Factorial experimental design for optimizing the roasting condition of banana peel ( Musa paradisiaca var Semeru): characteristics and antioxidant activity

Banana peel is an organic waste that has not been widely used. The rich content of vitamins, minerals, antioxidants and tryptophan is the background for research utilizing organic waste for something useful. This study aimed to use a factorial design to find the optimum conditions for the banana peel roasting process. The variables analyzed were roasting temperature (180°C and 200°C) and roasting time (15 and 20 mins). Responses to determine the optimum conditions were water content, color similarity with local coffee products and antioxidant activity (IC 50 ). Antioxidant activity was measured using the 2,2 - diphenyl - 1 - picrylhydrazyl method. The characteristics of banana peel powder and roasted banana peel powder showed the presence of beneficial substances and even richness in vitamins and minerals, as well as the presence of tryptophan. The scavenging activity of roasted banana peel powder was higher at high temperatures and longer during roasting. The lowest water content was at 200°C and 20 mins of roasting conditions. A brownish color resembling ground coffee was obtained with a roasting temperature of 200°C. The interaction between temperature and roasting time affects the moisture content and color similarity, while the roasting time dominantly affects IC 50 . The optimum conditions selected were 200°C roasting conditions; 20 mins will produce a water content of 2.00%, color similarity 11.80 and IC 50 1979.2 (ppm). In addition, the presence of tryptophan in banana peel powder has the opportunity to be used as an antidepressant compound.


Introduction
Banana peel from Musa paradisiaca L. var.Semeru is an organic waste produced from the processing of bananas.Banana peel is a third part of the banana fruit, with various nutritious substances, including carbohydrates, fats, proteins, vitamins, minerals, polyphenolic compounds, and tryptophan.Polyphenol compounds can be used as natural antioxidant, antibacterial, and anti-inflammatory compounds, while tryptophan is a compound with mood-improving effects (Someya et al., 2002;Khawas and Deka, 2016).
During the roasting process, heat transfer occurs from the roaster into the material and water mass is transferred, affecting the moisture content of the roasted powder.The length of the roasting time will affect the water content in the roasted powder.In addition to the water content, the roasting process will also affect the color of the roasted material.The higher the roasting temperature and the longer the roasting time, the darker the material's color.
Free radicals are unstable, reactive molecules or atoms that can cause the degradation of body cells.Those radicals participate in the aging and pathogenesis of diseases (Halliwell, 2012).Our body needs antioxidants to catch those radicals through catalyzing the destruction of free radicals (binding free radicals), acting as a primary antioxidant (donates hydrogen protons), reducing agent (donates electrons), reducing the formation of singlet oxygen ( 1 O 2 ) and binds metals (metal chelators) (Sharma et al., 2018).
Banana peel has been made into a coffee analog drink through roasting, but the optimum roasting condition has not been determined yet (Mentari et al., 2019;Sofa et al., 2019).A previous study showed that cocoa bean shells roasted at temperatures of 110 o C, 140 o C and 190 o C produced higher antioxidant activity than those not roasted (room temperature, 25 o C).The optimum antioxidant activity was reached by roasting at 140 o C, not at the highest temperature.Moreover, the temperature and roasting time greatly influence sensory properties.This study will determine the optimum RESEARCH PAPER roasting temperature of Agung banana peel (Utami et al., 2017).
To date, the utilization of banana peel waste still has low economic value.One previous study made starch from banana peels (Hadisoewignyo et al., 2017).This study used banana peels as an ingredient that works as a source of antioxidants and antidepressants.Banana peel is made into powder and roasted.The roasting process can affect the air content, color, and antioxidant activity of the ingredients, it is necessary to determine the optimal conditions of the roasting process.This study aimed to optimize the conditions for roasting agung banana peels with high antioxidant activity.

Characteristics of banana peel powder and roasted banana peel powder
The characteristics of banana peel powder and roasted banana peel powder were carried out at the Center of Agro-Based Industry, Ministry of Industry of the Republic of Indonesia, including microbiological testing.

