Background

Kiwfruit is one of the newest fruit crops gaining international commercial importance. In New Zealand, the first commercial plantations of Actinidia deliciosa were established around 1930 (Ferguson and Stanley, 2003). It has different varieties such as Hayward, Bruno, Monty, Allison and Abbott having different taste, odor, shape, and chemical compositions (Samadi-Maybodi and Shariat, 2003). It is produced in temperate areas located between the latitudes 25° and 45°. China, Italy, New Zealand, Chile and Greece are the biggest producers in the world (Ferguson and Huang, 2007; Nishiyama, 2007). The production of these five countries accounts for 87% of the world kiwifruit production (Ferguson and Huang, 2007). In context of Nepal, kiwifruit is an emerging commercial crop as well as its popularity has also been increased.

Several research works has been reported to characterize its nutrients, chemical compositions and medicinal uses (Samadi-Maybodi and Shariat, 2003). The chemical constituent of kiwifruit is a matter of great interest to those who wish to figure out the basis of the nutritional value and health benefits of consuming kiwifruit (Drummond, 2013). The nutritional values and appearance are usually influenced by the vitamins and pigments present in the fruit whereas taste is primarily affected by the acidity, sweetness and volatiles (Pal et al., 2015). Kiwifruits are healthy due to the presence of high ascorbic acid levels (Ferguson and Huang, 2007), polyphenols (Sheng et al., 2015), and flavonoids (Atkinson and Macrae, 2007). It can be utilized for the curing different types of cancers, e.g., stomach, lung, and liver cancer (Yang, 1981) in traditional medicine. It is reported that the extracts of kiwifruits inhibit cancer cell growth (Song, 1984) and exhibit cell protection against oxidative DNA damage in vitro (Collins et al., 2001).

This work was aimed to investigate comparative study on the nutrient and chemical composition of different varieties of fresh kiwifruits. As they lost their characteristics color and flavor during processing, only fresh kiwifruits are commercialized. The analysis is performed only on the edible flesh portion of kiwifruits since these are eaten with the skin removed only.

1. Results

Three different species were taken as sample fruits and different macronutrients, elemental composition, moisture content, acidity present in the given samples were determined.

Water content was analyzed as the largest amount of constituent found in kiwifruit. Titrated acidity was measured by taking malic acid as a reference as it is found in more amount than others. A significant variation in TSS content was noted in different species of kiwi fruit. The amount of protein was found comparatively less in kiwi whereas vitamin C content was found to be more in kiwifruit as compared to other nutrients. The elemental analysis of kiwifruit shows that the amount of calcium is more than that of iron.

2. Discussion

2.1 Water content

Figure 1 showed that there was not much difference in water content in three species of kiwifruit. It was found that Hayward has the highest amount of water (82.6%) and least by Allision (81.7%) which is almost similar value reported 83.6% (Nishiyama, 2007) and 83.32% (Celik et al., 2007) in Hayward. In previous studies, it was found that the water content of Hayward kiwifruit cultivar was between 80% and 85% (Snelgar et al., 1993). Therefore our result was in accordance with the above-mentioned literature.

Figure 1 Water content in different cultivars of kiwifruit

2.2 Titratable acidity

Figure 2 showed the titrated acidity among the three species of kiwifruit. From this result it is observed that Monty (1.75%) is more acidic followed by Allison (1.18%) and Hayward (1.12%) respectively. Celik et al., 2007, reported titratable acidity of kiwifruit cv. Hayward at harvest time 1.64% which is closely matched with our result. In contrast, Pal et al., 2015 studied that titrated acidity (TAD) varied significantly among most of the cultivars and found 0.54% in both Hayward and Allison but 0.52% in Monty in the harvesting month of November. This is due to the fact that TAD depends on the maturity stage of kiwifruits; higher at the start of fruit development, but it decreased as the fruit matured.

