Effects of microwave blanching conditions on the quality of green asparagus (Asparagus officinalis L.) butt segment

Abstract Blanching is a pretreatment method that is often applied in fruit and vegetable processing to inhibit enzyme activity and reduce loss of food quality. It was recently discovered that well‐controlled microwave volumetric heating could improve the blanching efficiency and retain nutritional and sensorial values of product. This study was conducted to investigate effects of microwave blanching conditions on the quality of green asparagus (Asparagus officinalis L.) butt segments, a rich source of fiber and antioxidants but are often discarded during processing. The experiments were designed by one‐factor‐at‐a‐time method with two varying factors including blanching time (2, 4, 6, and 8 min) and microwave power output (150, 300, 450, and 600 W). Quality of product was evaluated by sensory, retention of phenolics, and free‐radical scavenging activity retention. The results showed that longer blanching time or higher microwave power was associated with reduced quality of green asparagus butt segment. Besides, the appropriate parameters for microwave blanching of the green asparagus butt segment was found at 300 W for 4 min.

time and energy to heat the blanching solution to maintain nutrient content and antioxidant ability of the plant materials. Recently, microwave radiation was developed and applied in blanching operation to overcome drawback of conventional methods (Giami, 1991;Ramesh, Wolf, Tevini, & Bognar, 2002;Schirack, Drake, Sanders, & Sandeep, 2006). A comparative study that evaluates impact of hot-water and microwave blanching technique on quality of green beans suggested that microwave processing of green beans can be a good alternative to conventional blanching methods because of shorter processing times (Ruiz-Ojeda & Peñas, 2013). Novel blanching treatments such as blanching by high-humidity hot-air impingement, microwave, and Ohmic heating could reportedly reduce the nutrition loss and are more efficient in drying (Deng et al., 2017). Even though investigations regarding microwave blanching on different raw materials, such as peanut (Schirack et al., 2006) and lettuce (Wang, Zhang, Mujumdar, Mothibe, & Roknul Azam, 2012), have been reported, there is limitation in study of microwave blanching on asparagus, especially in butt segment of green asparagus.
Given the promising nutritional value of asparagus butt segment and the scarcity of studies related to asparagus blanching, this work aims to investigate effects of microwave blanching conditions on quality of the green asparagus butt segment. The aim of this study is to determine appropriate blanching parameters to retain the high quality of the sample.
Quality was determined by indicators including total phenolic content, DPPH radical scavenging capacity, and sensory properties.

| Sample preparation
Green asparagus spears (Asparagus officinalis L.) were purchased at An Phu Dong market, district 12, Ho Chi Minh City, Vietnam. Selected asparagus spears were intact and green. The green asparagus spears were washed. The butt segments were cut into parts, which are 5.0 ± 0.5 cm in length and 0.5 ± 0.2 cm in diameter ( Figure 1).

| Microwave blanching
First, 10 g of the sample were placed in a pottery bowl and 100 ml of water was added. The bowl was then placed in the microwave oven cavity for blanching. The blanching was performed at four microwave power levels of 150, 300, 450, and 600 W and four duration levels of 2, 4, 6, and 8 min. After blanching, samples were cooled promptly with cold water at 5.0°C ± 0.5°C and then analyzed for total phenolic content and antioxidant capacity.

Determination of total phenolic
The total phenolic content was measured by the Folin-Ciocalteu colorimetric methods, using gallic acid as a standard as described previously (Velioglu, Mazza, Gao, & Oomah, 1998). The blanched sample (5 grams) was ground and extracted with distilled water at the ratio of 1:10. The extract was placed at room temperature about 30 min and then filtered through Whatman No.1. The residue was extracted twice in the same way. The extracts were taken to the norm and analyzed. Extracts (1 ml) were put in a dark tube and added with 1 ml Folin-Ciocalteu reagent (diluted 10 times with F I G U R E 1 Green asparagus material used in the study (from left to right: materials (a) before cutting, after washing, and (b) after cutting) distilled water) and 1 ml sodium carbonate solution (20% w/v). The sample was placed in dark space before being taken to a photometer at an absorption of 765 nm. The total phenolic content was expressed in mg of gallic acid equivalent per gram of dry matter (mg GAE/g dry matter).

Determination of antioxidant capacity by DPPH
Antioxidant capacity was measured by scavenging free radical (DPPH), which was performed based on the method described by Braca et al. (2001). Since antioxidant compounds had the ability to scavenge free radicals, discoloration of purple color would occur in the DPPH solution. The blanched sample (5 g) was ground and extracted with methanol. The diluted extract (0.2 ml) was mixed with 3 ml DPPH solution. The sample was kept in the dark for about 30 min and then measured for absorbance at 515 nm. The result was expressed by mg of Trolox equivalent per gram of dry matter (mg TE/g dry matter).

Sensory evaluation
After being blanched at different modes, samples will be evaluated with respect to color and texture and compared to the fresh sample. Sensory evaluation was performed by description methods (Li, Zhang, & Yu, 2006). The results were expressed by image and sensory description.

