EFFECT OF STEAM BLANCHING AND CONVECTION DRYING CONDITIONS ON COLOR, VITAMIN C, TOTAL PHENOLIC CONTENT, FLAVONOID CONTENT AND ANTIOXIDATION ACTIVITY IN SOURSOP ( ANNONA

Blanching and drying techniques in food processing greatly affect the bioactive values. Research into this effect on soursop leaves raw material in the processing of tea which is a plant with anti-inflammatory, anti-cancer properties. Through a steam blanching process, 4 minutes were selected for compliance with TPC (208.43 ± 0.80 mgTE/g DM) and TFC (43.21 ± 0.71 mgQuercitin/mg DM). Along with the typical antioxidant activities such as TPC, TFC, the TAA, DPPH, ABTS content decreased sharply when heated by convection drying oven at 50-65°C. About 80% of the content is lost at the highest drying temperature. In addition, the Lab* color space of the fresh sample is dark brown transformed by heat, producing a characteristic yellowish hue when extracting custard leaf tea in hot water.


INTRODUCTION
Reactions by free radicals in the human body cause significant adverse effects on health (Formagio et al. 2015). Nowadays, studies extract natural compounds are increasingly being promoted, technologies that help to store ingredients with antioxidant activity in processing are of interest ( (Hasrat et al. 1997), sterol,… In addition, it contains carbohydrates (leaf glucose; sucrose, glucose, fructose in fruits; galactomannan in seeds), lipid (semi-dry oil with fatty acids in seeds and leaves), amino acids (prolin, -amino-butyric acid in fruits), polyphenols (caffeic acid, para-coumaric acid, procyanidin, tanins), vitamins (vitamin C in fruits), cyano glycosides (Moghadamtousi et al. 2015). Some compound polyphenols, such as flavonoid quercetin and flavone (2-phenyl-4H-1-benzopyran-4-one) found in leaves, have a high antioxidant capacity (Indrawati et al. 2018). Leaves of A. muricata have a potent antioxidant -a flavonoid found to be more potent apoptosis than clinically established camptothecin anti-cancer agent. These phytochemicals are highly degraded through processing, known to evaporate in some studies to inactivate oxidase enzymes and retain TPC content better than blanching and have limited expense (Sotome et al. 2009). At the same time, the drying process has also been extensively studied on plants and their antioxidant activit. Convection drying technology was chosen because of its ease of implementation, cost savings and high commercial applicability. This study aims to determine the influence of tea processing processes on the content of vitamin C, total phenolic, and antioxidant capacity in soursop leaves.

Preparations of sample
Soursop leaves were purchased in Tan Phu Dong area, Tien Giang province, Vietnam. Leaves were not pests, torn creased. 1g of sample was extracted with solvent to 50 mL volume. The volumetric flask was placed in a dark space to prepare for analysis.

Determination of total ascorbic acid
The method of titration with 2,6 dichlorophenolindophenol (DCPIP) was previously described by Manas Denre 2014et al. 2014Dao et al. 2021 based on dehydroascorbic acid forming reaction and colorless lenco derivative of oxidation of Vitamin C. 10 mL of solution was extracted with distilled water into an erlen flask, add 1 mL of 0.04M HCl. Titration under burettes containing DCPIP to light pink persists for 30s. Record volume and process data.

Determination of total phenolic content
Optical absorbance at 765 nm on UV-Vis spectrophotometer of 0.5 mL of custard leaf sample with 2.5 mL of 10% Folin-Ciocalteu solution, uniform with Votex and let stand for 5 min in the dark with 2.0 mL Na2CO3 solution 7.5% left to stand for 1 h was measured. The polyphenol content was expressed in milligrams of gallic acid equivalent in 1g of dry matter (mgGAE /g DM) (Nhi et al. 2020; Nguyen et al. 2020).

Determination of total flavonoid content
The total flavonoids were determined by the colorimetric method described by

2006, Nhi et al. 2020).
The scavenging ability of the tea pulp was calculated using the standard equation: y = 0.126914x + 0.6672952 with Regression coefficients R=0.99936.