Figure 2 Titratable acidity in different cultivars of kiwifruit

2.3 Total soluble solid (TSS)

A significant variation in TSS content was noted in different species of kiwi fruit (Figure 3).  Soluble solids varied from (14-19) °Bx. Allison had the highest TSS among other which can be illustrated by Figure 3. Hayward had 18 °Bx while Monty had 14 °Bx. The TSS increases as the maturity stage of fruit increases (Pal et al., 2015). In contrast, Pal et al., 2015 reported that TSS was more in Monty (12.67%) followed by Allison (11.89%) and Hayward (10.79%) harvested in the month of November. This variation in result is because of the environmental effect.

Figure 3 Total soluble solid content in different varieties of kiwifruits

The ratio of TSS/TAD represents the desirable flavor of varities (Pal et al., 2015). TSS/TAD ratio of Allision, Hayward and Monty was found 16.07, 16.10 and 8.00 respectively. Allision and Hayward had the almost similar TSS/TAD ratio followed by Monty which indicated that more Allision and Hayward have more desirable flavor than that of Monty.

2.4 Protein content

Protein is one of the macronutrients in the fruit. The amount of protein was found comparatively less in kiwi. Figure 4 represents the variation in protein content among given sample fruit. The total nitrogen content of cv. Hayward and Monty was 0.67% and of cv. Allison was 0.22%. Celik et al., 2007 reported that the total nitrogen content of cv. Hayward was 0.84%. It is well known that the cultivar, soil characteristics, climate and sample preparation method affects plant nutrient concentrations in fruit species (Salunkhe and Kadam, 1995).

Figure 4 Variation of protein content in kiwifruits

On comparing among three species (i.e. Hayward, Monty and Allison), Hayward and Monty were contain equal amount of protein (4.19%) while Allison was found to have least value (1.43%). Whereas, Richardson et al., 2018 reported 1.14% protein content in Green Kiwifruit and 1.02% in Gold Kiwifruit. These results indicated that Hayward and Monty are better sources of protein as compared to other varieties of kiwifruit i.e. Allison, Green Kiwifruit and Gold Kiwifruit.

2.5 Vitamin C content

The amount of vitamin C decreases with the ripening of the fruit. Among three species of kiwi fruit, Allison was found to contain highest amount of vitamin C (53.35 mg/100 g) and least by Hayward variety (33.20 mg/100 g) (Figure 5). This natural variation of amounts of vitamin C in kiwifruits is due to several factors such as growing region and conditions, use of fertilizers, maturity at harvest, time of harvest, storage and ripening conditions etc. (Lee and Kader, 2000; Richardson et al., 2018). The Allison kiwifruit contains 53.35 mg vitamin C per 100 g which is similar to that of oranges (53.20 mg/100 g) however, this content almost six times greater than that in bananas (8.70 mg/100 g) and watermelon (8.10 mg/100 g) on an edible flesh weight basis (Richardson et al., 2018). Lintas et al., 1991 found the variation in the vitamin C content of the 10 different cultivars where major differences between cultivars were observed. Fruit of 'Abbott' contain the lowest (73 mg/100 g) and fruit of Bruno with the highest (241 mg/l00 g) vitamin C concentrations.

Figure 5 Vitamin C content in kiwifruits

Carr and Frei, 1999 explored in vitro as well as animal and human intervention studies and explained the role of vitamin C in the functioning of the immune system. Leukocytes, which are cells responsible for defending the body against invading pathogens, contain high levels of vitamin C, indicating a vital function in the immune system. Similarly, the vitamin C content of kiwifruit has the highest correlation with total antioxidant activity of kiwifruit (Lim et al., 2014). Thus it is concluded that Kiwifruit are one of the richest sources of vitamin C available; a single fruit contains enough to satisfy the minimum daily requirement (Ferguson, 1991).

2.6 Elemental composition

Calcium and iron are important elements for fruit quality. The elemental analysis of kiwifruit shows that the amount of calcium is more than that of iron. The comparative study between the elemental compositions in kiwifruit is given by Figure 6. Monty species has largest amount of calcium (350.7 mg/kg) and iron (11.2 mg/kg) than other two species. Samadi-Maybodi and Shariat, 2003 studied the concentrations of calcium in the four varieties of kiwifruit are comparable to one another; Bruno and Monty have the same concentration of calcium. Analysis of magnesium in the four varieties of kiwifruits reveals that Monty has the greatest concentration, and Bruno contains the least concentration of this element; Hayward and Monty have comparable concentrations of the corresponding element (Samadi-Maybodi and Shariat, 2003).

