Formula Optimization and Product Design Analysis of Nougat with Purple Sweet Potato (Ipomoea batatas L), Cowpea (Vigna unguiculata L.) and Stevia Leaves (Stevia rebaudiana)

This study aims to determine the priority attributes of nougat with purple sweet potato, cowpea, and stevia leaves, optimize the nougat formulation from the organoleptic aspect, and determine the physicochemical characteristics of nougat. The research method used the Kano model and the central composite design (CCD) response surface methodology (RSM). The results of this study indicate that the attributes that influence consumer satisfaction are purple color, block shape, and low-calorie natural sweeteners. The optimum formulations obtained from purple sweet potato, cowpea, and stevia leaves were 20%, 1.5%, and 0.15% (Formula A14). Nougat products produce a moisture content of 6.09%-7.35%, ash content of 0.73%-1.95%, reducing sugar content of 11.50-17.11%, sucrose content of 35.14-49.38%, total color difference of 38.15-85.40; and hardness 0.357835-10.455 N. All formulations comply with SNI 3547.2-2008 (soft candy not jelly).


Introduction
Sustainable Development Goals (SDGs) are a framework for the next 15 years until 2030.SDGs contain 17 transformative goals that apply and are agreed upon by all nations without exception.One of these goals is to end hunger, achieve food security, improve nutrition, and promote sustainable agriculture, which is SDG goal number 2. Food diversification is related to improving the quality of human resources, agricultural development in food, and enhancing community nutrition, including production, consumption, marketing, and distribution.The importance of national food security through food diversification because it can strengthen Indonesia's food security, can be achieved by developing and introducing alternative local food ingredients with high nutritional value [1].Therefore, it is necessary to diversify food products by utilizing the potential of local wisdom to become nutritious food products ready for consumption.
Purple sweet potatoes, cowpeas, and stevia leaves were chosen because cowpea production is relatively high in Indonesia, 1.5-2 tons/ha depending on variety, location, season, and cultivation [2].Cowpeas contain protein, fat, and carbohydrates, which are suitable for the body [3].Purple sweet potatoes are also one of the abundant commodities, and their potential use as a food ingredient is extensive in Indonesia.According to Statistics Indonesia (2015), sweet potato production in Indonesia reaches 2,297,634 tons annually.Purple sweet potatoes contain higher anthocyanin than other sweet potato, resulting in an attractive purple color [4].Stevia was chosen because it is a natural sweetener without calories that comes from Stevia rebaudiana leaves.Stevia has been widely used as a commercial sweetener, with a market share of over 50%.Stevia can produce a sweetness level 200-300 times that of cane sugar.Stevia is not toxic and does not cause side effects [5].
Nougat is a type of candy made from nuts, egg whites and sugar.The taste, chewiness, and elasticity of nougat candy are characteristics that are distinctive in the product [6].Based on data from the Indonesian Food and Drug Authority Number 30 of 2018 shown in Table 1, nougat is more consumed when compared to soft and hard confectionery [7].

Table 1. Consumption of confectionery in Indonesia
Various types of candy, such as jelly candy, marshmallows, and nougat, dominate about 50% of the candy market [8].Most candies on the market contain high calories and use sweeteners, dyes, and synthetic flavors, which harm the body [9].Not many nougat candy products use natural sweeteners natural dyes and contain low sugar levels.Therefore, it is necessary to develop nougat with the natural purple color of sweet potato, cowpea, and the natural sweetener of stevia leaves to overcome such a problem.The purpose of this study was to determine the priority attributes of nougat products with purple sweet potato, cowpea, and stevia leaves desired by consumers to make the optimum nougat product formulation from organoleptic aspects (aroma, texture, taste, color, and overall liking), and knowing the physicochemical characteristics of the nougat product (moisture content, ash content, reducing sugar content, sucrose content, color, and hardness).This research will search for the best formulation with the Response Surface Methodology (RSM) method using the type of Central Composite Design (CCD) model from the organoleptic aspect.This design was chosen because it is the most widely used, easy, flexible, efficient, and has perfect predictions for all design areas [10].The quality attribute analysis was carried out using the Kano model, which aims to find the attribute components of nougat candy products that consumers can accept [11].

