Effects of Blanching and Natural Convection Solar Drying on Quality Characteristics of Red Pepper (Capsicum annuum L.)

The objective of this work was to determine the effects of blanching and two drying methods, open-sun drying and natural convection solar drying, on the quality characteristics of red pepper. A 2 × 3 factorial design with experimental factors as 2 drying methods (open-sun drying and use of solar dryer) and 3 levels of pepper blanching (unblanched, blanched in plain water, and blanched in 2% NaCl) was conducted. Dried pepper samples were analysed for chemical composition, microbial load, and consumer sensory acceptability. Blanching of pepper in 2% NaCl solution followed by drying in a natural convection solar dryer reduced drying time by 15 hours. Similarly, a combination of blanching and drying in the solar dryer improved microbial quality of dried pepper. However, blanching and drying processes resulted in reduction in nutrients such as vitamin C and minerals content of pepper. Blanching followed by drying in natural convection solar dryer had the highest consumer acceptability scores for colour and overall acceptability, while texture and aroma were not significantly (p > 0.05) affected by the different treatments. Therefore, natural convection solar dryer can be used to dry pepper with acceptable microbial and sensory qualities, as an alternative to open-sun drying.


Introduction
Red pepper (Capsicum annuum L.) is used as a spice or major ingredient in dishes around worldwide and perhaps considered the first spice to have been used by man with archaeological evidence of pepper and other fossil foods dating back to 6000 years old [1]. Red pepper is generally known to be cholesterol free, have low sodium and caloric contents, and serve as good source of vitamins A and C [2]. Additionally, red pepper is known to possess antimicrobial activity [3] and reduces the risk of diseases such as arthritis, cancer, and diabetes [4][5][6][7][8]. In food processing, red pepper is also used as colouring and flavouring agent in sauces, soups, pickles, and pizzas [9].
Ghana contributes about 1% of world pepper production and was ranked the eleventh largest producer of pepper in the world and second in Africa with an estimated total production of 88,000 metric tons [10]. Like other fresh fruits and vegetables, fresh pepper is a perishable produce and deteriorates within a few days after harvest without proper storage or preservation measures. The perishable nature of pepper can lead to economic losses which is further aggravated by storage and marketing problems and lack of appropriate processing technologies [11]. To prevent losses due to postharvest deterioration, dehydration is one important method often adopted for preservation and value addition to perishable agricultural products through moisture control. The major goal in drying fruits and vegetables such as pepper is to reduce the moisture content to desirable levels, usually 5-10%, which allows for safe storage over an extended period of time [12].
Unlike many industrialized countries where mechanized drying of fruits and vegetables is practiced, traditional opensun drying of fruits and vegetables is a common practice in developing countries. However, traditional open-sun drying of fruits and vegetables can be time-consuming and less hygienic. Although modern mechanized drying of food commodities is faster than open-sun drying and uses much less land space, the equipment cost as well as the continued recurring cost of fuel or energy to operate these systems is very high. Therefore, small-scale farmers and other players in  the pepper value chain in rural communities in Ghana resort to the traditional open-sun drying method of preservation which results in low-quality products. As an alternative to mechanized and open-sun drying methods, solar dryers are being investigated and used for drying various fruits and vegetables [12], especially at geographic locations where there is enough sunshine during the harvest season [13]. Further advantages of solar drying over conventional open sun drying are the improvement in hygienic quality of the dried products, safe moisture content, colour and taste, and the protection of produce from rain, dust, and insects [14].
To obtain dried pepper with the best organoleptic and nutritional qualities, pretreatment such as blanching may be applied prior to drying [15][16][17]. Blanching is considered a pretreatment or unit operation prior to freezing, canning, or drying in which fruits or vegetables are heated for the purpose of inactivating enzymes; modifying texture; preserving colour, flavour, and nutritional value. Therefore, the objective of this work was to determine the effects of aquathermal blanching and natural convection solar drying methods on the quality characteristics of pepper.

