Black plum peel as a useful by-product for the production of new foods: chemical, textural, and sensory characteristics of Halva Masghati

ABSTRACT

Plum peel is the by-product of plum processing and has all the functional properties of the plum fruit. The aim of this study was to investigate chemical composition of black plum peel and to use it as a functional ingredient to produce Masghati. The components of black plum peel in 100g were 3.40 g protein, 0.66 g fat, 13.73 g total sugars, and 2.50 g crude fiber. Plum peel is a good source of Ca, P, Zn, and ascorbic acid (201.29, 88.60, 0.97, and 25 mg/100 g raw materials, respectively). The amount of antioxidant and phenolic compounds in black plum peel was significant (88.59% and 105.91 mg/g GA, respectively). Heavy metals contained Hg, Cd, and Pb and chlorine and organophosphorus pesticides were very low and unrecognizable in black plum peel. Gelatin (1.5, 1.8, and 2.3%) and black plum peel puree (14, 27, and 41%) used for Masghati formulations and chemical properties, sensory evaluations, and texture profile analysis (TPA) of Halva Masghati samples were investigated. The results of texture evaluation showed that hardness and chewiness increased by increasing gelatin and puree, while adhesiveness decreased (P < .05). Cohesiveness increased by addition of the gelatin (P < .05). Sensory evaluations showed that total acceptance increased by black plum peel puree, while decreased by gelatin concentration (P < .05). Masghati including 1.8% gelatin and 41% puree had the lowest amount of hardness, adhesiveness, chewiness, and suitable color characteristics, was determined as the best formulation among the other investigated samples.

KEYWORDS:

• Black plum peel
• Masghati
• Texture
• PLS methodology

Introduction

Masghati (Halva Masghati) is one of the traditional products of high consumption confectionery in Iranian families, which has the possibility of industrial production in large quantities. This product is made from starch, water, sugar, citric acid, and natural flavoring substances such as saffron, rose, and cardamom, as well as almond slices and coconut powder (for decoration). Masghati is similar to Turkish delight and has soft and sticky texture. The special point of the production of these products is the use of colorants, flavorings, and natural decorations native to Iran. Different types of Masghati are produced in Iran, including Shirazi Masghati, Masghati pointed to the flavors of saffron, cherry, orange, pomegranate, lemon, rose water, cocoa, milk, and grape juice.^[1]

Plums are the stone fruits of trees in the genus Prunus, subgenus Prunus, and include a number of related species. Due to the different varieties of plums, this fruit has different flavors and textures. This fruit has low calories and high nutritional contain carbohydrates like sucrose, glucose, fructose, and sorbitol, organic acids including citric acid and malic acid, soluble fiber (pectin), tannins, volatile substances, and enzymes. These compounds play an important role in creating the flavor and nutritional properties of plums .^[2] In terms of nutritional properties, plums contain a variety of minerals including Fe, Ca, P, Mn, S, Mg, K as well as vitamins A, B, C, PP.^[3] Also due to high amounts of dietary fiber, sorbitol, anthocyanins, carotenoids, phenolics, help in the regulation of both digestion and sugar metabolism and may be involved in lowering plasma cholesterol concentration and risk of bowel cancer.^[4]

According to the FAO, in 2017, the amount of plums produced in the world was more than 18.5 million tons. China, Romania and the United States are the largest producers of plumes in the world. Iran is the fifth producer in the world with a production of 313,103 tons of plums.^[5] About 20% of plum being marketed fresh and 80% are used to dry. Generally, out of every five kilograms of plums, one kilogram of plum peel is produced. Therefore, about 60,000 tons of plum peel is produced every year in Iran that accumulate in the gardens and villages.^[6] Moisture content, bulk of this material and its spoilage can pollute the environment and impose environmental cleanup costs. It contains valuable compounds (similar to the black plum) and it can be used to improve texture, flavor and color in food industry. Although plum is one of the most important agricultural products in Iran and is widely used by people, but so far this fruit has not been used for Masghati production. While plum and plum peel have an attractive color and flavor and can be used to produce Masghati. Black plum peel Masghati is a novel product. Innovative aspects of this product include using plum by-product and reducing environmental pollution, low cost, high profitability, high nutritional properties, and production of no synthetic additives and production of a special food as Iranian souvenirs.

