Effects of Infrared Drying Conditions and Maltodextrin Addition on Some Physicochemical Characteristics of Avocado (Persea americana) Pulp Powder

Featured Application

Avocado powder was first produced from avocado pulp using infrared drying. Drying with 9% maltodextrin at 70 °C retained 95% antioxidant contents and activity.

Abstract

In this study, avocado pulp with a good nutritional profile and economic value was dehydrated using infrared drying to produce pulp powder, which shows potential application in nutritional supplements. An experimental design with two factors, namely maltodextrin level (0% and 9%) and infrared temperature (ranging from 65 to 80 °C), was used. Responses related to the physicochemical properties of the resulted powder were observed, including peroxide value, total polyphenols, total chlorophylls, antioxidant activity, and color parameters (L*, a*, and b* values). The quality of dried products may be harmed by drying either at a high temperature or for an extended period of time. The coating substance maltodextrin was found to be beneficial in limiting unexpected changes in avocado pulp subjected to infrared drying. The highest quality of dried avocado could be obtained via infrared drying of avocado pulp with 9% maltodextrin at 70 °C, as illustrated by the exceptional retention of total polyphenols, total chlorophylls, and antioxidant activity, being 95.1, 95.2, and 94.4%, respectively. Moreover, the short drying time (35–55 min) led to the minimization of lipid oxidation and the absence of peroxide compounds in all samples.

1. Introduction

Avocado (Persea americana) fruit originating from Central Mexico have been dubbed “God’s greatest gift to humanity” or “superfoods” with a high commercial value due to their high nutritious content, distinct flavor, and aroma [1]. Nutritional value of avocados has been proven through their chemical composition, including proteins, carbohydrates, lipids, vitamins (B2, B3, B5, B6, B7, B9, C, E, A, etc.), minerals (calcium, copper, iron, magnesium, manganese, and phosphorus), and natural pigments (chlorophylls and carotenoids) [1,2,3,4,5]. Furthermore, avocados contain bioactive compounds related to health benefits, such as sterols, polyhydroxylated fatty alcohols, alkaloids, acetogenins, and essential oils [6]. Lye et al. (2020) characterized and summarized various therapeutic properties of avocados, including antioxidant, anticancer, antidiabetic, antiatherogenic, and antibacterial effects, as well as anti-inflammatory effects [1]. However, because the avocado is a climacteric fruit with high moisture content [2], it is difficult to store, resulting in a short shelf life and increased postharvest loss. According to the literature review of Bill et al. (2014), when ripe and unripe avocados were preserved at high relative humidity (85–90%) with corresponding storage temperatures of 2–7 °C and 4–13 °C, avocados had a shelf life of 1–4 weeks for ripe avocados and 2–8 weeks for unripe [7]. As a result, it was difficult to ensure both economic value and trade stability by using avocados as fresh products. The application of food processing and preservation on avocados could extend shelf life, retain nutritional value, and increase economic and trade value. In addition, a variety of new food products could be developed and produced.

Among various unit operations in food processing, drying technology is the best method to remove moisture from material to preserve and produce products [8]. Eliminating moisture could prevent or limit unexpected changes under the activity of enzymes and microorganisms, resulting in product quality degradation [9]. In addition, the volume of products was reduced, which helped to minimize packaging, delivery, and storage costs [9]. In the literature, some drying methods were applied to dehydrate avocado, such as freeze-drying [10,11,12], hot air drying [13], heat pump drying [14], and superheated steam drying [11]. Freeze-drying technology was proven to be the best for retaining the highest quality of the product [15]. Nevertheless, there are some critical drawbacks, such as long operating time (more than 20 h), high implementation cost, and excessive energy consumption; in consequence, freeze-drying technology has been limited in its industrial use [16]. The other drying methods required less time than freeze-drying, with hot air drying taking approximately 5 to 8 h at temperatures ranging from 50 to 80 °C [13], heat pump drying taking about 5.8 h at 70 °C [14], and superheated steam drying taking 3 h at temperatures more than 130 °C [11]. However, drying avocados for an excessive amount of time might degrade the quality of dried goods. Furthermore, it was reported that when avocados are thermally processed, the higher temperature or longer treatment time lead to the production of bitter-tasting molecules [17]. However, Degenhardt and Hofmann (2010) found that thermal processing of avocados within 30 min (ranging from 80 to 120 °C) or 60 min (at 80 °C) only slightly increased bitter taste in comparison with untreated avocados [17]. Thus, the higher quality of dried avocados will be obtained if the temperature and duration of drying are well-regulated.