Sample Preparation
The ingredients used to produce nougat are stevia leaf powder, cowpea, purple sweet potato, unsalted butter, skimmed milk powder, gelatin, vanilla powder, cornstarch, and water.The process of making nougat candy begins with melting the butter.Marshmallows and powdered milk are added.Purple sweet potato puree, cowpea, and stevia leaf powder were added according to treatment.All the ingredients were stirred for 20 minutes at 123℃-129℃.Nougat is poured onto a baking sheet lined with parchment paper and then pressed until solid and cooled at room temperature (20℃-25℃) for 6 hours.After hardening, the nougat is cut into pieces 9 cm long and 6.5 cm wide.[12,13,14,15].

Kano Method
Analysis of quality attribute was conducted using Kano Model [11].The Kano model aims to find priority attribute components of nougat candy products that consumers can accept.The resulting attributes from the Kano model were used as formulation in optimizing nougat.The assessment used in the questionnaire has five satisfaction level scales consisting of functional and dysfunctional questions.The number of samples involved in filling out the Kano questionnaire was 50 students of Bina Nusantara University.The questions consist of five questions for each functional and dysfunctional question.The response to the Kano questionnaire was analyzed by calculating the number of each category.The consumer satisfaction coefficient consisting of if better than (IBT) in the Y axis and if worse than (IWT) in the X axis.The calculation according to the formula can then be mapped through the Kano quadrant [16].

Validation method
The validity test is carried out by comparing the r count's value with the r table's value.The question is valid if the r count is more significant than the r table.The reliability test was carried out through statistical tests using Cronbach's Alpha (α) value.A variable reliable if Cronbach's Alpha > 0.60 [17].

Physicochemical characteristic
Analysis of moisture and ash content was carried out by using the gravimetric method.While the reducing sugar and sucrose content was carried out by the luff schoorl method, which all these methods, refers to the AOAC (1990) and SNI 3547.2-2008(soft candy not jelly).The hardness value was calculated by texture analyzer, and the total color difference value was analyzed by colorimeter.The sensory evaluation is used a 9-point hedonic scale based on aroma, color, texture, taste, and overall liking.Fifty semi-trained panelists carried out the assessment.The rating scale for all sensory attributes ranged from 1 (very not preferred) to 9 (strongly preferred).

RSM
Formulation optimization was carried out using the response surface methodology (RSM) with a central composite design (CCD) using Design Expert 13.The RSM begins with determining the lower limit (-1) and upper limit (+1) for each factor.In this study, three factors were used, the purple sweet potatoes ( ! ), cowpeas ( " ), and stevia leaves ( # ) with a combination of five composition variables.The central point used to determine the optimal value of the three factors is one so that 15 experiments are obtained with the α value defined in the CCD design is 1.68.The variety of 15 treatments is shown in Table 2.

Kano Model
Determination of the value of the product satisfaction coefficient is carried out by using the formula for the level of satisfaction (IBT) and the level of dissatisfaction (IWT).The results of the product satisfaction coefficient values of various attribute components are shown in Figure 1.

Figure 1. Customer satisfaction mapping
Figure 1 and Table 3 show the five nougat candy attributes mapping.Three attributes fall into quadrant II of the attractive category.Namely, the nougat candy product has a purple color and uses low-calorie natural sweeteners in the form of blocks.The purple color in nougat candy is caused by purple sweet potato, attracting consumers' attention.The use of low-calorie sweeteners is also included in the attractive category.This is likely due to increased consumer awareness of calorie intake so that consumers demand low-calorie sweeteners.Low-calorie sweeteners such as stevia are included in the safe sweetener, so it is preferred by consumers who reduce their consumption of sucrose sugar.The shape of a product is an essential basis for determining product identity by consumers so that it is included in the attractive category.Nougat candy is in the form of blocks because it has gone through a dough compaction process, making it easier to cut [18].The cowpea grains that are evenly distributed and the whole shape of the cowpea grains are included in quadrant III, indifferent category.This category has no effect on consumer acceptance.

Physicochemical Characteristics
The analysis of the physicochemical characteristics of nougat consisted of moisture content, ash content, reducing sugar content, sucrose content, hardness, and color (total color difference) as shown in Table 4.