Literature review shows that there have been some researches into the quality of Turkish delight.^[7–19] However, to our knowledge, there is just one study available concerning the physicochemical and sensory properties of Masghati.^[20] Black plum peel is by-product of plum processing and no research has been done about the properties of this nutritional material. So, in this research, some characteristics of black plum peel include chemical properties (brix, pH, moisture, protein, fat, total sugar, ascorbic acid, ash and crude fiber), major, minor and heavy metals content (Ca, P, Cu, Zn, As, Al, Ni, Hg, Cd, and Pb), microbial properties (presence of mold), pesticides (chlorine and organophosphorus), antioxidant capacity, and phenolic compounds of black plum peel were determined. In the next step, black plum peel was used to produce Masghati. Then, chemical properties (i.e. brix, pH, and moisture content), sensory attributes (color, flavor, hardness, chewiness, adhesiveness, and total acceptance), and texture properties (hardness, adhesiveness, chewiness, gumminess, resilience, springiness and cohesiveness) of black plum peel Masghati was measured. In the end, PLS methodology was used to find relations between textural and sensory attributes of black plum peel Masghati. In Iran, gelatin and sometimes pectin and their different percentages are used to produce Halva Masghati, which in some cases its texture is soft and pasty or hard and brittle. Also, in some cases, artificial flavoring and coloring materials are used for this purpose. The aim of this research was to find the appropriate amount of gelatin concentration to increase the quality of the product. In addition, black plum peel puree used to create a new flavor and color in this product. The results of this research improves the texture, flavor, and color of product in the factories.

Materials and methods

Raw materials for Halava Masghati

The materials included starch, water, rose water (Attar, Iran), gelatin (bloom gel 240–270), sugar, citric acid (Jovin company, Sabzevar, Iran), sunflower oil, vanilla, and plum peel, which was prepared from plum processing factories, Neyshabur, Iran.

Sample preparation

After rinsing with cold water and removing impurities and excess materials, plum peel was crushed in an industrial crusher and stored in the freezer (−18°C). To make black plum peel puree, the chopped peel mixed in a ratio equal to water and passed through the filter to obtain a smooth and uniform composition. The obtained solution named black plum puree.^[21]

To produce Masghati, starch washed to remove impurities. In the next step, water and sugar added. The mixture was stirred until become homogenous. After that, black plum peel puree added and cooked. Then, citric acid added. Simultaneously with increasing consistency, rose water and vanilla added. In the next step, gelatin gradually added to get the glassy and gel-like texture. In the end, oil was added. During the production process, brix regularly measured to reach about 70%. The time required to reach the desired brix, depending on the type of formulations, was 40 to 60 minutes. In the end, they were filled in the mold and after cooling in the refrigerator, they were cut into pieces and packed. The formulation of Masghati samples was determined based on the initial tests. In this research, the effect of black plum peel puree (14, 27, and 41%) and gelatin (1.5, 1.8, and 2.3%) concentrations on chemical composition, texture characterization, and sensory properties of Halva Masghati was studied. Other components of Halva Masghati formulation included water (0, 14 and 27% according to the black plum peel puree), sugar 26%, rose water 15%, starch 16%, oil 2%, and citric acid 0.25%. Figure 1 shows a picture of black plum, plum peel, and the process of Halva Masghati production.

