Physicochemical properties of chili padi Centil (Capsicum frutescens) powder dried at different drying temperatures

F U L L P A P E R Physicochemical properties of chili padi Centil (Capsicum frutescens) powder dried at different drying temperatures Hussain, N., Abd Rahim, N.A. and Azhar, N. Department of Food Technology, Faculty of Food Science and Technology, Universiti Putra Malaysia, 43400 UPM Serdang, Selangor, Malaysia Halal Products Research Institute, Putra Infoport, 43400 UPM Serdang, Selangor, Malaysia


Introduction
Chili is a type of herb that belongs to the genus Capsicum, under the family Solanaceae. It is a native fruit originated from America and later introduced into Europe, Africa and Asia by Christopher Columbus (Kollmannsberger et al., 2011;Giufrrida et al., 2013). There are five major species of Caspsicum which include Capsicum frutescens, Capsicum annuum, Capsicum chinense, Capsicum baccatum and Capsicum pubescens each distinguished by their own trait, color, aroma and pungency level (Kollmannsberger et al., 2011). In Malaysia, Capsicum annuum and Capsicum frutescence are widely grown and used by locals as one of many cooking ingredients or consumed as fresh vegetables. One of a common chili cultivate in Malaysia is Chili padi Centil, a type classified under Capsicum frutescence that is small in size and has a strong pungent smell. Its size ranges from 2 to 3 cm and it contains nutrients including minerals, vitamins and phytochemicals such as flavonoids, phenolic acids, carotenoids and capsaicinoids that act as health-promoting components in consumers' diet (Jayaprakasha et al., 2012;Ogunlade et al., 2013).
Chili is popularly known for its hot and spicy flavor, thus suitable to be used for culinary purposes, spice to dishes and sauces and also as a condiment in cooking (Ogunlade et al., 2013). The spicy flavor is due to the presence of capsaicinoids which accumulate in the placenta of Capsicum species (Juangsamoot et al., 2012). Fresh chili has a short shelf life due to its high moisture content (more than 40%), thus chili has also been made into various forms include dried, flakes, paste and powder to extend its shelf life by the food manufacturing companies mainly through drying process.
Drying is one of the common preservation methods that will inhibit or slow down microbial growth and certain biochemical changes within the plant tissue, altering the plant's aroma and appearance (Kubra and Rao, 2012). Traditional drying involves exposing the chilies under open air and sunlight which is considered as time-consuming and not contamination free (Condori et al., 2001). Alternative drying method such as hot air drying offers a shorter drying time with uniform heating and more hygienic. Toontom et al. (2012) reported that the temperature needed to achieve approximately 10% of moisture content ranges between 50°C to 70°C. However, thermal process such as drying significantly affect the visual and nutritional quality of chili. A study was done on the effect of thermal treatment on bioactive compounds in green chili (Capsicum annum) powder found that the vitamin C content of the powder has reduced approximately 50% (from 110 mg/ 100 g to 59.08 mg/ 100 g) due to thermal processing (Sarker et al., 2013). Since different species compose of different proximate composition, the drying temperature that is compatible for each species is different and need to be adequately specified.
This study is aimed to determine the composition of local chili padi Centil and characterize the powders dried at different drying temperatures. This is important to produce shelf-life stable chili powder with minimum discoloration to meet consumer's demand.

Sample preparation and drying process
Fresh chili padi Centil (Capsicum frutescence) was collected from Agro Sembilan Belas farm at Batu Pahat, Johor. The color and degree of maturity of chili were selected according to the Malaysia Agricultural Commodity and Food Standards (2016). The chilies were washed and their pods were removed, then blanched in 100°C of boiling water for 6 mins (Aminah and Permatasari, 2013). The blanched chilies were cut vertically and oven-dried using different temperatures of 50°C (B), 60°C (C) and 70°C (D) for 4 hrs and 30 mins. Fresh chili padi Centil that did not undergo any processing was used as control (A). The dried chilies were ground for 3 mins into powder, weighed and stored in polypropylene bag at -20°C for further analysis.

