online version of this article at http://www.sciencedirect.com/journal/rice-science;http://www.ricescience.org.

File S1.Methods.

Fig.S1.Antioxidant activity of wild-rice grass at various roasting temperatures.

Table S1.Total phenol contents reported for various teas.

Table S2.Components of wild-rice grass and grain.

Table S3.Contents of elements in roasted wild-rice grass and grain samples.

Table S4.Preferences of tea panel,sensory attributes and purchase intentions for wild-rice tea-bag samples.

Table S5.Characteristics of test panel that conducted the sensory evaluation of wild-rice teas.

Table S6.Importance scores for factors affecting the intent to purchase tea.