Figure 1. Black plum and plum peel, masghti production process and an example of TPA curve of black plum peel Masghati

Chemical analysis of black plum peel and Halva Masghati

Chemical compositions were analyzed as soluble solid (Refractometer, DANA, Iran), pH (pH meter, Aqua lytic, Taiwan), moisture content (by an oven method, Fan-Azma-Gostar, Iran at 105°C), protein (Kjeldhal method), lipid (Soxhlet method), total sugars (Lane & Inon method), and ash content (by an oven at 550°C, Fan-Azma-Gostar, Iran) .^[22] To measure ascorbic acid, black plum peel was mixed with 4% Oxalic acid solution. Then, it was titrated with 2,6-Dichlorophenol-indopheno 0.01% until a stable pink color. Crude fiber was measured by Weende method. At first, separating of lipids with alkali, proteins with acid and soluble carbohydrates using water were done. After that, the sample turned to ashes, dried and then, fiber content was determined.^[23] For measuring heavy metals Varian AA240, atomic absorption was used.^[22,24] Pesticides by HPLC, Agilent Technologies, Part No:5982–5550, USA were determined.^[25] To measure antioxidant activity, 1 g of sample was mixed with 10 ml of 80% methanol/water solvent mixture for 2 hours at room temperature and was centrifuged. Antioxidant activity was determined using DPPH method. Total phenolic compounds were determined using the Folin–Ciocalteu method reported by Spanos and Wrolstad.^[26] The absorption of sample for antioxidant activity and total phenolic compounds at wavelengths 517 nm and 765 nm were read by UV/Vis spectrophotometer (Jenway 7315 model, U.K).^[27] Measurements performed at 20 ± 2°C and two replications.

Instrumental texture measurement of Halva Masghati

Texture properties of the Masghati samples were obtained TPA test using a TA.XT Plus Texture Analyzer (Stable Micro Systems, England, UK). The cubes of Masghati (10 mm×30 mm×10 mm) that were filled during the production, were removed from the molds. The test conditions were uniaxial two-bite compression; 75-mm diameter cylindrical flat probe; compression 50%; pretest speed 1 mm/s, test speed 5 mm/s, posttest speed 5 mm/s, and trigger force 5 g.^[28,29] The textural parameters of Masghati expressed as hardness (N) (the peak force during the first compression cycle), cohesiveness (the force of internal bond holding a food structure or the ratio of the positive force area during the second compression to that during the first compression), adhesiveness (N.s) (the negative force area for the first bite representing the work required to pull the plunger away from the food), resilience (degree to which the sample returns to the original shape (elasticity)), springiness (the height to which the food recovers during the time that elapses between the end of the first bite and start of the second bite) and chewiness (N.mm) (the product of hardness×cohesiveness×springiness index).^[30] TPA test was performed in four replications. Figure 1 shows a typical TPA curve of black plum peel Masghati.

Sensory evaluation of Halva Masghati

Black plum peel Masghati examined organoleptically by 14 panelists (7 men and 7 women, 20–40 years old) from students and staff of Neyshabur University of Medical Sciences, Iran). Sensory attributes were evaluated by panelists according to Bourne (2002).^[30] Initially, the sensory characteristics of plum peel Masghati was explained to the panelists. As regards, the panelists were familiar with Masghati and plum fruit, so, they knew the sensory properties of the black plum peel Masghati, very well. Evaluations were performed at room temperature (22 ± 2°C) and under white fluorescent light. The nine-point hedonic scale^[31] (1 = Dislike extremely, 9 = like extremely) was used to assess the sensory attributes of black plum peel’s Masghati. Throughout panel sessions, panelists were instructed to rinse their mouths with water before testing each sample. Therefore, seven black plum peel Masghati factors including color, flavor, hardness, chewiness, adhesiveness, and total acceptance were evaluated.

Statistical analysis

A completely randomized factorial design used to evaluate the results where analysis of variance (ANOVA) carried out to compare the mean values. All significant differences were reported at P ≤ 0.05 level. Tukey test employed to determine difference among the formulations. Partial Least Square (PLS) was applied to explore relationships between sensory attributes and textural properties of black plum peel Masghati. Minitab statistical software (Version 16, USA, 2010) used for all statistical analyses in the present research. GraphPad Prism (Version 8.0.1, USA) also utilized to plot the curves.