Infrared drying, a novel drying technology, was recently proved to be effective in drying fruits and vegetables [18]. The most significant benefits of this approach include reduced drying time, reduced energy consumption, consistent temperature distribution, improved dried product quality, ease of application, high heat transfer coefficients, environmental friendliness, and space savings [19,20]. Heat-trapped water is created by the ability of infrared radiation to penetrate deep inside moist materials [21], resulting in more efficient moisture removal than traditional heating [22,23]. In comparison to conventional drying processes, the infrared drying system’s shorter drying time and consistent temperature distribution contribute to higher dried product quality [24]. Specifically, in the study of Olsson et al. (2005) on heat transfer on crust formation of bread, the jet impingement and infrared heating improved the color development of products compared to conventional heating [25]. Another study on efficiency of different drying of bitter gourd slices showed that although freeze-drying provided better results in terms of color, appearance, rehydration ratio, and microstructure than infrared drying of bitter gourd slices, infrared drying gave the best results in the bioactivities and physicochemical characteristics of dried bitter gourd slices in comparison with other drying methods such as hot air drying or freeze-drying [26]. Infrared drying, on the other hand, does not appear to be suitable for drying essential oil-rich materials. Zhang et al. (2018) reported that infrared drying caused more loss in yield as well as antioxidant and antibacterial activity of essential oil than hot air drying and natural drying [27]. According to a comprehensive review of Salehi (2020), many reports of infrared drying of fruits and vegetables were published, but there is still no publication on avocados [28]. The advantages of infrared drying make it ideal for producing high-quality dried avocados.

Therefore, the aims of this study were to investigate the impact of infrared drying conditions on the quality of avocado pulp with different formulations. The experimental data were used to analyze and evaluate the efficiency of the infrared drying method based on nutritional value in the drying of superfoods such as avocado flesh. To dry food with high nutritional content, a new drying technique with suitable drying parameters was applied.

2. Materials and Methods

2.1. Chemicals

Gallic acid, 2,2-diphenyl-1-picrylhydrazyl (DPPH), 6-hydroxy-2,5,7,8 tetramethylchroman-2-carboxylic acid (Trolox), and maltodextrin (DE of 16.5–19.5) were purchased from Sigma-Aldrich (Ascent, 118222 Singapore). The Folin–Ciocalteu reagent (2 N) was prepared basically from solid sodium tungstate, sodium molybdate, and lithium sulfate.

Sodium bicarbonate, acetone, glacial acetic acid, sodium carbonate, methanol, chloroform, potassium iodide, sodium thiosulfate pentahydrate, starch, and other chemicals were of analytical grade.

2.2. Pretreatment of Avocado Fruits

Fresh avocados (Persea americana) were provided by local farms in Dak Lak Province, Vietnam. In this study, the ripe avocado fruits were chosen to produce avocado pulp. The ripe, ready-to-eat avocados had a green surface, slightly soft, and the seeds removed from the flesh easily. The materials were then cleaned, peeled, and deseeded before being cut to slices of 3.00 ± 0.03 mm thickness. Avocado slices were immersed in a 0.05% sodium bicarbonate solution for 30 min to stabilize natural colors in the flesh and then steam blanched for 3 min to inactivate the enzyme accountable for browning reactions. Following steam blanching, the pieces were quickly chilled in cold water at a temperature of 5.0 ± 0.5 °C, and avocado pulp was generated using the BJY-CB2L60-A commercial blender (Berjaya Steel Product Sdn Bhd, Kuala Lumpur, Malaysia). Before infrared drying, the maltodextrin as carrier agent was added to the pulp and stirred to reach a homogeneous mixture at investigated solid concentrations. This mix was then spread into 2 mm thick layers on Mylar films (Jiangsu, China) and put into the drying chamber.

