Roselle (Hibiscus sabdariffa L.) is considered an underutilized plant with high potential of nutritional value. Roselle can be used as a functional food, mainly due to rich in vitamin C, anthocyanin, phytochemicals, and natural colorant. Although Roselle has been widely planted and consumed, little is known about their volatile composition which contributes to important quality characteristics. The role of Roselle’s aroma and flavor in the food industry is given special attention in the support of growing Roselle industry. Thus, the present study provides the volatile profiles of the calyces and the seeds, and chemical characterization, including physicochemical properties, taste components, microstructure, nutritional value, and also the influence of drying methods to produce the optimum quality of dried calyx. Furthermore, the optimal storage stability of milled seeds and its application in food formulation were further explored.
Firstly, in order to obtain more knowledge about the aroma of Roselle, the influence of different methods of sample preparation on aroma profiles of the dried calyx and milled seeds were studied in Paper I and Paper VI, respectively. Five different procedures (whole dry, WD; ground dry, GD; ground and then mixed with water, GMW; ground and then mixed with water and kept in water bath (2hr/40 °C), GMWKB; and blended together with water, BTW), were prepared for dried calyx whereas two procedures (ground dry, GD; and ground, mixed with water, GMW) were prepared for milled seeds. Different sampling techniques were performed and were hypothesized to potentially result in different aroma profiles of calyx and seeds and therefore, we investigated which methods give the optimum volatile profile. In Paper I, results revealed that least amounts of volatile compounds were recovered by analysis of WD, GD, BTW, and GMW. The highest number of volatile compounds was found in Roselle treated as GMWKB. GMW was chosen as the preparation method for Roselle calyx because it was to be an efficient extraction method without the possibility of excessive chemical changes of the sample. In contrast, results revealed that GD would therefore, be the preferred sample preparation technique compared to GMW (Paper VI). It seems that addition of water during sample preparation does not improve the efficiency of sampling of Roselle seeds. GD was assumed to cause less chemical changes of the samples, it was simple, and it was easy to handle. Thus, it was decided to use the GD preparation method in further experiments for the seeds.
By using the optimum sample preparation method described in Paper I, volatile profiles of dried Roselle calyx of different varieties grown in various countries were investigated. Seventeen dried calyx samples collected from eight different countries were studied with regard to their physicochemical properties, taste components, and volatile profiles as presented in Paper II. The volatile analyses showed that a total of 156 volatiles were identified mainly terpenes, aldehydes, esters, furans, and ketones. Thirty-seven odorants considered as potentially important aroma compounds based on GC-olfactometry. Significant differences were found in total soluble solids, pH, and color L* a* b*. Oxalic, tartaric and citric acids were the dominant organic acids in Roselle tea. Differences between samples were mainly due to processing conditions (sun drying vs. oven drying), variety and particle size (powder or whole calyx). This study provides valuable information for future commercial utilization of Roselle in the food industry.
Fresh and dried Roselle calyx were studied to their physicochemical properties, volatile compounds, and microstructure during different drying methods (oven drying, freeze drying, vacuum drying, and sun drying) in Paper III. Oven drying and freeze drying reduced moisture content most while vacuum drying and sun drying were not as efficient. However, all drying methods except sun drying resulted in water activities low enough to ensure safety and quality. The color after freeze drying was the one closed to the color of fresh samples, both when evaluated in ground and liquid form. All drying methods reduced terpenes, some aldehydes, and esters which are expected to contribute with fruity, floral, spicy, and green odors whilst furan compounds increased causing caramel-like aroma. Sun drying produced more ketones, alcohols, and esters. Drying significantly affected the microstructure. Freeze drying preserved the structure best and freeze-dried samples were those that most resembled the fresh. The study concludes that freeze drying should be considered as a suitable drying method, especially with respect to preservation of color. Additional precaution must be considered when applying sun drying to ensure samples produced are of high quality.
In paper IV, different levels (5 %, 10 %, 15 %, 20 %) of milled Roselle seeds were incorporated in wheat bread and the quality of Roselle seed bread was evaluated in term of proximate composition, physical characteristics, volatile profile and sensory quality. Results showed that nutritional values (ash, protein, lipid, and dietary fiber) increase as the seeds added increased. The color of crumb, crust, and specific volume of bread were significantly affected. A total of 50 volatile compounds were identified including alcohols, aldehydes, pyrazines, acids, esters, furans, ketones, terpenes, and lactone. All pyrazines and most of furans were increased with increased addition of milled Roselle seeds. In sensory analysis, the color of bread (‘Yellow’, ‘Dark brown’, and ‘Grey’) changes significantly with increased levels of the seeds. Moreover, other sensory attributes including ‘Malt’ and ‘Nutty’ odor, ‘Grainy’ texture, ‘Bitter’, ‘Beany’, and ‘Malt’ flavor, and ‘Moldy’ aftertaste were markedly increased as higher percentages of the seeds were added. However, only small changes were observed in ‘Sweet’ and ‘Stale’ flavor, ‘Toasted’ and ‘Yeasty’ odor, and ‘Sponginess’ and ‘Stickiness’ texture. This study suggested that bread produced from a by-product, Roselle seeds could improve supply with valuable nutrients, create versatility and add value for the use of the seeds but sensory quality will change. Further consumer studies are needed in order to evaluate the acceptability and preferences of the population towards Roselle seeds bread.
Milled Roselle seeds were further analyzed for proximate composition, water and oil absorption capacity, and the influence of storage conditions on storage stability in Paper V. Four types of storage conditions were conducted and were monitored for seven consecutive months: light/oxygen (air) (LO), light/nitrogen (LN), darkness/oxygen (air) (DO), and darkness/nitrogen (DN). Results demonstrated that milled seeds are a potential ingredient for food purposes because they contain high levels of protein, lipid, and total dietary fiber. Besides that, milled seeds exhibited good water and oil absorption capacity, which can be beneficial if included in food products. Results also revealed that storage conditions had an influence on the formation of volatiles during storage of packaged milled seeds. A total of 85 volatiles were identified, mainly aldehydes, alcohols, ketones, furans, and acids indicating lipid oxidation. It is recommended that milled Roselle seeds should be flushed with nitrogen and stored in darkness. Under these conditions, the seeds can be stored for at least three months without changes in volatile profile. This is important to ensure the good quality of milled Roselle seeds for further commercialization.