Results and discussion

Chemical composition of black plum peel

Table 1 shows the chemical composition and antioxidant activity of black plum peel. The soluble solids and pH of black plum peel were 24% ° Brix and 3.79, respectively. The moisture content of black plum peel was 78.1% that is consist with the values reported with Stacewicz-Sapuntzakis et al.^[32] Protein, lipid and total sugars of black plum peel were 3.4, 0.66 and 13.73% respectively. The results of protein and lipid are lower than the values obtained with Milala et al.^[33] for plum pomaces and higher than Sapuntzakis et al.^[32] for fresh plum. Ash content and crude fiber of black plum peel were 2.7 and 2.5%, respectively. Milala et al.^[33] reported 1.7–2.1% for the ash content of plum pomace that is lower than the result of this study. Crude fiber of black plum peel is higher than the results of Marlett JA, Vollendorf^[34] for plum fruit. Perhaps, plum peel contains more total solids, so, the ash and fiber content in the plum peel are higher than fresh fruit and pomace. Ascorbic acid content of black plum peel was 25 mg/100g that is higher than the values reported with Stacewicz-Sapuntzakis .^[4] Total phenolic compounds and antioxidant activity of black plum peel were 105.91 (mg/g GA) and 88.59%. Stacewicz-Sapuntzakis et al.^[32] reported 184 mg/100g total phenolic compounds for fresh plum.

Table 1. Chemical composition and antioxidant activity of black plum peel

Parameter	Amount
°Brix (%)	24
pH	3.79
Moisture content (%)	78.1
Ash (%)	2.7
Protein (%)	3.4
Fat (%)	0.66
Total sugars (%)	13.73
Crude fiber (%)	2.5
Ascorbic acid (mg/100 g)	25
Antioxidant activity (%)	88.59
Total phenolic compounds (mg/g GA)	105.91

The concentration of major and minor elements and heavy metals in black plum peel were determined on the fresh base and summarized in Table 2. The amount of Ca and P were 201.29 and 88.60 mg/100g wet weight, respectively. Grembecka and Szefer^[35] determined Ca and P for plum fruit 12.1 and 7.75 mg/100g wet weight. These values are much lower than the values obtained for black plum peel in this study. Maybe a large amount of calcium and phosphorus of plum are in the peel.

Table 2. Heavy metals, major and minor content, pesticides, and mold content of black plum peel

Parameter	Amount
Pesticides
Organophosphorus	ND
Chlorine	ND
Mold (CFU/g)	<10
Ca*	201.29
P	88.60
Cu	1.54
Zn	0.97
As	0.12
Al	0.97
Ni	0.28
Hg	ND**
Cd	ND
Pb	ND

^{* mg/100g for all metals}

^{**ND: Not Detection}

The concentrations of Cu and Zn of black plum peel were 1.54 and 0.97 mg/100 g wet weight. The values obtained in this study are higher than the values obtained with Grembecka and Szefer^[35] for plum fruit. Hamurcu et al.^[36] reported the Cu and Zn of plum fruit 0.27 and 1.68 mg/kg dry weight. In the present work, As, Al, and Ni content of black plum peel were 0.12, 0.97, and 0.28 mg/100g, respectively. Grembecka and Szefer^[35] and Hamurcu et al.^[36] determined Ni concentration in plum fruit 0.01 and 0.008 mg/100g fresh weight, respectively.

The results of this study showed that the amount of Hg, Cd, and Pb in black plum peel were very low and indistinguishable. Hamurcu et al.^[36] obtained Cu, Cd, and Pb in plum fruit 0.27, 0.14, and 2.82 mg/100g, respectively. To determine the quality and safety of black plum peel and to ensure its use in food production, the amount of chlorine and organophosphorus pesticides and mold in it were determined. Based on the results, the amounts of chlorine and organophosphorus pesticides were very low and indistinguishable. The mold content of black plum peel was less than 10 CFU/g.