2.3. Infrared Drying Equipment

In this study, the customized infrared drying apparatus at laboratory scale was used to dry avocado pulp. The module of infrared radiation, composed of two lamps (ITT 1000 W 235V-01X0) with maximum power of 1000 W, was put on top of the drying chamber (L × W × H = 64 × 23 × 27 cm). In this apparatus, the surface of avocado pulp was exposed to infrared radiation for heat generation and mass transfer during drying. Moreover, a couple of fans were mounted to the equipment to increase driving force for moisture removal. The control of infrared temperature (IR) relied on the thermal sensor with an absolute error of ± 0.5 °C and temperature controller with a time delay of 0.5 s.

2.4. Infrared Drying of Avocado Pulp and Experimental Design

In this study, two factors, including maltodextrin addition and infrared drying temperature, were investigated. The maltodextrin was set at two levels: avocado pulp with 9% (w/w) maltodextrin (APM) and avocado pulp without maltodextrin (AP). The temperature was set at four different settings, ranging from 65 to 80 °C, with an interval of 5 °C. Infrared light was used to dry each sample (100 g) until the moisture content reached an estimate of 0.04 g/g d.b. It took 55, 45, 40 and 35 min to dry avocado pulp when the temperature changed from 65 to 80 °C. Then, the dried avocado sheets were collected and determined for color parameters, including L*, a*, and b* values. Furthermore, the above sheets were ground into powder for measuring total polyphenols (TPC), total chlorophylls (TCC), antioxidant activity (AA), and peroxide value (PV).

For each run, the fresh pulp also was determined quality before drying. The result was used to calculate the retention percentage of each quality criterion as follows (the quality criterion (such as TPC, TCC, or AA) in powdered pulp/this criterion in pulp before drying) × 100.

2.5. Analytical Methods

2.5.1. Preparation of Extracts

The preparation of extracts was conducted according to the procedure described in the literature with some modifications [29]. The dried avocado pulp powder (0.2 g) was mixed with 10 mL of solvent (7 mL acetone + 0.03 mL glacial acetic acid + 2.97 mL water) and then vortexed for 30 s at 2000 rpm using the VELP ZX4 Advanced IR Vortex Mixer (VELP Scientifica, Milan, Italy) to start the extraction step. After sonication (40 KHz, 240 W, 5 min) in a PRO 100 ultrasonic cleaner (ASonic, Ljubljana, Slovenia), the sample was again vortexed for 10 s and cooled for 20 min at 10 °C. The cooled sample was then sonicated for a second time at the previous condition and centrifuged in the PLC-05 centrifuge (Gemmy Industrial Corp., Taipei, Taiwan) at 1220× g/10 min. The supernatant was collected and diluted to 25 mL for the analysis of total polyphenol content (TPC) and antioxidant activity by DPPH radical scavenging assay.

2.5.2. Determination of Total Polyphenol Content (TPC)

The total polyphenol content of the extract was determined according to ISO 14502-1:2005 [30]. Briefly, 0.6 mL of extract was mixed with 1.5 mL of Folin–Ciocalteu reagent (10-fold dilution) and allowed to stand for 5 min at room temperature. Then, 1.2 mL of 7.5% sodium carbonate solution was added, and the mixture was incubated for 1 h in the dark condition. Finally, the sample was measured for absorbance at 765 nm using the UV-9000 spectrophotometer (Metash, Shanghai, China). TPC was determined through the gallic acid standard curve and expressed as mg gallic acid equivalent/g sample on the dried basis (mg GAE/g d.b.), and the retention (%) of TPC was also calculated by the percentage of TPC in avocado powder over TPC in the avocado pulp.