Moisture content
The moisture content of chili powders (A, B, C and D) was done according to the standard Official Methods of Analysis (Association of Official Analytical Chemists (AOAC), 2006).

Determination of color
The fresh chili padi Centil and dried chili padi Centil powder was spread out in a transparent zipper bag (5cm by 5cm) for color measurement using chroma meter CR-410. Instrumental color data was provided by the CIE system in terms of L*(lightness), a*(redness and greenness) and b*(yellowness and blueness) (Pathare et al., 2012).

Determination of particle size
The particle size of chili padi Centil powder was determined using a particle size analyzer according to methods described by Phat et al. (2015).

Determination of water and oil holding capacity
Water holding capacity (WHC) and oil holding capacity (OHC) of chili padi Centil powder were determined using the method described by Zhang et al. (2012).

Determination of water solubility index
Water solubility index (WSI) of chili padi Centil powder was determined using the method described by Zhang et al. (2012). A 5 g of chili powder was added into a centrifuged tube containing 10 mL distilled water at ambient temperature. The dispersion was placed in a water bath at 80°C for 30 mins, followed by centrifugation at 6000 rpm for 10 mins. The supernatant was carefully collected in a pre-weighed evaporating dish and left to dry at 105˚C. The evaporating dish with residue was weighed. The WSI was calculated using Equation 1.

Determination of total phenolic content
Extraction of total phenolic content from chili padi Centil powder was done according to the method described by Alam et al. (2015) with some modifications. Approximately 2 g powders were mixed with 20 mL methanol and incubated in 40°C water bath shaker for 2 hrs. The supernatant was collected using filter paper and the residue was extracted again. The supernatants were pooled together and subjected to a rotary evaporator. The concentrated extract was stored at −20°C for further analysis.
The total phenolic content (TPC) was determined using the Folin-Ciocalteu method described by Alam et al. (2015). Approximately 0.5 mL extract was mixed with 0.5 mL Folin-Ciocalteu reagent and 10 mL of 7% Na 2 CO 3 solution. The mixture was incubated for 1 hr at 25°C in the dark. The absorbance was measured at 765 nm using a UV-vis spectrophotometer. The amount of TPC was expressed as milligram of Gallic acid equivalents (GAE) per g of sample (mg GAE*g−1 DW).

Statistical analysis
All results were expressed as mean ± standard deviation (SD) and statistics were performed using Minitab version 17 statistical package. One-way analysis of variance (ANOVA) and Turkey's multiple comparison tests were used to determine the significant difference between the different drying temperatures on chili padi powder when the calculated p-value is lower than 0.05. drying process at different temperatures. Based on preliminary study, the drying temperatures were chosen from 50˚C, 60˚C, 70˚C at a fixed time to achieve approximately 10% moisture content. The highest drying temperature took the shortest time to dry and achieved moisture content up to 10%. Following the kinetic theory, an increase in temperature can cause the energy molecules to increase. The higher state of energy allowed the water molecules to escape easier and faster from the product, resulting in shorter drying time (Maskan et al., 2002). The moisture content of chili powder should be in the range of 8-10% per 100 g. According to Indian Standard, the maximum moisture content for chili powder is 11% and values higher than 15% was critical with respect to mold growth (Mamun, 2016).

Color
Table 2 presents the color values of fresh chili padi Centil and dried chili padi Centil powders after drying process at different temperatures. L* and a* values are very important in order to observe the changed and the degradation of chili padi Centil powder color after drying process. From the results, it is observed that L* and a* values differ significantly for every sample treated at different drying temperatures. The different drying temperatures significantly affect the color of chili padi Centil powder, whereby high drying temperature produces orange chili padi Centil powder with low L* and a* values. The red color of chili comes from the redpigmented carotenoids including capsanthin, casorubin, zeaxanthin and cryptoxanthin which are all very stable in intact tissue plant. However, the subjection of plant tissue to friction and heat during grinding and drying causes the carotenoids to easily auto-oxidize. The reaction leads to a more orange (Figure 1 (a) and (b)) and less intense coloration that devalues the spice powder (Gupta et al., 2017). The chili powder dried at the lowest temperature (50˚C) showed intense in red color while chili powder at highest temperature (70˚C) showed less intense in red color where it degraded into orange color.

