Product development of passion fruit and citrus peel dark chocolate

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Introduction
Fruit wastes such as fruit pomace and seeds are produced as by-products because of large consumption and industrial processing of the edible parts of the fruits (Ibrahim et al., 2017).These by-products are a good source of bioactive compounds including polyphenols, flavonoids, carotenoids as well as dietary fibre.Dietary fibre is well-known for its health benefits such as evading hydrolysis, helping in intestinal and bowel health, enhancing colonic fermentation, reducing preprandial cholesterol levels and maintaining insulin levels (López, et al., 2010).Chocolate is one of the most popular sweet snacks.Dark chocolate contains relatively high amounts of flavonoids including catechin, epicatechin, and procyanidin.Dark chocolates possess antioxidants and a positive effect on cognitive function and mood (Scholey and Owen, 2013).This study aimed to develop dark chocolate with a flavour combination using passion fruit and orange peels as functional ingredients.Dietary fibre content, sensory quality, proximate composition, and antioxidant activity of the developed product were determined.

Preparation of passion fruit powder
The passion fruit pulp (passion fruit with seeds) was collected and blended using Waring blender for a 1minute interval with 10 s rest to prevent overheating.The mixture was then spread on a plate, covered with food plastic wrap, and stored at -80°C for 2 hrs.The frozen sample was then freeze-dried for 3 days.The freezedried sample was then ground and sieved with a 150 μm sieve.

Preparation of orange peel powder
The oranges were first dewaxed using the boiling water method.The oranges were placed into the colander eISSN: 2550-2166 © 2022 The Authors.Published by Rynnye Lyan Resources FULL PAPER and boiled water was poured over the oranges.Then the oranges were scrubbed using a vegetable brush under cool running water to remove any wax residue.Finally, the oranges were wiped gently using clean paper towels.The orange peel was separated from the whole fruit with the albedo part removed.The orange peel was stored at -80°C freezer for 2 hrs before freeze-drying for 1 day.The freeze-dried sample was then ground and sieved with a 150 μm sieve.

Preparation of dark chocolate
The formulations of dark chocolate are displayed in Table 1.The cocoa butter in a bowl was heated by applying the double-boil method.Once the cocoa butter was melted and the temperature reduced to 50°C, unsweetened cocoa powder was sieved, slowly added to the melted cocoa butter, and mixed well with a balloon whisk to give a smooth chocolate liquid mixture.Honey, passion fruit and orange peel powder was added slowly before the tempering step to achieve a temperature of 26°C , followed by a reheating step until the temperature reaches 31°C to obtain a stable fat crystallization of the cocoa butter.The chocolate mixture was mixed well and poured into a mould.The chocolate was set at room temperature for 20 mins and then transferred to a refrigerator.The chocolate was removed from the mould and stored at 4°C.The control dark chocolate was prepared in the same way without adding passion fruit and orange peel powder.

Proximate analysis
Proximate analysis was done in duplicate for moisture (Hot air oven method), protein (Kjeldahl method), ash, total dietary fibre (Fibretec method), and fat contents (Soxhlet method) according to the AOAC method (William and George, 2005).

Sensory evaluation
Control and sample were evaluated by 35 panellists using a 9-point hedonic scale (1: dislike extremely to 9: like extremely).The panellists were asked to evaluate the degree of liking for the various sensory attributes (surface smoothness, aroma, texture, melting properties, and overall acceptance) of dark chocolates.All samples were labelled with 3-digit random digits and were arranged on trays based on a random permutation table.A preference test was also conducted.The acceptance Index of control and sample was calculated using the formula below.

Data analysis
All data obtained were tabulated using Microsoft ® Excel 2016 and analyzed with IBM SPSS ® software.The mean and standard deviation values (x̄±SD) were determined while a t-test with a confidence level of 95% was used to determine significant differences.

Results and discussion
Table 2 presents the proximate composition of control and sample, the sample had significantly higher fat content (51.15%) and dietary fibre content (3.06%) while lower in moisture content (9.55%), protein content (8.04%), and ash content (3.56%) compared to control.The increased dietary fibre content of the sample was attributed to the addition of passion fruit seeds and orange peels.The passion fruit raw seeds were rich in total dietary fibre (64.8 g/100 g) (Chau and Huang, 2004), while orange peels contain 64.3% total dietary fibre (Figuerola et al., 2005).
Table 3 shows the DPPH radical scavenging activity of the sample (86.10±0.13%)was significantly higher than that of the control (61.10±0.13%).The addition of passion fruit powder and orange peels powder caused an increase in the antioxidant activity of dark chocolate.Polyphenols in passion fruit powder and orange peel powder are major contributors to antioxidant activity.The main flavonoids in cocoa and chocolate are the and proanthocyanidins, which are polymeric compounds comprising catechin and epicatechin subunits (Steinberg et al., 2003).The citrus flavonoids include a class of glycosides, namely, hesperidin and naringin, and another class of O-methylatedaglycones of flavones such as nobiletin and tangeretin (Rafiq et al., 2018).
The mean sensory scores were analysed using a ttest.The sample scored higher in colour, surface smoothness, the aroma of passion fruit and orange peel, overall acceptability, and lower in texture and melting property compared to the control (Table 4).However, there was no significant difference between the sensory quality of the control and the sample.Figure 1 shows the comparison of the overall acceptance between the control and sample.The acceptance index of the sample is higher (77%) compared to the control (75%).

Conclusion
The addition of passion fruit powder and orange peels powder into dark chocolate significantly increased its dietary fibre content (3.06±0.02%)and DPPH radical scavenging activity (86.10±0.13%).The product meets the nutritional claim as a "source of" dietary fibre.The development of passion fruit and orange peel dark chocolate provides an alternative way to deliver healthpromoting antioxidants and dietary fibre to consumers.The product demonstrated acceptable sensory quality with an Acceptance Index of 77%.The utilization of agricultural waste such as fruit seeds and peels in functional food production has significant implications for population health and environmental protection.

Table 1 .
Formulations of dark chocolate (control) and passion fruit and citrus peel dark chocolate (sample).

Table 3 .
Antioxidant activity of control and sample Values are presented as mean±SD.Values with different superscript within the same row are significantly different (p<0.05).

Table 4 .
Mean scores for different sensory attributes of control and sample Values are presented as mean±SD (n = 35).Values with different superscript within the same row are significantly different (p<0.05).Figure 1.Comparison on the overall acceptance of control and sample