Chemical properties of black plum peel Masghati

The results of ANOVA for chemical properties of black plum peel Masghati are presented in Table 3 and Figure 2. Increasing the concentration of black plum peel puree caused to decrease pH that can be due to the acidity property of the black plum peel. As shown in Figure 2, no significant difference (P > .05) in pH of samples with gelatin concentration. The pH values of Masghati samples varied from 3.64 to 3.82. Other researchers have reported different amounts of pH for Turkish delights, Uslu et al.^[37] 3.90–5.10 for soapwort extract delight, İpek^[38] 4.49 for black grape and sour cherry delight, Dirik^[39] 4.79 for pomegranate Turkish delight and OeZEN et al.^[11] 3.99 for black carrot juice delight. These differences can be due to diversity in the acidity of the fruits.

Table 3. Analysis of variance for different characteristics of black plum peel Halva Masghati

Properties	DF	Seq SS	Adj SS	Adj MS	F	P
Chemical composition
Brix	2	48.11	48.11	24.06	8.33	0.009
	2	164.78	164.78	82.39	28.52	0.000
	4	28.89	28.89	7.22	2.50	0.117
pH	2	0.02	0.02	0.01	5.33	0.030
	2	0.02	0.02	0.01	5.22	0.310
	4	0.01	0.01	0.00	1.35	0.323
Moisture content	2	18.09	18.09	9.05	2.81	0.013
	2	5.47	5.47	2.74	0.85	0.042
	4	27.52	27.52	6.88	2.13	0.159
Texture measurement
Hardness	2	72.22	78.34	39.17	25.13	0.000
	2	13.50	12.01	6.00	3.85	0.034
	4	180.84	180.84	45.21	29.01	0.000
Adhesiveness	2	0.56	0.51	0.26	6.91	0.004
	2	0.07	0.07	0.04	0.98	0.038
	4	0.12	0.12	0.03	0.78	0.548
Cohesiveness	2	0.01	0.01	0.01	0.75	0.484
	2	0.12	0.13	0.06	6.57	0.005
	4	0.05	0.05	0.01	1.33	0.287
Gumminess	2	17.32	18.44	9.22	8.37	0.002
	2	26.40	26.19	14.00	11.89	0.000
	4	27.54	27.54	6.88	6.25	0.001
Resilience	2	0.01	0.01	0.01	8.58	0.001
	2	0.06	0.54	0.03	45.64	0.000
	4	0.03	0.02	0.01	14.29	0.000
Sensory properties
Color	2	19.48	20.17	10.08	3.44	0.036
	2	13.87	14.27	7.14	2.43	0.003
	4	5.75	5.75	1.44	0.49	0.743
Flavor	2	4.32	3.84	1.92	0.84	0.043
	2	4.42	4.13	2.07	0.90	0.041
	4	7.01	7.01	1.75	0.76	0.551
Hardness	2	11.95	12.54	6.27	1.44	0.024
	2	0.48	0.77	0.38	0.09	0.044
	4	13.84	13.84	3.46	0.79	0.553
Chewiness	2	83.59	79.02	39.51	1.56	0.021
	2	21.60	20.95	10.48	0.41	0.662
	4	68.45	68.45	17.11	0.68	0.610
Adhesiveness	2	20.58	20.55	10.28	2.29	0.010
	2	0.84	0.96	0.48	0.11	0.042
	4	3.70	3.70	0.92	0.21	0.934
Total acceptance	2	3.31	3.31	1.66	0.87	0.042
	2	0.30	0.26	0.13	0.07	0.034
	4	3.26	3.26	0.81	0.43	0.787

Figure 2. The effect of black plum peel puree and gelatin concentrations on chemical properties of black plum peel Masghati

The results indicated that significant decrease (P < .05) in moisture content when the gelatin and black plum peel levels are increased that is due to tighter texture and stronger gel network formation (Figure 2). Moisture content of Masghati samples varied from 14.60 to 17.20%. Gelatin and black plum peel puree concentrations had significant impact (P < .05) on °Brix that ranged from 50 to 60. It seems that high °Brix of black plum puree, the presence of fibers especially pectin in black plum peel and its interaction with added gelatin it can be effective in the formation of gel network and texture of product after cooling. Kaya et al.^[18] reported 71.25–81.90 °Brix for pomegranate Turkish delight. The result of their study was different from the results obtained in this study, which is probably due to the difference in composition of black plum peel puree and pomegranate juice and the presence of gelatin in this study.