Optimization of banana peel powder processing with factorial design
The roasting process was optimized using the factorial design method with two factors and two levels.The factors used are the temperature of the roaster and the roasting time.The temperature levels were 180 o C and 200 o C and the roasting time was 15 mins and 20 mins.Water content, powder color, and antioxidant activity (IC 50 ) were the observed responses to obtain the optimum process.The optimum condition was analyzed using Design Expert software ver 10.0.1.
Banana peels are washed, chopped (1×1 cm), dried, ground and sieved with 80 mesh.Banana peel powder is roasted at a roasting temperature of 180 and 200°C for 15 and 20 mins, as mentioned in Table 1.

Moisture content analysis
The moisture content of the banana powder was determined after the roasting process using a moisture balance analyzer.

Color analysis
The color of roasted banana peel powder was measured using a colorimeter.

DPPH radical scavenging activity
The DPPH radical scavenging activity (DPPH-RSA) test was carried out using the Burda and Oleszek (2001) method.A weight of 12.5 mg of the sample was dissolved in 25 mL of methanol.Then 1 mL of the solution was pipetted, and 2 mL of 0.1 mM DPPH solution was added.Then it was incubated in a dark room for 30 mins at room temperature, and then the absorbance was measured using a UV-Vis spectrophotometer at a wavelength (λ) of 517 nm.Antioxidant activity as a free radical scavenger was expressed as IC 50 (μg/mL), which indicates the sample's concentration that can inhibit free radicals by 50%.

Characteristics of banana peel powder and roasted banana peel powder
The characteristics of banana peel powder and roasted banana peel powder (Figure 1) were determined by several parameters, as shown in Table 2. Banana peels contain high amounts of minerals, especially sodium, potassium, calcium, magnesium, iron and zinc.In addition, there are also several vitamins, including vitamin C and beta-carotene, in a reasonably high amount.Dietary and insoluble dietary fiber is available in relatively high quantities in roasted and non-roasted banana peel powder.The insoluble dietary fiber in banana peels was high enough to benefit the body, especially the digestive system.In addition, heavy metal ( contamination is determined for roasted and non-roasted banana peel powder.The content of lead, cadmium, copper, tin, and mercury does not exceed the required provisions. The test on microbial contamination (Table 3) showed that the total plate count and the number of yeast molds were lower in the roasted banana peel powder.That was related to the use of high temperatures in the roasting process.
The presence of high carbohydrate content in banana peel powder and the heat during processing allows the formation of acrylamide, which is formed through various reactions, one of which is the Millard reaction (reaction between amino acids and reducing sugars).In the roasting process, the temperature used was 200 o C, where high temperatures can increase the formation of acrylamide, so the acrylamide content is higher in roasted banana peel powder (0.475 µg/g).
The presence of tryptophan in banana peels can relieve symptoms of depression and mood disorders.Tryptophan turns into serotonin when it breaks down, which can improve mood.The presence of tryptophan in banana peels can be used as an antidepressant.The tryptophan content in unroasted banana peel powder was higher than that of roasted banana peel 0.1% and 0.01% (w/w), respectively.The roasting process can reduce the tryptophan content because tryptophan is unstable at high temperatures.

Moisture content and color determination
The moisture content of the roasted banana peel was determined using a moisture content analyzer (Table 4).The moisture content is lowest in the fourth roasting condition (200 o C, 20 mins).Controlling moisture content was essential for optimal process control, maintaining high product quality, and compliance with regulations (Wernecke and Wernecke, 2014).Although roasting can kill microbe's toxins and deactivate enzymes, the process can affect the nutrition status.A roasting process can influence color, aroma, taste and antioxidant properties (Sruthi et al., 2021).
The brownish color resembling ground coffee is achieved with 200 degrees of roasting (Table 4, Figure 2).The visual appearance of the color could be an indicator of the roasting degrees and play as an indicator to stop the roasting process.During roasting, some important reactions are the Maillard reaction which induces aroma, the caramelization that creates the color, and pyrolysis, which creates a signature aroma and taste (Tarigan et al., 2022).Maillard reaction has melanoidins as the end product with reducing properties, absorbing oxygen, and metal chelating capacity.Color can be an index of the antioxidant properties of food (Pokorný and Schmidt, 2003).