Figure 6 Elemental composition (Iron and Calcium)

According to USDA National Nutrient Database for Standard Reference Release 28, Green and Gold raw kiwifruit contain calcium 340 mg per Kg and 170 mg per Kg respectively. Hence, the value of calcium content was found more in Monty (350.7 mg/Kg) and Hayward (348.9 mg/Kg), than that in Green kiwifruit and Gold kiwifruits cultivars whereas, that of Allision (263.6 mg/Kg) was observed more than Gold kiwifruits but less than that in Green kiwifruit. This result concludes that the Monty and Hayward cultivars are better option for calcium micro nutrients. 

Sharaf et al., 1979 studied the chemical composition of banana fruit (Musa cavendishii) grown in Egypt and found that calcium and iron content in banana fruit was found 701.9 mg/Kg and 4.7 mg/Kg respectively. It is concluded that kiwifruit contain less calcium but more iron than that of banana.

3 Materials and Methods

3.1 Materials

Samples of kiwifruit i.e. Hayward, Monty and Allison were brought from Bhotechaur Agricultural Organic farm, Sindhupalchowk, Nepal. The cultivation conditions were same for all the species. The sample fruits were washed in cold water, peeled, sliced and stored for further analysis.

Ascorbic acid (Himedia), Dye solution (sodium salt and 2,6­ dichlorophenol-idophenol) and magnesium oxide (Loba Chemie), sodium bicarbonate, Phosphoric acid and sulphuric acid (Fisher Scientific), boric acid powder (Emplura), Iron and Calcium standard for AAS (Fluka) were used during the experiments. All the reagents were of analytical grade and double-distilled water was used throughout the analysis.

3.2 Biochemical analysis

3.2.1 Water content

The sample was ground in the mixer and was put into the beaker with the help of spatula. The weight of aluminum dish was weighed. The sample was added on the dish and weight was measured. Then the sample was put on the hot air oven for 2-3 hours to dry. The sample was again weighed in the balance and result was calculated using the equation (1) (Mohammed et al., 2009). All the tests were performed in triplicate.

Water content =[ (W_w – W_d)/ W_d]×100%  (1)

where W_w is the wet weight and W_d the dried weight.

3.2.2 Titratable acidity

Titratable acidity indicates the total or potential acidity such that it includes the total number of acid molecules (Pal et al., 2015). The sample paste was taken in a volumetric flask and its weight was measured. The volume was made up to 100 mL with distilled water. The solution was filtered and 5 mL of the sample solution was taken in a conical flask which was mixed with 20 mL of distilled water. 2-3 drops of phenolphthalein were added on it and titration was done with NaOH having 0.0875N normality. The reading of end point was noted carefully. Finally, acidity of the sample fruit was calculated using the following equation (2) (Garner et al., 2008). All the tests were performed in triplicate.

Acidity = (titre×normalityof NaOH × equivalent weight of malic acid × volume made × 100) /(weight of sample ×aliquot of sample solution ×1000)   (2) 

3.2.3 Total soluble solid (TSS) content

The molecules which are completely soluble in an aqueous sample are called total soluble solids (TSS) content. In commercial standards, the TSS of a ripe fruit is used to indicate its sweetness (Crisosto and Crisosto, 2001). It is measured as the units of Brix value that is defined as percent sucrose by weight. TSS has been used to reflect the eating quality of ripe fruit. To measure TSS, the fruit was peeled and sample juice was extracted from it. Few drops of sample juice were placed on the refractometer. The cover plate was lowered and the reading was noted. Measurement was performed in triplicate.