Table 4. Physicochemical characteristics
For moisture content, all nougat candy formulations have met the quality standards of soft candy (not jelly) SNI 3547.2-2008 with a maximum limit of 7.5% [19].Moisture content resulting in nougat candy ranges from 6.09% to 7.35%.The highest moisture content is found in the A4 formulation, which is 7.35%.The higher the purple sweet potato, the higher the water content will increase.The high moisture content in purple sweet potatoes is caused by fiber-containing cellulose, pectin, and lignin, so it easily binds water [20].The higher the water content, the softer and stickier the texture will be [21].For ash content, all nougat candy formulations have met the quality standard of SNI 3547.2-2008 with a maximum limit of 2% [19].Ash content resulting in nougat candy ranges between 0.73% -1.95%.The highest ash content value is in the A4 formulation, which is 1.95%.The higher the purple sweet potato, the higher the ash content.This is because of the high mineral content in purple sweet potatoes [22].For reducing sugar content, all nougat candy formulations have met the quality standards of SNI (3547.2-2008)with a maximum limit of 20% [19].The value of reducing sugar content produced in nougat candy ranges from 11.50-17.11%.The highest reducing sugar content was found in the A9 formulation, which was 17.1%.The higher the purple sweet potato, the reducing sugar level increases because the carbohydrates in purple sweet potato contain glucose [23].The higher the stevia leaves, the reducing sugar levels also increase [24].
All of the nougat candy formulations for sucrose content met the quality standards of SNI 3547.2-2008 with a minimum limit of 35% [19].The resulting sucrose content ranges from 35.14-49.38%.The highest sucrose content value was found in the A8 formulation, which was 49.38%.The higher the reducing sugar content, the lower the sucrose level, and vice versa.This is due to the hydrolysis of sucrose by water and high temperatures into inverted sugars, namely glucose, and fructose, so that they can be involved in the Maillard reaction [25].The hardness value produced in nougat candy ranges from 0.357835-10.455N. The highest hardness value is found in formulation A1, which is 10.455 N. The higher the hardness value, the more complex the candy texture [26].The higher the purple sweet potato, the softer the texture because it contains a high moisture content [27].

Formula
The total color difference (∆E) value produced in nougat candy ranges from 38.15-85.40.The highest ∆E value is found in formulation A12, namely 85.40.The more purple sweet potatoes, the darker purple because it contains more anthocyanins [28].Meanwhile, stevia leaves are brownish-green and become darker as the concentration increases.The green color is produced from chlorophyll, tannins, flavonoids, steroids, and alkaloids [29].Reducing sugar levels also affects the color of candy nougat because of the reaction of reducing sugars with amino acids (Maillard reaction) to produce a brown color [30].

RSM
Optimization of the formula of nougat was carried out using a Response Surface Methodology of the Central Composite Design (CCD) type with a second-order quadratic polynomial curve model.In this study, three factors were used, the concentration of purple sweet potato (A), cowpea (B), and stevia leaves (C).The midpoint utilized to determine the optimum value of the combination of the three factors is one so that 15 trials are obtained.The α value set in the CCD design with three elements is 1.68.The equations obtained from each response are shown in Table 5.

Table 5. Equation of formula optimization results on hedonic values
Optimization is carried out after obtaining a mathematical model for each response.Optimization of the nougat was carried out using an overlay plot based on the effect of a combination of purple sweet potato (A), cowpea (B), and stevia leaves (C) on the variety of response values, namely hedonic aroma, texture, color, taste, and overall liking.In Figure 2(a), (b), and (c), it is shown that one optimum point is predicted with a combination of purple sweet potato, cowpea, and leaves of 20%, 1.5%, and 0.15% with aroma, texture, color, taste and overall liking of 6.14; 5.84; 6.49; 6.31; and 6.35 respectively.

Conclusion
The conclusion of this study is that the attributes that influence consumer satisfaction based on the level of importance in product design are purple color, block shape, and low-calorie natural sweeteners as attractive components.The cowpea granules, which are spread evenly and form whole cowpea grains, are indifferent components.The optimum formulation of nougat candy obtained from concentrations of purple sweet potato, cowpea, and stevia leaves was 20%, 1.5%, and 0.15%, respectively (Formula A14).Based on the physicochemical characteristics test, nougat products produced a moisture content in the range of 6.09%-7.35%,ash content in the range of 0.73%-1.95%;reducing sugar content in the range of 11.50-17.11%;sucrose content in the range of 35.14

Figure 2 .
Figure 2. (a) Overlay plot showing optimum region for hedonic responses of purple sweet potato and cowpea, (b) purple sweet potato and stevia leaves, (c) stevia leaves and cowpea

Table 3 .
Product satisfaction coefficient value -49.38%; total color difference value (∆E) in the range of 38.15-85.40;hardness values in the range of 0.357835-10.455N.All formulations are in accordance with the quality standards of SNI 3547.2-2008(soft candy not jelly).