Design and Fabrication of Natural Convection Solar Dryer.
A direct natural convection solar dryer used in this study was designed and fabricated as shown in Figure 1. It was constructed mainly with wood on the sides and a transparent glass (166 cm × 76 cm) covering the top inclined at an angle of 10 ∘ to face the equator. A metal plate, painted black to improve absorption, was fitted at the bottom of the box with cardboard (insulator) beneath it. The inner side walls of the box were lined with aluminum foil for the reflection of radiation back to the interior space of box. Three inlet and outlet vents were created to allow air convectional current take place.
Further details and measurements of the solar collector are shown in Table 1. The constructed direct solar drying system was installed in an open place at the Navrongo Campus of the University for Development Studies. The drier representing "dislike extremely," "neither like nor dislike," and "like extremely," respectively [19]. All products were presented to the panelists randomly and placed side by side, with each panelist receiving two rounds of each product.

Ambient and Solar Dryer Temperatures.
Variations in ambient temperature and temperatures in the drying chamber of solar dryer and absorber plate during the experimental period are shown in Figure 2. The average ambient temperature increased from 28.6 ∘ C at 9:00 GMT to 40.5 ∘ C at 14:00 GMT and decreased thereafter to 31 ∘ C at 17:00 GMT. In a similar trend, temperature of the drying chamber within the solar dryer increased from an average 55.0 ∘ C at 9:00 to 69 ∘ C at 12:00 GMT and decreased thereafter to average of 42 ∘ C at 17:00 GMT. The highest average temperatures of drying chamber and bottom absorber plate of the solar dryer, 69.0 ∘ C and 77.5 ∘ C, respectively, were attained at 12:00 GMT. Figure 3. Fresh pepper samples had an average moisture content of 75% prior to pretreatment and drying. Pepper samples that were blanched in 2% NaCl solution and subsequently dried in the solar dryer had moisture content reduced to 5% after 13 h of drying. Pepper samples that were blanched in plain water and unblanched pepper attained moisture contents of 5% after drying in a period of 16 and 17 h, respectively, in solar dryer. On the other hand, all pepper samples that were dried in the open sun, irrespective of pretreatment, attained moisture contents of 5% after 28 h of drying. Thus, blanching in 2% NaCl followed by use of the natural convection solar dryer reduced the drying time for pepper by 15 hours. In general,  average drying time for pepper samples that were dried in the natural convection solar dryer was significantly lower ( < 0.05) than pepper samples dried in the open sun. Blanching either in 2% NaCl solution or in water significantly ( < 0.05) improved drying rate of pepper (Figure 3). In general, drying pepper in the solar dryer reduced drying time by about 49-54% depending on pretreatment method applied to the pepper.

Drying Characteristics of Pepper. The reduction in moisture contents of pepper samples during drying in the open sun and in solar dryer is shown in
In a review of various types of solar drying systems for agricultural commodities, Fudholi et al. [20] reported that the moisture content of fresh chili decreased from 80% to 5% under solar drying in 48 h. Improvement in the drying rates of blanched pepper has previously been reported [5,16,[21][22][23], and the observation has been attributed to possible rupturing of cell membrane making pepper tender and thus facilitating faster removal of moisture in the drying process [22].

Effect of Blanching and Drying Method on Nutrient
Composition of Pepper. The nutrient composition of pepper following blanching treatments and drying is shown in Table 2. Total proteins, ash, and fibre contents of fresh pepper were 2.8 ± 0.04, 2.6 ± 0.10, and 4.8 ± 1.00 (g/100 g), respectively, while vitamin C, calcium (Ca), and iron (Fe) contents were 175.6±1.5, 18.5±0.9, and 2.1±0.10 (mg/100 g), respectively. Generally, crude proteins and fibre contents were not significantly affected by the blanching treatments and drying methods. However, crude ash content significantly reduced following blanching and drying. Similarly, vitamin C, Ca, and Fe contents reduced significantly ( < 0.05) after blanching treatments and drying (Table 2).
Generally, vegetables and fruits serve as good sources of energy, minerals, and vitamins. However, during dehydration processes, changes in nutritional quality of heat sensitive vitamins and other nutrients occur [12,24]. It has previously been reported that significant losses in the content of vitamin C, minerals, and polyphenols in vegetables occur after aquathermal blanching [25,26], an observation which may be due to their sensitivity to heat and/or leaching of these compounds in water [24,27]. However, steam blanching was reported to retain higher amounts of vitamin C in spinach compared with hot water blanching [28]. The present results show that neither blanching in NaCl solution nor plain water could significantly retain the vitamin C and mineral contents of pepper as there were significant losses in these nutrients after blanching and drying.