3.2.4 Protein content

Kiwifruit are not a significant source of protein. Specific protein fractions and enzymes naturally present within kiwifruit are of much interest in relation to health effects. Therefore, protein content was determined by following ways:

Protein content of the samples was analyzed by Kjeldahl jar method. The sample of 0.5g (homogenized) was accurately weighed and transferred carefully in Kjeldahl flask. 2.5g of digestion mixture was added and 10 mL of conc. sulphuric acid was added. The flask was adjusted on the hot plate, in the exhaust tube and heated first at low temperature until the frothing ceases. Then the heat was increased to boil the acid vigorously and digest for about 3 hours. After the mixture is clear, the contents of the flask were cooled, diluted with 10 mL distilled water and transferred in a 100 mL volumetric flask. The digestion flask was rinsed three times with distilled water and collected in the same flask. The volume makeup was done up to the mark after bringing at room temperature. The blank digestion was carried out without the sample and made the digest to 100 mL.

Protein content of the samples was also verified by distillation method. The automatic distillation unit was used to distillate the sample. The distillated sample was taken in 100 mL Erlenmeyer flask with 10 mL of boric acid solution and the distillation process was continued for 2 minutes or till the receiver changes into greenish. The content was then titrated in the receiver with standard acid (0.05N H₂SO₄), till the indicator changed to pink from green and reading was noted. The calculation was done by using the equation (3, 4) (Nwanekezi et al., 2010).

Total Nitrogen % by weight = (titre ×normality ×1.4 ×volume made) / (aliquot of digest ×weight of sample ×100)   (3)

Total Protein (%) = Nitrogen ×6.25  (4)

3.2.5 Vitamin C content

Vitamin C (total ascorbic acid) content is the most important and distinctive nutritional value of kiwifruit. The method used to determine vitamin C content in the fruit sample is described as:

The sample was prepared by weighed it in a watch glass and then ground with the help of mortar and pestle. It was blended with 3% HPO₃ and made up to 100 mL with HPO₃. An aliquot (10 mL) of the HPO₃ extract of sample was taken in a conical flask. It was then titrated with standard dye to a pink end point. The reading was noted carefully. The calculation is done by using the equation (6) (Ranganna, 1986).

mg of ascorbic acid per 100 mg= (titre×dye factor×volume made×100) / (aliquot of extract×weight of sample)  (6)

Dye factor was determined using the equation (5) (Ranganna, 1986).

Dye factor = 0.5 / titre  (5)

3.2.6 Elemental analysis

Elemental Analysis of kiwifruit was performed by Flame Atomic Absorption Spectroscopy (FAAS) technique. Kiwifruit consists of various minerals such as calcium, magnesium, iron, phosphorous, potassium, zinc, copper etc. To determine the calcium and iron, the paste of the sample was kept in crucible, weighed it and kept in a muffle furnace for 8 hours. The solution of conc. HNO₃ was added in it and was kept in fume hood to heat the solution mixture for 10-15 minutes. The solution mixture was poured into the volumetric flask and volume was made up to 100 mL with HNO₃ then standardized by calibration method. Then after, the elements iron and calcium was determined by FAAS technique.

3 Conclusion

This research work explores the biochemical analysis of varieties of kiwifruit grown in Nepal. These data indicate that kiwifruit is nutritionally dense, driven largely by the high amount of vitamin C. Results indicated that the amount of elements, nutrients, TSS and acidity varies in different species. Among all the chemical constituents, water content was found more in all three species of kiwifruit. Based on the elemental composition, between iron and calcium, amount of calcium was found more than iron. Both the micronutrients (Fe and Ca) were found more in Monty than that in Allision and Hayward. It is concluded that Kiwifruit are one of the richest sources of vitamin C available; a single fruit contains enough to satisfy the minimum daily requirement. Amount of Vitamin C decreases with the maturity and ripening of the fruit and was found more in Allison. There was not much difference in TSS and TAD among the species. The TSS/TAD ratio in Allision and Hayward had the almost similar higher value than in Monty which indicated that Allision and Hayward have more desirable flavor than that of Monty. These results support that kiwifruit is highly nutritious, low-calorie fruit with the potential to deliver a range of health benefits.

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