Texture measurement

Table 3 demonstrates the analysis of variance for texture characterization of black plum peel Halva Masghati.

Hardness

Significant increase (P < .05) in the hardness of samples was observed with increase in gelatin and black plum peel puree concentrations (Figure 3). It seems that crud fiber especially pectin in black plum peel caused to increase in hardness. The high protein content of gelatin causes to improve the structure bonding and the formation of stronger gel network. In addition, the interaction of gelatin and pectin in black plum puree is also effective in this increasing. These results were similar to the findings of Azimi and Mortazavi, ^[29] Shafiur Rahman and Al-Mahrouqi^[40] and Collar et al. .^[41] In this research, the hardness of samples varied from 4.96 to 14.11 N. Uslu et al.^[9] studied the effect of soapwort extract and starch levels on texture properties of Turkish delight. The hardness of samples was very higher than the result of this research. Gelatin gels provide elasticity, while the addition of starch increases the gel strength and the hardness of gelatin-based gummy confections.^[42]

Figure 3. The effect of black plum peel puree and gelatin concentrations on texture properties of black plum peel Masghati

Adhesiveness

Adhesiveness is defined amount of work needed to overcome the forces between the surface of the food product and the surface of the material with which it comes in contact and it’s depends on the combination of adhesive and cohesive forces and other parameters like viscosity and viscoelasticity .^[43,44] The results showed that adhesiveness decreased with increasing the levels of gelatin and puree (P < .05) (Figure 3). It maybe, increasing the levels of gelatin and black plum peel (due to the carbohydrate and fiber content) increased the amount of bonding water and components in the formulation were placed together and the structure became denser and eventually adhesiveness reduced. Increasing the gelatin concentration reduces adhesiveness and stringy of gels.^[45] The adhesiveness of samples varied from 0.11 to 0.62 (N.s).

Cohesiveness

Cohesiveness results from the interaction of structural forces acting at a molecular level.^[46] The results of texture measurement showed that increasing the gelatin concentration increased the cohesiveness (Figure 3), while the increase in the percentage of black plum peel puree did not have a significant effect on cohesiveness. Cohesiveness values varied from 0.52 to 0.77. This finding is similar to results of Uslu et al.^[9] who reported 0.729 for the best sample of soapwort delight.

Resilience

Significant decrease in the resilience of samples was observed with increase in black plum peel puree concentration, while, it was growing up with increasing the gelatin level (P < .05) (Figure 3). The higher the amount of gelatin, the higher the elasticity of the samples. The results of this study showed that resilience of samples was from 0.20 to 0.34.

Springiness

When springiness is high, it requires more mastication energy in the mouth. High springiness was resulted when the gel structure was broken into few large pieces during the first compression TPA whereas low springiness resulted from the gel breaking into many small pieces .^[29] There was no difference (P > .05) in springiness of Masghaties that ranged from 0.997 to 0.999. Marshall and Marion^[47] reported gels of corn starch and gelatin were more cohesive, springy, chewy, and gummy than those of agar, low methoxyl pectin and carrageenan. Uslu et al.^[9] reported 0.390–0.828 for the samples of soapwort delight that is lower than the results of this study. It is probably due to the interaction of gelatin and starch in the black plum peel Masghati.

Gumminess and chewiness

Results of ANOVA showed that gelatin and puree levels had significant effect (P < .05) on the gumminess of samples. Generally, the more the gelatin and puree concentrations, the higher the gumminess (Figure 3). It may be due to reduce in moisture content and increase in hardness of samples with increasing the concentrations of gelatin and puree. Because the springiness of the samples was approximately equal to 1.00; therefore, the amount of gumminess in the samples is equal to their chewiness and all the results of the gumminess are true for chewiness.

