# Comparative Study on Physicochemical and Nutritional Qualities of Kiwifruit Varieties
## 2 Materials and Methods
### Methods
#### 2.2.6 Determinations of Soluble Sugar and Organic Acid Content  
Determination of soluble sugar (sucrose content + glucose content + fructose content) as referred to in the literature [16]. The kiwifruit samples were ground into a fine powder in liquid nitrogen and 1 g of powder was dissolved in 6 mL deionized water that was generated by a Milli-Q Element water purification system (Millipore, Bed ford, MA, USA). The mixture was centrifuged at 5000\(\times\)\(g\) for 15 min at 4 \({}^{\circ}\)C and the supernatants were filtered through a SPE-C18 cartridge (Supelclean ENVI C18 SPE) and a 0.22 \(\upmu\)m millipore membrane. A Dionex P680 HPLC system was used to measure the amount of soluble sugar in the filtered solution (Dionex Corporation, Sunnyvale, CA, USA). The amount of fructose, glucose and sucrose contents were determined using the refractive index detector. The mobile phase was degassed, distilled and deionized water flowing at a rate of 0.6 mL/min. The total injection volume was 20 \(\upmu\)L. Samples were eluted with 0.02 mol/L KH\({}_{2}\)PO\({}_{4}\) solution (pH 2.4). The flow rate was 0.8 mL/min. Eluted compounds were detected by UV absorbance at 210 nm.  
Determination of organic acid (malic acid content + citric acid content + tartaric acid content + quinic acid content) as referred to in the literature [17]. A 50 mL centrifuge tube containing 20 g of kiwifruit pulp was then filled with 25 mL of NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) (40 mmol/L, pH 2.5 was adjusted using H\({}_{3}\)PO\({}_{4}\)), followed by 15 min of centrifugation at 12,000\(\times\)\(g\) at 4 \({}^{\circ}\)C. The supernatants were collected and stored at 4 \({}^{\circ}\)C for further analysis. A total of 8 organic acid standards were detected, including oxal acid, tartaric acid, quinic acid, isocitric acid, malic acid, shikimic acid, lactic acid and citric acid. A Waters Alliance 2695 HPLC system with a 2996 PDA Detector (Waters Corp., Milford, MA, USA) was used to simultaneously separate and analyze the organic acids. The system was run at 1.0 mL/min using a ST-C18 column (250 mm \(\times\) 4.6 mm, 5 \(\upmu\)m) (TechMate, Beijing, China) and a guard column RP-18 (10 mm \(\times\) 4 mm) (Merck, Darmstadt, Germany). The column temperature was set at 25 \({}^{\circ}\)C, and 10 \(\upmu\)L of sample was injected. The detection wavelength was 210 nm. The mobile phase was NH\({}_{4}\)H\({}_{2}\)PO\({}_{4}\) (40 mmol/L, pH 2.5 adjusted using H\({}_{3}\)PO\({}_{4}\)).  
2.7 Determinations of Vitamin C (VC), Vitamin B1 (VB1), Vitamin B2 (VB2), Vitamin B6 (VB6) and Vitamin E (VE)  
The VC content was determined using a 2,6-dichlorophenol titration method based on CNS GB/T 6195-1986. Put 100 g kiwifruit fruits into a masher, add 100 mL extract, and quickly mash them into a homogenate. Then 20 g of the slurry sample was weighed, the sample was transferred to a 100 mL volumetric flask with extractant, and dilute to a certain extent, and then shaken and filtered. If the filtrate is colored, it can be decolorized by adding 0.4 g porcellanite per gram sample and then filtered. A total of 10 mL of filtrate was aspirated into a 50 mL conical flask with calibrated 2,6-dichlorophenol solution and was titrated until the solution was pink and did not fade for 15 s. A blank test was performed at the same time. Measurement of VB1 content referred to CNS GB 5009.84-2016. Measurement of VB6 Measurement of VB2 content referred to CNS GB 5009.154-2016. Measurement of VE content referred to CNS GB 5009.82-2016.