Particle size
Based on Table 3, the chili dried at 70˚C had the smallest value of particle size compared to other dried chili powders. The different moisture content of the dried chili after drying at different temperatures could have an effect on the performance of the grinding process (Jung et al., 2018). Powders with lower moisture content are harder and more brittle, thus less resistant to grinding process (Lee et al., 2014). According to Moon and Yoon (2018), the grinding kinetics model of powder assumes the force of aggregation of each particle during grinding is zero, however, in a high moisture content powder, agglomeration is highly expected to happen which disrupt the grinding process, thus producing larger particle size powder. The lowest moisture content of sample D may be caused by the high temperature exerted during the drying process, thus less energy is required to grind the chili into smaller powder size (   Yoon, 2017). A similar report was narrated on the relationship between low moisture content influenced a shorter grinding process time which produced finer or smaller particle size of kernel wheat powder and black soybean powder (Walde et al., 2002;Lee et al., 2014).

Water holding capacity, oil holding capacity and water solubility index
Based on Table 4, the ability of chili padi Centil powder to retain water and oil increased as the particle size decreased. Water and oil holding capacities are important functional properties. Oil holding capacity influences textural and sensory attributes of food (Haque et al., 2016). The water holding capacity involves a protein-water interaction in the food system (Damodaran and Paraf, 1997). It affects the texture, juiciness, taste and the shelf life of bakery products (Jideani, 2011). The heating treatment applied on food can either reduce the availability of polar amino groups for hydrogen bonding with water molecules by enzyme denaturation or improve water holding capacity by unfolding and exposing the side protein side chains to bind with water molecules (Cheftel et al., 1985). The smallest particle sized powder may have the largest surface area for reaction to occur and more hydraulic groups of chili powder exposed causing an easier integration of powder with water (Zhao et al., 2011). The water solubility index of chili powder also the highest when the particle size is the smallest. The finer particle sized powders have more ability and stability to be mixed with other spices or ingredients (Zhao et al., 2011).

Total phenolic content
Total phenolic content is an assay to assess the amount of phenolic content in a sample which contributes to the functional properties of the food. Previous studies reported a strong and positive correlation (p≤0.05) between the total phenolic compound contents and the antioxidant potential of fruits and vegetables (Reddy et al., 2010). Food containing antioxidative compounds reduces lipid oxidation in animal tissues, therefore decreasing the risk of potential oxidative stress-based diseases (Eliassen et al., 2012;Tanaka et al., 2012;Pojer et al., 2013).
Based on Table 5, the fresh chili showed sample D showed the lowest phenolic content as compared to others. Deepa et al. (2005) reported that the high thermal treatment subjected during drying process may disrupt the cell compartments and causing enzymatic degradation that leads to the reduction of phenolic content. The high thermal treatment applied can cause degradation of the thermal-labile compounds including phenolic compounds (Baharuddin et al., 2018).

Conclusion
The application of drying treatment at temperature starting from 60˚C can produce chili padi Centil powder with required 10% moisture content, reducing the potential of microorganism growth, thus extending its shelf life. The drying treatment at 70˚C produced chili padi Centil powder with the highest water holding capacity of 2.67%, oil holding capacity of 1.3% and water solubility index of 7.65% which promotes the powder's solubility, enhancing its application efficiency. However, further study should be conducted to overcome the less intense orange color produced which is less preferred by consumers and the degradation of total phenolic content resulted from the thermal treatment.

Conflict of Interest
The authors declare no conflict of interest.