Microbial Load in Dried Pepper.
Mean aerobic mesophilic counts (AMC) and E. coli/coliform (ECC) counts in dried pepper are shown in Figure 4. Aerobic mesophilic counts ranged between 4.6±1.2 and 6.7±0.8 cfu/g, while ECC counts ranged between 1.8 ± 0.5 and 3.5 ± 1.0 cfu/g of dried pepper samples.
Generally, pepper samples that were blanched and dried in solar collector had significantly ( < 0.05) lower microbial load when compared to samples that were dried in the open sun with or without blanching ( Figure 4). Thus, a combination of blanching and the solar collector drying processes can significantly reduce the microbial load in dried pepper. Through blanching and drying to control moisture levels (water activity, ) in pepper, microbial load can be controlled although spores may not be killed. These may bring microbial load of dried pepper to allowable safe limits while supporting long shelf life of these products. 6.4 ± 2.2 a 6.1 ± 1.9 a 6.3 ± 1.9 a 6.0 ± 1.8 a 6.3 ± 1.6 a 6.2 ± 1.8 a Texture 5.9 ± 1.8 a 5.7 ± 1.5 a 6.1 ± 2.0 a 5.7 ± 1.7 a 5.8 ± 1.7 a 6.1 ± 1.7 a Overall acceptability 6.2 ± 1.4 a 6.4 ± 1.2 a 4.6 ± 1.7 b 6.9 ± 1.7 c 7.1 ± 1.

Consumer Sensory Evaluation of Dried Pepper.
Consumer sensory evaluation of the differently treated and dried pepper samples assessed using a 9-point hedonic scale is shown in Table 3. Consumer evaluation of dried pepper covered sensory characteristics such as colour, aroma, texture, and overall acceptability. Results of consumer evaluation generally showed no significant difference ( > 0.05) in aroma and texture between the different pepper samples. However, significant differences ( < 0.05) were observed in colour and overall acceptability between samples. For colour, pepper samples that were blanched either in plain water or in 2% NaCl solution and subsequently dried using the solar dryer scored significantly higher (7.0), whereas pepper samples that were not blanched and subsequently dried in the open sun scored significantly ( < 0.05) lower (3.8). For overall acceptability, the highest score of 7.1 was attained for pepper samples that were blanched in 2% NaCl solution and subsequently dried in the solar dryer, indicating the panelist likeness for the product.
It has been suggested that inactivating the enzymes responsible for browning (polyphenoloxidase, lipoxygenase, and peroxidase), during blanching improves both colour and flavour of vegetables [29]. However, if not carefully managed, blanching may also be accompanied by a reduction in sensory and nutrient qualities in many foods, principally due to Maillard reaction [30].

Conclusions
Blanching of pepper in 2% NaCl solution followed by drying in a natural convection solar dryer reduced drying time by 15 hours when compared to drying in the open sun which takes at least 28 hours. Similarly, a combination of blanching and drying in the solar dryer improved microbial quality of the dried pepper. However, the blanching and drying processes resulted in reduction of nutrients such as vitamin C and minerals contents. Consumer sensory analysis showed that blanching, followed by drying in natural convection solar dryer, had the highest scores for colour and overall acceptability, while texture and aroma were not significantly ( > 0.05) affected by the various treatments. Therefore, natural convection solar dryer has the potential for commercial small-medium scale industrial application for drying pepper with acceptable microbial and sensory qualities, especially in developing rural communities where equipment cost as well as recurring cost of fuel or energy to operate mechanical dryers can prove to be high. This in turn would ensure reduction in postharvest losses of pepper and thus better economic returns for farmers and processors.

Conflicts of Interest
The authors declare that they have no conflicts of interest.