2.5.3. Determination of Antioxidant Activity (AA)

The DPPH radical scavenging activity of the extract was determined according to the method described in the literature with minor changes [31]. To prepare the DPPH stock solution, 24 mg of solid DPPH was dissolved in 100 mL of methanol and stored at 4 °C in the dark condition for 24 h. The DPPH working solution was prepared by diluting the DPPH stock solution to the absorbance of 1.1 units at 515 nm using methanol. Then, 150 μL of the sample was reacted with 2850 μL of the DPPH working solution for 30 min under dark conditions. Finally, the sample was measured for absorbance at 515 nm over a methanol blank. DPPH radical scavenging activity was determined using the %DPPH radical inhibition–standard concentration curve (%DPPH radical inhibition = (1 − Absorbance of sample/Absorbance of blank) × 100) and expressed as mg Trolox equivalent/g sample on the dried basis (mg TE/g d.b.), and the retention (%) of AA was also calculated by the percentage of AA in avocado powder over AA in the avocado pulp.

2.5.4. Pigment Extraction and Determination of Total Chlorophyll Content (TCC)

The extraction of pigment and determination of total chlorophyll content were conducted according to the method described in the literature with some changes [32]. The sample (0.2 g) was mixed with 5 mL of solvent (chloroform:methanol 2:1, v/v) and then vortexed for 30 s at 2000 rpm to start the extraction step. After sonication (40 KHz, 240 W, 5 min), the sample was again vortexed for 10 s and cooled for 20 min at 4 °C. The cooled sample was then centrifuged at 2095× g/10 min, and the supernatant was collected. The solvent in the supernatant was evaporated in the petri dishes using the LO-FS100 Forced Convection Oven (LK Lab, Namyangju, Korea) at 50 °C for 30 min and finally redissolved to 10 mL by pure diethyl ether for analysis of total chlorophyll content (mg/g d.b.) by measuring absorbance at 660 and 642 nm, and the retention (%) of TCC was also calculated by the percentage of TCC in avocado powder over TCC in the avocado pulp.

2.5.5. Determination of Peroxide Value (PV)

For peroxide value determination, the sample (5 g) was weighed in a 250 mL flask. Acetic acid–chloroform solution (30 mL), saturated potassium iodide (0.5 mL), and distilled water (30 mL) were then added with occasional agitation. The solution was titrated with 0.1 N of sodium thiosulfate with an additional aliquot of 1% starch solution (0.5 mL) as an indicator, and titration continued with vigorous agitation until the dark color disappeared. Peroxide value was calculated using the following formula: PV (meq/kg sample) = V × N × 1000/m (g), where V is the corrected volume of sodium thiosulfate solution used (mL), N is the normality of the sodium thiosulfate solution (mEq/mL), and m is the sample mass (g).

2.5.6. Determination of Color Attributes

The values of L*, a*, and b* were measured by the NR110 Precision Colorimeter (Shenzhen 3nh Technology Co., Shenzhen, China) using the Hunter CIELAB color system. L* indicates lightness, with a scale ranging from 0 (black) to 100 (white). Positives and negatives in a* represent red and green, whereas positives and negatives in b* represent yellow and blue, respectively.

2.6. Statistical Analysis

Using basic statistical techniques, experimental data were analyzed using SPSS 15 software (SPSS Inc., Chicago, IL, USA). One-way ANOVA was used to determine the differences between samples, and Tukey’s multiple range test was applied to determine significant differences between mean values at a significance level of 5%. All experiments were conducted in triplicate.

3. Results

3.1. The Quality of Avocado Flesh

It was necessary to standardize the materials and examine their quality before conducting experiments.

Table 1. The physicochemical properties of avocado pulp powder obtained by infrared drying.