Chewiness was found to increase with the increase of solids content.^[48] Gelatin directly and significantly affects the chewiness, texture and assessment.^[49] Chewiness values varied from 3.63 to 8.63 N.mm. This finding is similar to the results of Uslu et al.^[9] for soapwort delight.

Sensory evaluation

The results of analysis of variance for sensory properties of black plum peel Halva Masghati are given in Table 3.

Color

The color of foods, which is one of the initial properties noticed in foods, is one of the most important quality parameters affecting consumers.^[50] Today, there is a great demand for natural food colorants, especially red. These colorants include betanin, cochineal (carmine and carminic acid), anthocyanins, and carotenoids (paprika, canthaxanthin).^[51] Especially, anthocyanins, well-known natural red colorants, are used in foods, because of their role in reducing the risk of different diseases .^[52]

Results of ANOVA showed that gelatin and black plum peel puree concentrations had significant effect (P < .05) on color acceptance of samples. Generally, the more the gelatin concentration, the less the color acceptance Masghaties. It probably due to the stronger gel network formation and reduced product transparency (Figure 4a). Significant increase (P < .05) in color score of samples was observed with increase in black plum peel puree (Figure 4b). The increase in black plum peel puree (due to the red color) had a more prominent color, which has been more acceptable to the panelists. The color score of Masghaties ranged from 5.45 to 7.54.

Figure 4. The effect of gelatin (a) and black plum peel puree (b) concentrations on sensory properties of black plum peel Masghati

Flavor

The aroma of products results from volatile substances in the fresh food such as esters, ketones, terpenes, aldehydes, and others. The loss of these volatiles leads to a decrease in aroma detection.^[53] Increase in black plum peel puree significantly augmented the flavor of samples (Figure 4a), while it decreased with increasing the gelatin concentration (Figure 4b) (P < .05). Gel consistency seems to play a significant role in the flavor release. It seems that the stronger gel network, reduces the movement of aroma and reduces the flavor. Guinard and Marty^[54] reported flavor release decreases as gel strength increases for gelatin. The flavor score of Masghati samples ranged from 5.85 to 7.54. The highest flavor score was for the sample with 41% black plum peel puree and 1.8% gelatin and the lowest score was for the sample by 27% black plum peel puree and 1.5% gelatin.

Hardness

To evaluate the hardness of solid foods, the item is placed between the molar teeth and the panelist bites down evenly, evaluating the force to compress the food.^[30] As shown in Figure 4a-b, increase in puree and gelatin levels significantly augmented the hardness of samples (P < .05). These results are consistent with the results of instrumental test.

Chewiness

Chewiness is the energy required to disintegrate a solid food so that it is ready for swallowing.^[30] Similar to instrumental measurement, panelists’ perception of chewiness increased significantly (P < .05) with increase in black plum peel puree. Effect of gelatin concentration on chewiness of samples was not significantly important (P > .05). The chewiness score of Masghaties ranged from 5.59 to 7.20.

Adhesiveness

Sensory adhesiveness can be perceived in the palate, teeth, and tongue when the food is being masticated.^[44] The results indicated that significant increase (P < .05) in adhesiveness occurred when the gelatin and black plum peel puree concentrations increased (Figure 4a-b). The adhesiveness score of samples varied from 5.42 to 7.00. The highest adhesiveness was for the sample with 41% black plum peel puree and 1.8% gelatin and the lowest score was for the sample by 27% black plum peel puree and 1.5% gelatin. These results are not consistent with the results of Azimi et al.^[29] as increasing the amount of gelatin didn’t show significant effect on the adhesiveness of mulberry pastille.