Data analysis
All experiments were conducted in triplicate. The mean and standard deviation of the results were calculated using Microsoft Excel program (Microsoft Inc., Redmond, WA, USA). Experiment data were analyzed using one-way analysis of variance (ANOVA) test in SPSS software (IBM Company) with the level of significance at 5%.

| Investigating asparagus material
Green asparagus materials were analyzed for total phenolic content and antioxidant activity (the DPPH assay). Analytical results were showed in Table 1. The results showed that the total phenolic content and antioxidant activity in butt segment were lower than those in bud segment by 51.50% and 52.26%, respectively.

| Effects of microwave blanching time on sensory
The texture and color changes in asparagus butt segment after microwave blanching at 300W for different durations were recorded and displayed in Table 2. It was found that in samples, which were TA B L E 1 Comparison of phenolic content and free-radical scavenging ability (the DPPH assay) per dry matter of the green asparagus bud segment and butt segment   (Nisha, Singhal, & Pandit, 2006).
However, softer materials also makes further processes such as drying, and extraction become easier as it could reduce drying time and improve yield of extraction. with results from a previous report (Jaiswal, Gupta, & Abu-Ghannam, 2012). This result can be explained as after blanching at 300 W for 2 min, and the temperature of blanching solution reached 70°C

| Effects of microwave blanching time on the percent retention of phenolics and antioxidant capacity
( Figure 2) that broke down the phenolic compounds into simple isomers by polyphenol oxidase activity. Furthermore, losses in phenolic content could also be caused by diffusion into the blanching solution and self-degradation.
For antioxidant capacity, overall, the free-radical scavenging ability

| Effects of microwave power on sensory characteristics
Texture and color of the asparagus butt segment with regards to the microwave power are described in Table 4. The results showed that after being blanched at 150 W, the hardness and the bright green color of the fresh sample remained unchanged. This is because the wavelength at this power supply provides a low value of 150 J. At microwave power of 150 W, temperature in the sample is at 56°C, which is inadequate to cause noticeable damage to the cell wall as well as the color components of the green asparagus butt segment.
As the samples were blanched at 300, 450, and 600 W, the asparagus butt segment becomes softer and the wrinkles were observed at the epidermis layer. Moreover, the initial green color of the material became darker due to the detriment of chlorophyll at high temperature. In addition, high temperatures could promote cell wall disruption, cell depletion, and the formation of the wrinkles in the epidermis layer. These results are commensurate with another kinetics study of asparagus. According to Weemaes, Ooms, Van Loey and Hendrickx (1999), green pigments (chlorophyll) decomposed when being heated at temperature of above 80°C. In addition, the pigment content suffered a loss of 3.2% with every one-minute exposure to microwave of 650 W (Muftugil, 1986). Brewer and Begum (2003) also found that different radiation at microwave power of 385 W, 490 W, and 700 W for 4 min significantly reduced the brightness (L* value) of the sample (Brewer & Begum, 2003). Note: Data are expressed as mean (standard deviation) and values within a column with the same letter are not significantly different (P > .05) tannin and nontannin) content of green asparagus was found to be increasing by approximately 23% after heat treatment (Fanasca et al., 2009). This may be due to the inactivation of the polyphenol oxidase enzyme during the precipitation process leading to polyphenol degradation (Yamaguchi et al., 2003). The increase in microwave capacity elevates electrical energy absorbed into the solution and the raw material. This causes an increase in the motion of the polar molecules and in turn the temperature of the solution, thus promoting the decomposition reaction. Previous disclosures indicate that increased temperatures and blanching times may lead to the loss of phytochemicals in cabbage (Jaiswal et al., 2012), carrots, green beans, broccoli (Patras, Tiwari, & Brunton, 2011), and green asparagus (Drinkwater, Tsao, Liu, Defelice, & Wolyn, 2015;Fuentes-Alventosa, Jaramillo-Carmona, et al., 2009). For antioxidant capacity, the results in Table 5 Papetti, Daglia and Gazzani (2002) showed that the free-radicals scavenging ability in vegetables decreases when exposed to various types of thermal treatment, such as blanching. These results are consistent with Wu et al. (2004), who found that the antioxidant capacity of broccoli and carrot after cooking decreased by 22% and 67%, respectively (Wu et al., 2004). The percentage of nutrient loss and antioxidant capacity depends on the type of material, the structure of the product (Ferracane et al., 2008), or the extraction solvent used (Sultana, Anwar, & Ashraf, 2009). for microwave blanching of the green asparagus butt segment are 300 W for 4 min. For further study, degradation kinetics of total phenolics degradation, antioxidant ability, and quality properties should be investigated to allow better processing control for manufacture of food products from the green asparagus butt segment.

ACK N OWLED G EM ENTS
This study is funded by NTTU (Nguyen Tat Thanh University) Foundation for Science and Technology Development under the grant 2018.01.50.

CO N FLI C T O F I NTE R E S T
All authors declare no conflict of interest with regard to the described research, the publication of the result, and financial issues.

AUTH O R S CO NTR I B UTI O N
Investigation, Thi Yen Nhi Tran; Supervision, Long Giang Bach; Writing -original draft, Thi Van Linh Nguyen; Writing -review & editing, Duy Chinh Nguyen.

E TH I C A L A PPROVA L
This study does not involve any human or animal testing.