Determination of Lab* system
Select collate values L*, a*, b* based on CIE Lab* color space previously described by Torres B et al. 2011. The brightness was measured using a colorimeter Chroma Scanner (NR60CP model). Results were displayed as numbers via L* (brightness ranges from 0-100), values a* (from green to red) and b* (from blue to yellow).

Statistical analysis
Microsoft Excel and Stagraphic software (The Plains corporate, Virginia) were used to support analysis of results based on three replicates. The difference is assessed ANOVA a factor with significance level between means with an error of 0.5%.

Effect of steam blanching on the antioxidant capacity and color of soursop leaves
The content of mg/g dry material of TPC and DPPH affected by blanching time is shown in Figure 1. The results show that blanching time dramatically affects the content of TPC and DPPH. TPC tended to decrease at the heating time of 2 min and 8 min with 159.41 ± 1.55 and 129.76 ± 0.34 mg/g DM, respectively, about 10-26.61% reduction compared to fresh samples. However, in a 4-6 minute heat treatment sample, TPC increased to 17.43 and 9.42%. It can be understood that during heating, 2 min of peroxidase (PE) enzyme treatment is not inactivated, leading to reduced content. In contrast, at 8 min of heating, the longtime lead to the gradual decomposition of a natural compound by heat (Pinheiro et al. 2018). While at 4-6 min provided enough energy to release polyphenols bound to proteins in plant cells, inactivation of PE resulted in increased content extraction. This explanation also correlates with the DPPH content. The concentration after heat treatment with steam at 8 minutes was 35.73 ± 0.72 mg/g DM increased by 15.81 mg compared to fresh samples. In a report from Zin et al. 2020, DPPH increased due to blocking the initiation of free radical scavenging reaction chains. The antioxidant activity of soursop leaves was assessed according to the free radical scans ABTS model and the total flavonoid content along with vitamin C is shown in Figure 2. The typical decrease in vitamin C content over 2 min (30.44 ± 0.55 mg/gDM) to 8 min (6.00 ± 0.44) decreased by 19.71%. The sensitivity to heat and light of ascorbic acids was previously reported by Adefegha et al. 2011. Through the process of cooking, a large amount of TAA has been identified. Remarkably TFC and ABTS decreased at 2 min of blanching, tended to increase at 4 min and continued to decrease with extended heating time. Significantly increase the content at 4 min 43.21 ± 0.71 and 30.96 ± 0.77. The difference at 2 min and 6 min of TFC is negligible. Indeed, the increase of TPC, TFC, ABTS during blanching, prominent at 4 min, could be due to a decrease in enzyme-mediated polyphenol degradation (complete inactivation of natural polyphenol oxidase). The increase in total phenolic content may also be due to the release of bound phenolic acids from the breakdown of cellular components of the plant cell walls in leafy vegetables (

Effect of convection drying temperature on the antioxidant capacity and color of soursop leaves
From the survey studies of Figures 1 and 2, the blanching process parameter was selected for soursop leaves for 4 minutes. The drying process was finished after the leaf moisture content was less than 10%. The results of the antioxidant content retention through blanching and drying processes are shown in Figure 3. In particular, phytochemical compounds are significantly reduced through heating. The total phenolic content decreased by 30.92% after ANOVA treatment with a significance level of α = 0.05 through blanching and continued to decline sharply after drying at 50°C (31.69), reducing by 68.31%. No significant difference in TPC and ABTS between samples 55 and 60 °C. The lowest retention content after drying at 65°C is 12.70% (TPC), 43.36% (TFC), 18.60% (DPPH), and 17.12% (ABTS). The result is consistent with a few studies when making comments that, after processing, the ability to catch free radicals in soursop leaves decreased

CONCLUSION
The retention of color values and the content of antioxidants has been significantly reduced after blanching and convection drying. For small sample volumes, blanching at 4 minutes resulted in a higher TPC and TFC content than fresh samples because of the inactivation of oxidase enzymes. However, during the same simultaneous drying process, this content was reduced by a large total sample mass, and decreased after drying, with losses of more than 50% for TAA and 80% for chemicals that differs by hot air stream agent for a long time. The color varies slightly in low a* and TDC values between blanching and high drying samples. The limitation of the study is the need for an assessment of the effect of drying time on the quality of soursop tea leaves and the content of antioxidants after extracting tea leaves with hot water of users.