Characteristics	Unit	Value
Moisture	g/100 g	82.4 ± 0.5
TPC	mg GAE/g d.b	5.33 ± 0.06
TCC	μg/g d.b	260.38 ± 5.52
AA	mg TE/g d.b	1.60 ± 0.05
PV	meq/kg	Not detected
Color parameters
L* value		60.87 ± 0.59
a* value		−23.11 ± 0.96
b* value		55.82 ± 1.13

shows the results of the chemical composition of fresh avocado fruit. In this study, the moisture content of avocado pulp (82.4 ± 0.5%) was significantly different from previous studies. Specifically, Lye et al. (2020) reported the moisture content in the edible part was 73.23%, based on 14% Florida and 86% California ecotypes [1], while Kassim et al. (2013) observed a moisture content of 73 to 80% in the cultivar “Pinkerton” exported from South Africa [33]. It has been noted that materials in this study had higher moisture contents than those in the literature. Differences in variety, culture, soil, ripeness, harvesting, and post-harvest could all contribute to this variation. Moreover, peroxide compounds were not detected, indicating the oxidative deterioration of lipids had not occurred in fresh materials yet.

3.2. Peroxide Value (PV)

PV was confirmed to be undetectable in both fresh and dried samples in this investigation. It is a positive sign that represented the good quality of dried products, although the material was rich in lipids, particularly the unsaturated fatty acids, which are highly susceptible to oxidation [34,35]. The most active site in the structure of unsaturated fatty acids is the unsaturated bond, where autoxidation and enzymatic reactions, the principal mechanisms of lipid oxidation, occur, with the primary products of autoxidation being non-radical products, mainly peroxides and hydroperoxides [36]. Lipoxygenases (LOX) have been shown to catalyze the oxidation of polyunsaturated fatty acids, boosting the initiation and propagation of the free radical chain reaction in lipid oxidation governed by enzymatic reactions [37]. In this study, PV of all dried samples was not detected. Therefore, the lipid oxidation in these samples could be limited during IR drying of avocado pulp. The activity of LOX in avocados was characterized in the study of Jacobo-Velázquez et al. (2010), which reported that LOX activity decreased considerably at temperatures below 30 °C and above 40 °C [38]. As a consequence, the effectiveness of thermal pretreatment and drying at high temperatures (65–80 °C) inactivated these enzyme groups completely, leading to the termination of the start of the initiation and propagation steps in the free radical chain reactions. In IR drying of avocado pulp, it was indicated that the drying process was completed in a short time (from 35 to 55 min). The results implied that moisture would be quickly removed from the materials; thus, hydrolysis of lipids hardly occurred to release fatty acids, which were the materials in the chain of lipid oxidation.

3.3. Total Polyphenols, Total Chlorophylls, and Antioxidant Activity

Figure 1 shows the percentage retention of TPC, total chlorophylls, and antioxidant activity in all dried samples at different drying conditions. Avocado quality was shown to be diminished after IR drying, which implied that polyphenols and chlorophylls were degraded during IR drying of avocado pulp with various formulations.

When the IR temperature was raised from 65 to 70 °C, the concentration of polyphenol and chlorophyll in the dried sample rose for each formulation (p < 0.05). Then, if the IR temperature rose to 80 °C, both polyphenols and chlorophylls would dramatically decrease (p < 0.05 for AP and p > 0.05 for APM). Consequently, it was recognized that the impact of IR temperature on APM and AP was similar. When comparing TPC and total chlorophylls in avocado pulp formulations dried at 70 °C, it was discovered that APM had higher TPC and total chlorophylls retention than AP. The multifactor ANOVA showed that investigated factors and their interaction significantly impacted TPC (p = 0.010) and TCC (p = 0.004). In proportional relation to the changes in TPC and total chlorophylls, the antioxidant activity was highest at the IR temperature of 70 °C, and APM was also more antioxidant-active than AP.

