- Materials plus methods
2.1. Material
Sweet potato tuberous roots of cultivar ‘Longshu No. 9’ were obtained from a plantation located in Xiongxian County, Hebei Province, China, plus transported to the laboratory on the same day. Roots that were disease-free with uniform size plus shape (weighing about 350–400 g), plus indicating nomor mechanical damage were selected for fresh-cut slices processing. Sweet potato roots were flushed under tap water for 1 min, air-dried at room temperature (25 °C), plus then peeled plus cut into slices with a thickness of 4–5 mm. Peeler, knife plus cutting board were disinfected with 0.1% (v/v) sodium hypochlorite solution before usage. A keseluruhan of 300 sweet potato slices were obtained after 2 h of processing, then all slices were mixed plus randomly allocated into two groups of 150 slices each for subsequent treatment.
2.2. Ultrasound treatment
Ultrasound treatment was conducted in 230 mm × 140 mm × 100 mm (length × width × height) ultrasound bath with a KQ2200DE ultrasound apparatus (Kunshan Ultrasonic Instrument Co., Ltd, China), operating at 40 kHz frequency plus 100 W power density for 10 min at 25 °C. This ultrasound exposure time was chosen as the best one based on preliminary experiments for 5, 10, 15 plus 20 min, plus resulted in significant reduction of spoilage microorganisms enumerated from sweet potato slices. Fresh-cut sweet potatoes immersed in distilled water for 10 min at 25 °C served as the control. Subsequently, all treated slices were drained by gauze, then packed into polyethylene bags (280 mm × 180 mm) plus stored at 4 °C for 10 d. All treatments were replicated three times plus changes in relevant indicators were measured every 2 d.
2.3. Color evaluation
The color of fresh-cut sweet potato was determined using a HP-200 automatic colorimeter (Shanghai Hanpu Photoelectric Technology Co., Ltd, China). Three points were randomly selected from each side of all slices for measurement (6 times for each sample). Numerical values of L* (light/dark), a* (red/green) plus b* (yellow/blue) were recorded, where L* value represents luminosity, while a* plus b* were converted into chroma (C) according to the equation: C = (a2 + b2)1/2.29
2.4. Total phenolic content determination
The keseluruhan phenolic content was determined using Folin–Ciocalteu method as described by Liu et al.30 with some modifications. Briefly, ten grams of the segar sweet potato sample were ground into liquid nitrogen, then the powder was added with 10.0 mL of 80% (v/v) acetone plus centrifuged at 10 000 × g for 10 min. After that, 1.0 mL supernatant was added with 2.0 mL of Folin-phenol reagent plus 10.0 mL of 10% (w/w) sodium carbonate solution, shake well plus placed in a dark place for 60 min. The absorbance value at 765 nm was measured using an UV-spectrophotometer (Shimadzu UV-1800, Japan) plus keseluruhan phenolic content was expressed as chlorogenic acid equivalent (CAE) values in mg g−1 segar weight (FW).
2.5. Phenolic metabolism enzymes analysis
Crude enzyme extraction of PPO plus POD was performed at 4 °C by a previous method with slight modifications.30 Tissue (5 g) from triplicate samples was homogenized in 5 mL of 0.1 mol L−1 sodium acetate buffer (pH 5.5) containing 1 mM polyethylene glycol 6000, 4% (w/v) polyvinylpolypyrrolidone plus 1% (v/v) Triton X-100. After centrifugation at 10 000 × g for 15 min, the supernatant was used for enzyme assay.
PPO activity was analyzed according to the method described by Zhou et al.31 with slight modifications. The crude enzyme (1.0 mL) was reacted with 3.9 mL of 100 mM sodium phosphate buffer (pH 6.5) plus 1.0 mL of 100 mM catechol. Immediately after, the absorbance at 420 nm (A420) was recorded every 30 s for 3 min. PPO activity was defined as the change of 0.01 in A420 per g per min plus expressed as U g−1 FW.
POD activity was analyzed according to the method used by Xu et al.32 with slight modifications. The crude enzyme (1.0 mL) was reacted with 6.0 mL of 25 mM sodium phosphate buffer (pH 7.8), 2.0 mL of 0.5 M guaiacol plus 1.0 mL of 2% H2O2. Absorbance at 470 nm (A470) was measured every 1 min for a keseluruhan of 6 times. POD activity was defined as a change of 0.01 in A470 per g per min plus expressed as U g−1 FW.
PAL activity was measured according to the previous method with some modifications.33 About 2 g of sweet potato slices were homogenized with 5 mL of boric acid buffer (pH 8.8) containing 40 g L−1 PVP, 2 mM EDTA plus 5 mM β-mercaptoethanol in an ice bath. The homogenate was centrifuged at 5000 × g for 25 min at 4 °C plus the supernatant was collected as crude enzyme extract. PAL activity was measured by incubating 1 mL enzyme extract at 37 °C for 60 min with 5 mL boric acid buffer mentioned above plus 1 mL of 20 mM l-phenylalanine solution. The substrate was added after 10 min of preincubation plus the reaction ceased with 0.1 mL of 6 N HCl. Finally, the increase in absorbance at 290 nm (A290) was measured. PAL activity was defined as a change of 0.01 in A290 per g per h plus expressed as U g−1 FW.
2.6. O2−˙ plus H2O2 concentrations measurement
O2−˙ was measured using the method of Xu et al.34 with slight modifications. First, 5 g of segar sweet potato sample was homogenized in 5.0 mL of 50 mM sodium phosphate buffer (pH 7.8) plus centrifuged at 12 000 × g for 20 min at 4 °C. Next, 1.0 mL of supernatant was mixed with 1.0 mL of 50 mM sodium phosphate buffer (pH 7.8) plus 1.0 mL of 1 mM hydroxylamine hydrochloride. After incubation at 25 °C for 1 h, 1.0 mL of 17 mM p-aminobenzene sulfonic acid plus 1.0 mL of α-naphthylamine were added to the incubation mixture; the mixture was incubated for a further 20 min for color reaction. Finally, the absorbance at 530 nm was measured plus O2−˙ concentration was expressed as μmol g−1 FW.
H2O2 was extracted by homogenizing segar tissue (5 g) with 5.0 mL of cold acetone plus centrifuged at 12 000 × g for 20 min at 4 °C. Its concentration was measured according to the method employed by Patterson et al.35 with slight modifications. It involved incubating 1.0 mL extracted supernatant with 0.1 mL of 10% titanium tetrachloride–hydrochloric acid plus 0.2 mL of concentrated NH4OH plus centrifuging at 12 000 × g for 15 min. Then the sediment was dissolved in 3.0 mL of 2 M H2SO4 plus the absorbance at 412 nm was measured. H2O2 concentration was expressed as μmol g−1 FW.