Fruit is an important part of human nourishment, as it contains a number of beneficial and essential compounds, including saccharides, vitamins, minerals, fiber, and natural pigments [1
]. To secure a fruit supply throughout the year, it is often necessary to preserve it [2
]. One of the oldest techniques of preserving foods is by lowering the water activity in foods, for example, by drying [3
]. However, through drying, fruit may lose much of its natural texture and color [4
]. Color change is caused by both enzymatic and non-enzymatic browning [7
]. One of the methods of suppressing enzymatic browning is by use of ascorbic acid [2
]. Thus, the main objective of this research was to investigate the effect of natural juices with ascorbic acid for suppressing enzymatic browning of apple (Malus domestica
Borkh.) slices during dehydration.
2. Experimental Section
Natural fruit juices of five species containing high amounts of vitamin C (Sorbus aucuparia (mountain-ash), Diospyros kaki (Japanese persimmon), Hippophae rhamnoides (sea buckthorn), Actinidia deliciosa (kiwifruit), Rosa canina, (dog-rose)) and a 1% solution of ascorbic acid (AA) were used in this study. Except for the juice from Rosa canina, all juices were prepared in a juicer and diluted with an equal amount of distilled water. The juice from fruits of Rosa canina was prepared by macerating the fruits in the dark at 18 °C.
Vitamin C content in the juices was measured by high-performance liquid chromatography (HPLC). Samples of juices were centrifuged in 1.5-mL Eppendorf test tubes at 14,500 rpm and then filtered through a PTFE filter with a 0.45-μm pore size. The samples were diluted with water at 20:1 and then analyzed via HLPC with a diode array detector (Dionex Summit, Dionex Inc., Sunnyvale, CA, USA). The chromatography conditions were as follows: isocratic elution with a mobile phase of 0.1% TFA, and a flow rate of 0.8 mL/min on a Gemini C-18, 250 × 4.6 mm, a particle size of 5 μm (Phenomenex, Inc., Torrance, CA, USA) column with a C-18 pre-column. Column temperature was 25 °C, and the injection volume was 20 μL. The program duration was 7.5 min with elution time of the ascorbic acid at 6.2 min. Sample ascorbic acid content was measured at 243 nm and quantified by external calibration using a linear calibration for ascorbic acid of 0.004–1.000 mg/mL.
A single juice or ascorbic acid solution was applied to five slices of “Idared” apple, cold stored prior to use, by soaking the slices for 10 min. The apples were purchased at a local fruit market in Prague, Czech Republic. The treated apple slices, as well as untreated slices, were then dried in a home fruit dryer. The dryer temperature was set to 57 °C with a constant air flow rate of 1.0 m/s. The drying was continued until a constant mass was obtained.
After drying, the degree of browning and the color change were measured on a colorimeter MiniScan® XE Plus. The results were reported using the L*–a*–b* system. Freshly cut apples were measured as a standard, and untreated dried samples measured as a control. After that, the samples treated with the fruit juices and 1% ascorbic acid were measured.
Parameters such as browning index (BI
), hue angle (h°), and saturation index (C*) were calculated. The BI
represents purity of brown color and is generally accepted as a significant parameter in color change. It is calculated as [8
The BI assumes values between 0 and 100, where 0 corresponds to white and 100 corresponds to black. Hue angle (h°) is defined as the angle between the hypotenuse and 0° on the a* (bluish-green/red-purple) axis, and is calculated from the arctangent of b*/a*. The hue angle value corresponds to whether the object is red, orange, yellow, green, blue, or violet. Saturation index C* represents the hypotenuse of a right triangle created by joining points (0, 0), (a*, b*), and (a*, 0). The greater the C* value, the purer the color appears. The total color difference ΔE* was calculated as ΔE* = (ΔL*2 + Δa*2 + Δb*2)1/2.
Besides colorimetry, the organoleptic properties of the apple slices were evaluated by 14 trained panelists. During panel tasting, participants filled out a questionnaire about the perceived color change, degree of browning, taste, and overall attractiveness of presented dried apples. The degree of browning and color change was evaluated in comparison with a standard of freshly cut apples. All tests were done in triplicate.