McSweeney and Seetharaman (2015) comprehensively reviewed the degradation of phenolic compounds during drying mainly due to the participation of the oxidation enzymes, namely polyphenol oxidases (PPOs), peroxidase (POD), and LOX [39]. As previously stated, LOX might be completely inactivated by IR drying avocado pulp. Some previous studies [40,41,42] reported the activity of PPOs and POD in avocado pulp. The PPOs in avocado pulp were more heat-resistant than those in other fruits and vegetables [43]. Under heat inactivation at 84 °C for 15 min, only around 0.2% of the relative activity of PPOs in avocado flesh was maintained [44]. Vanini et al. (2010) observed that fruit maturation, as well as changes in temperature and time, can reduce the activity of the POD soluble enzyme [42]. Water activity, in addition to heat inactivation, has a major impact on enzyme activity. Lower water activity was found to reduce enzyme activity. Thus, a considerable volume of water would accelerate enzymatic reactions, bringing about the loss of TPC at the start of the drying process. However, when water was promptly removed from the substances, the enzymes were rapidly inactivated [45]. The rate of reactions would rise with a higher IR temperature, causing more TPC loss, but the higher rate of moisture removal would reduce enzyme activity. On account of this, higher temperature or longer operation time could lead to higher TPC loss [46]. Therefore, the highest content of TPC was reached at 70 °C, which resulted from the complex interaction between the changes in biochemistry and physical chemistry of avocado pulp during IR drying. Between two avocado pulp formulations, APM preserved higher TPC more effectively than AP for each IR temperature. This is possibly because hydrocolloids worked as a coating material, creating a physical barrier to oxygen and light to protect against chemical and enzymatic degradation [47].

The degradation of chlorophyll could be caused by enzymatic and non-enzymatic reactions. For chemical degradation, Mg²⁺ ions are replaced by two H⁺ ions from the porphyrin ring to form pheophytin and pyropheophytin, under acidic or heat treatment or by the action of Mg-chelatase [48]. For enzymatic reactions, enzymes involved in the degradation of chlorophyll were characterized as chlorophyllase or POD. It was reported that suitable pretreatments, such as steam blanching, could effectively inactivate chlorophyllase [49]. Under POD action, peroxy radicals accelerate the oxidation of chlorophyll, causing the loss of chlorophyll in dried samples. Yamauchi et al. (2004) discovered the mechanism of in vitro chlorophyll degradation by POD, especially in the presence of phenolic compounds [50]. The phenoxy radical and superoxide anion are formed during the oxidation of phenolic compounds by POD, which attacks chlorophyll directly to create C13²-hydroxychlorophyll a. In this study, it was found that degradation of chlorophyll is directly related to TPC degradation. The Pearson correlation analysis (see

Table 2. Pearson correlation coefficients between the total phenolic content (TPC), total chlorophyll content (TCC), antioxidant activity (AA), and color parameters (L*, a*, b*) of avocado pulp powder obtained by infrared drying.

	TPC	TCC	AA	L*	a*	b*
TPC p-value	1 -
TCC p-value	0.83 7.1 × 10−7	1
AA p-value	0.77 9.6 × 10−6	0.90 2.4 × 10−9	1 -
L* p-value	0.93 6.7 × 10−11	0.72 7.0 × 10−5	0.71 1.1 × 10−4	1 -
a* p-value	−0.79 5.2 × 10−6	−0.82 7.8 × 10−7	−0.73 6.0 × 10−5	−0.78 5.7 × 10−6	1 -
b* p-value	0.93 2.5 × 10−11	0.91 8.1 × 10−10	0.79 3.9 × 10−6	0.87 4.5 × 10−8	−0.91 6.1 × 10−10	1 -

Correlations in bold are significant at the 1% level (2-tailed).

) presented strong positive correlations between total polyphenols and total chlorophylls (0.83), which in turn explained why the change in total chlorophylls at different drying temperatures was similar to the change in TPC in IR drying of avocado pulp.

Polyphenols and phytopigments, such as chlorophylls, could act as antioxidants based on their structure, ideal for acting as free radical reducing agents and inhibiting the formation of free radicals under the catalysis of transition metals [51]. As expected, in this study, the correlations between antioxidant activity of avocados with polyphenols and chlorophylls were very high, 0.77 and 0.90, respectively. The change of antioxidant activity in IR drying of avocado pulp was mainly dependent on polyphenols and chlorophylls, and drying of avocado pulp supplemented with maltodextrin at 70 °C retained the highest antioxidant activity (94.4%).