Antioxidant activity
The scavenging activity of roasted banana peel powder was higher in high temperatures and more prolonged during the duration of roasting (Table 4).This result was in agreement with a previous study, which reported a lower IC 50 value with a longer drying time for banana peel (Mentari et al., 2019).Some compounds with antioxidant properties can get lost during roasting, but new compounds with antioxidant activity were created during the Maillard reaction.Maillard reaction is also responsible for forming volatile compounds that contribute to the aroma of the food (Diaz- de-Cerio et al., 2019).Some antioxidant molecules are formed during the roasting process, such as phenylalanine and heterocyclic compounds (Poncet et al., 2021).Phenolic compounds were considered one of the quality markers of the roasting process (Diaz- de-Cerio et al., 2019).Although enzymatic or chemical oxidation of polyphenols can reduce the antioxidant capacity, higher antioxidant activity is shown by partially oxidized polyphenols (Pokorný and Schmidt, 2003).A previous study reported that the maximum antioxidant activity was in mediumroasted coffee and dark-roasted coffee has a lower antioxidant value (Nicoli et al., 1997;del Castillo et al., 2002).Other studies reported the highest antioxidant capacity in lightly roasted coffee (Duarte et al., 2005).

Optimization of the roasting condition using factorial design
Roasting temperature and time will significantly affect several parameters of the roasted powder, such as the resulting color and antioxidant content.The roasting conditions must be optimal per the standard criteria to produce the roasted powder.Therefore, optimizing the roasting process will provide optimal conditions to make the desired roasted powder.Optimization is carried out using a factorial design with two factors and two levels.The temperature and time of the roasting optimization condition were analyzed using the factorial design method using the software Design Expert ver.10.0.1.Polynomial equations (Table 5) were built based on the moisture content, color similarity and IC 50 value.Y is the response, and X is the value of the level of concentration.Roasting time and temperature influenced color similarity, moisture content and IC 50 value.A longer roasting time resulted in lower moisture content.
Based on the polynomial equation, it can be seen that the interaction between temperature and roasting time dominantly affects the moisture content and color similarity, while the roasting time dominantly affects IC 50 .The color change of the roasted powder depends on the length of the roasting time.The longer the roasting time, the darker color will appear, in this case, giving a darker brown color.The contour plot of moisture    3).
In Figure 4, the yellow area on the superimposed contour plot depicts the optimum area.The optimum condition chosen was 200 o C for 20 mins of roasting temperature.The optimum condition will result in moisture content of 2.00%, color similarity of 11.80 and IC 50 of 1979.2 ppm.The previous hedonic test study showed that panelists prefer a darker color, a more pungent aroma and a bitter coffee analog from the banana peel (Hasbullah et al., 2021).Hence, it is probable that panelists will also prefer this optimum condition of the Agung banana peel coffee analog.

Conclusion
Roasted banana peel can be considered a food raw material that has antioxidant and antidepressant activity.The optimum conditions selected based on the factorial design were 200°C; 20 mins.The presence of tryptophan in banana peel powder can be used as an antidepressant compound.It is recommended that further study should be carried out on a hedonic test for the taste and aroma of Agung banana peel coffee analog.
L*, a*, b* values obtained represent three dimentions:1.L* represents the light-dark spectrum with a range from 0 to 100 d(black to white), 2. a* value represents the green-re spectrum with a range from -60 to +60 (green to red), 3. b* value represents the blue-yellow spectrum with a range from -60 to +60 (blue to yellow).The L*, a*, and b* values were then converted into ∆E * ab values using equation 1. L*a*b* Color Difference Compare with Local Coffee Product

Figure 2 .
Figure 2. Color of banana peel after roasting.

Figure 3 .
Figure 3. Contour plot of moisture content (A), color similarity (B), and IC 50 (C) of roasting conditions (roasting time and roasting temperature).

Figure 4 .
Figure 4.The superimposed contour plot of roasting condition.

Table 1 .
Factorial design for the roasting condition.

Table 2 .
Characteristics of non-roasted and roasted banana peel powder.

Table 3 .
Microbial contamination on non-roasted and roasted banana peel powder.

Table 4 .
Color space, moisture content after roasted, and antioxidant activity after roasting.

Table 5 .
Polynomial equation X a : level value for temperature, X b : level value for time content, color similarity and IC 50 have been established based on polynomial equations (Figure