Total acceptance

The sensory acceptability factors of foods are extremely important because people obtain great enjoyment from eating their food and, furthermore, the enjoyment of food is a sensory pleasure that is appreciated from the cradle to the grave.^[30] Results of ANOVA showed that increasing the concentration of gelatin decreased the acceptability of samples (Figure 4a). This can be due to the formation of a stronger gel network, reduced flavor, and transparency in samples. Renard et al.^[55] stated that, with decreasing hardness gel, the understanding of the flavor increased. As shown in Figure 4b, increase in puree level significantly increased the total acceptance of samples (P < .05). Probably the main reason is the increase in red color and the flavor of black plum. It is reported that the acceptability of fruits and vegetables is influenced by their aroma.^[53] Three main acceptability factors of foods are appearance, flavor, and texture. The texture is less important than flavor in food acceptability.^[30] Therefore, although with increasing the percentage of black plum peel puree, the texture of the samples became harder, but the general acceptance of the samples increased. The results showed that the sample with 41% black plum peel puree and 1.8 % gelatin (score 7.38) had the most acceptability and the sample containing 14% puree and 1.8% gelatin gained the lowest acceptance (score 6.54). Batu et al.^[13] reported that the use of black grape and sour cherry syrups can improve the appearance, color and aroma of Turkish delight.

Correlation between sensory and instrumental variables

Texture is the attribute resulting from a combination of physical properties perceived by the senses of kinesthesis, touch (including mouth, feel, sight and hearing). The properties may include size, shape, number, nature and conformation of constituent structural elements.^[30] Texture certainly influencec the consumer and identifying the consumer prefernece is very important, especially for quality control and the design of new products.^[46] Obtaining a high correlation between textural and sensory properties has been the goal of scientists include food scientists, physicists, psychologists and dentists .^[30] Figure 5 shows the results from PLS2 regression analysis, which describe the relationship between texture and sensory properties. It can be seen,sensory properties contain chewiness-adhesiveness and instrumental properties contain gumminess-chewiness-cohesiveness are highly correlated, because angles between them is small. Instrumental hardness, sensory chewiness springiness and flavor are not very important in comparison with other parameters, because of shorter lines. On the second component, color has greater loadings than the rest. The first and second PLS represented 96% and 68% of the variations fore puree and gelatin, respectively. Sensory properties contain color, total acceptance, flavor, adhesiveness, and chewiness were positively correlated together and negatively correlated with sensory hardness and instrumental parameters. That is to say, the lower the instrumental values and sensory hardness, the higher acceptance scores and total acceptability. In other words, the panelists preferred samples with low hardness,gumminess, chewiness, springiness and cohesiveness. Adhesiveness didn't have any correlation with other parameters, because it was on the reference line. Periche et al.^[46] studied the correlation between sensory and texture properties of based-isomaltulose gummy confections. According to their results, sensory gumminess and hardness were negatively correlated with instrumental gumminess and hardness and positively correlated with instrumental springiness and cohesiveness.

Figure 5. Correlation between sensory and texture properties of black plum peel Masghati

Conclusion

Black plum peel is the by-product of black plum drying, which has nutritional properties such as vitamins, minerals, antioxidant capacity and phenolic compounds and it can be used to produce novel foods. Masghati has been produced in Iran since hundreds of years. Fruit flavored Masghati is produced by adding synthetic colorants and flavors. The results of this study showed that it is possible to produce fruit Masghati using a real fruit puree or concentrates instead of colorants and fruit flavors. Sensory evaluations showed that the sample containing 1.8% gelatin and 41% black plum peel puree had the highest acceptance. Increasing black plum puree increased the total acceptance of the samples, while it was reduced with gelatin concentration. Texture measurement showed that increasing puree and gelatin levels increased hardness and chewiness and reduced adhesiveness. Production of black plum peel Masghati in addition to produce a novel sweets, makes optimum use of plum processing by-products, which can play an important role in reducing environmental pollution.

Acknowledgments

We thank the Neyshabur University of Medical Sciences for funding the project (Project number 97263, code of ethics IR.NUMS.REC.1397.1). The authors declare that they do not have any conflict of interest. This study does not involve any human or animal testing.