3.4. Color Parameters

In this study, color parameters were changed after infrared drying of avocado pulp with different formulations. Primarily, it was observed that L* and b* values decreased; however, there was an increase in a* value after drying (see Figure 2). Furthermore, changes in color characteristics had strong correlations with total polyphenols and total chlorophylls (Table 2).

Color is a widely used criterion for evaluating the efficiency of operation, and it has a considerable influence on the purchasing intention. Figure 2 shows that the color of the dried product became darker (decrease in L* value), less green (increase in a* value), and more yellow (increase in b* value), indicating an increase in browning pigments and a loss of natural pigments in products (see Figure 3). For hot air drying of sliced avocados [52] and infrared-vacuum drying of grapefruit slices [53], the same trend was observed, with an increase in a* value and a decrease in L* and b* values. This study found each investigated factor showed a significant impact on the change of color parameters. Plus, their interaction also had significant effects on a* and b* values, but an insignificant effect on L* values (p = 0.333).

During processing, the discoloration of the product was directly related to the changes, such as pigment destruction and oxidation, enzymatic and non-enzymatic browning, and phenol polymerization [54]. Whereas the color shift in green-pigmented foods is mostly due to the biotransformation of chlorophylls to pheophytins, which replaces magnesium with hydrogen [55], the browning of fruit containing polyphenols is linked to the enzymatic breakdown of polyphenols. Some previous studies presented that the change in avocado pulp color had a strong correlation with an increase in PPO activity [40,56]. Table 2 also shows a strong relationship between total polyphenols, total chlorophylls, and color parameters with a positive correlation for L* and b* values and a negative correlation for a* value. It could be concluded that both polyphenol and chlorophyll degradation occurred in infrared drying of avocado pulp with different formulations, causing color damage.

Figure 2 further reveals that APM had greater L*, |a*|, and b* values than AP. When hydrocolloids were added to the avocado pulp, it suggested that the sample brightness would enhance. This was mostly attributable to the use of a high concentration of white maltodextrin [57]. As previously stated, maltodextrin as a physical barrier helps to preserve bioactive molecules from degradation reactions. Thus, color parameters were retained better in the presence of maltodextrin in the avocado pulp. Some previous studies confirmed that the addition of maltodextrin could decrease the browning index values of some dried foods [57].

4. Conclusions

In this study, the effects of different temperatures and formulations on the quality of IR dried avocado pulp were identified and examined. This study shows that maltodextrin concentration and IR temperature are important technical parameters in IR drying of avocado pulp. It was found that maltodextrin and IR temperature, as well as their interaction, have significant impacts on the quality of powdered avocado pulp, excepting L* value, which was insignificantly affected by the interaction between maltodextrin and IR temperature. The results indicate that during IR drying of avocado pulp, lipid oxidation appeared to be limited in this investigation, leading to PV not being detected in all dried samples. This could help to improve the quality of dried products. Polyphenol and chlorophyll oxidation occurred during IR drying of avocado pulp, causing the loss of antioxidant activity and natural pigments as well as discoloration, which produces browning pigments in products. Polyphenol degradation under POD activity occurred primarily in the beginning of the drying process, when the moisture content of material was still high. The degradation of chlorophyll has been correlated with the oxidation of phenolic compounds via POD, which provokes the destruction of polyphenols. In this study, higher loss in dried product quality was the result of drying avocado pulp at a high temperature or for a long time. Maltodextrin significantly retained the quality of avocado pulp during IR drying by acting as a covering substance. This study found that drying avocados supplemented with 9% maltodextrin at 70 °C produced the highest quality of dried avocado powder. Specifically, at this drying condition, total polyphenols, total chlorophylls, and antioxidant activity were retained at about 95.1, 95.2, and 94.4%, respectively. According to the findings, infrared drying could be considered a suitable drying method for developing new products with high quality from fruits and vegetables.