The drying time was longer for mango slices dried in OAD, MVD and OVD, respectively. The effect of the drying methods on the drying kinetics of mango slices (3 mm, 6 mm and 9 mm) and the effect of lemon juice pretreatment were investigated.
INTRODUCTION
Assessment of post-harvest fruit losses at the Tshakhuma fruit market in Limpopo Province, South Africa. Unpublished MSc Nutrition Thesis, Faculty of Medicine and Health Sciences, Stellenbosch University, Stellenbosch, South Africa.
LITERATURE REVIEW
Production of The Mango Fruit
Post-harvest Losses of Fruits and Vegetables
- Causes of postharvest losses
Furthermore, the presence of injuries, such as bruising, increases the rate of water loss and respiration, resulting in physiological deterioration, which increases the rate of respiration and water loss in a product. The changes that occur during respiration and water loss can result in drying, germination and increased susceptibility of the product to mechanical damage (Maremera, 2014).
Fruit Preservation Methods
A Review of Drying Principles and Factors Controlling the Drying Process
- Drying principles
- Climatic factors influencing the drying process
Therefore, this study attempts to consider these factors, to determine their influence in the drying process. In this case, higher temperatures lead to a higher drying rate during the decreasing rate of the drying curve, as shown in Figure 2.2.
Drying Methods
- Hot-air drying
- Freeze-drying
- Microwave drying
- Vacuum drying
At the end of the drying process, a temperature below 55℃ is recommended to prevent brown discoloration of the products (Schiavone, 2011). Moreover, microwave-assisted vacuum drying is reported to be one of the most successful applications in food drying (Feng et al., 2012).
Classification of Solar Dryers
- Open-air uncontrolled solar drying
- Direct solar dryers
- Indirect solar dryers
The crop is placed in a chamber that is used as a collector (Ogunkoya et al., 2011; Eswara and Ramkrishnarao, 2013). In addition, a solar greenhouse dryer is an attractive solution for drying large quantities of crops up to 1000 kg (Tiwari et al., 2016).
Comparison of Natural and Forced Ventilation Solar Drying systems
Indirect solar dryers have a higher drying speed than direct solar dryers and open sun dryers, due to their higher operating temperatures. When comparing the performance parameters as shown in Table 2.2, it was found that the efficiency of the drying systems is almost the same, however, the drying time varies.
Improving the Efficiency of Solar Dryers
- Wind ventilator
- Solar Chimney
- Heat storage
Thermal storage of heat either in the form of sensible heat or latent heat also enhances the performance of solar dryers. Heat storage maintains a consistent drying temperature and improves drying speed. 2012) evaluated the naturally ventilated solar dryer, with heat a storage provided by crushed basalt rock.
Electrical Energy for Drying Agricultural Products
According to Sharma et al. 2009), many countries of the world use solar thermal systems in agriculture for preserving fruits. The perception is that modified solar drying of fruits improves the quality of the products, compared to uncontrolled open-air solar drying, and contributes to the reduction of post-harvest losses (Wankhade et al., 2012).
Solar Energy Availability in South Africa
Therefore, solar energy is one of the best alternatives for use in drying processes in South Africa.
Drying Pre-treatments
- Sulphating
- Blanching
- Fruit juice
Research studies have found that pretreatment preserves the shape and shrinkage volume of products during drying. Sulfite pretreatment was found to increase the weight during drying of apples and peaches.
Quality Parameters
- Physical properties
- Sensory properties
- Chemical properties
It induces several reactions, such as pigment degradation (especially carotenoids), maillard reactions and the oxidation of ascorbic acid (Sivasankar, 2009; Korbel et al., 2013). Aroma, taste and chemical mouthfeel factors contribute to flavor and these are affected by thermal treatments, such as drying (Meilgaard et al., 2007). A consumer test determines whether consumers like a product, prefer it over another product, or find the product acceptable, based on its sensory characteristics (Tenenhaus et al., 2005).
Preference mapping indicates the sensory characteristics that contribute to a consumer's liking of a particular food product (Murray et al., 2001).
Microbial Properties
The denaturation of proteins during drying processes results in a lower dehydration ratio, due to the product's inability to reabsorb water (Perumal reported that the rehydration of sun tunnel dried tomato was higher (3.15) than open air uncontrolled dried tomato (3.10). Perumal (2007) investigated the rehydration ratio of tomato slices, of 4, 6 and 8 mm thickness dried under open air uncontrolled sun drying and a vacuum assisted sun dryer. Therefore higher drying temperatures, drying method and the thickness of dried product have an effect on the have rehydration ratio.
Fresh fruits are a significant source of plant and human pathogens (Gultie and Sahile, 2013; Tasirin et al., 2014).
Modelling of drying process
The Page model was best suited for forced convection drying and the Verma model was best suited for uncontrolled outdoor solar drying. This study intends to fit four drying models on the drying data to illustrate the drying process of mango fruit. As illustrated in the above studies, models have been successful in describing optimal conditions for operation of tumble dryers, as well as the drying process.
MR (dimensionless) is the predicted moisture ratio, k (hour-1) is the drying constant, t (hour) is the drying time and a, b, n are drying model coefficients.
Discussions and Conclusions
Freeze drying, microwave drying and vacuum drying are more efficient than uncontrolled open air solar dryers. Therefore, innovative options, such as the use of thermal storage, a solar chimney and a wind fan, were considered to improve the drying time and speed. The drying process will be analyzed, using models to establish optimal drying conditions.
Therefore, this study will also fill a research gap in illustrating the drying properties and quality attributes of dried mango.
Unpublished MSc Food Science Thesis, Department of Food Science, Stellenbosch University, Stellenbosch, South Africa. Unpublished PhD Dissertation in Hydrology, School of Bioresource Engineering and Environmental Hydrology, University of Kwazulu-Natal, Pietermaritzburg, South Africa. Unpublished MSc Food Safety Thesis, Department of Food Safety, University of KwaZulu-Natal, Pietermaritzburg, South Africa.
THE EFFECTS OF HOT AIR DRYING METHODS ON MANGO DRYING PROPERTIES AND MODELING.
THE EFFECTS OF HOT-AIR DRYING METHODS ON THE DRYING OF
Introduction
Studies have found that a greenhouse/tunnel solar dryer is satisfactory and competitive with uncontrolled open-air solar drying (Farhat et al., 2004; Fadhel et al., 2014). As a result, naturally ventilated cabinet solar dryers are a popular method of solar drying (Pangavhane et al., 2002; Fadhel et al., 2014). The challenge with using naturally ventilated cabinet solar dryers is their limited capacity and relatively high construction cost (Fadhel et al., 2014).
The conclusions of Fadhel et al. 2014) also indicate that a modified ventilation solar greenhouse dryer reduces drying time.
Materials and Methods
- Study area
- Sample preparation and treatments
- Description of drying methods
- Experimental design
- Data collection
- Temperature, relative humidity and solar radiation data analysis
- Drying characteristics data analysis
- Effective moisture diffusivity
Furthermore, the relative humidity values of the area are within the ranges tested by Kaya et al. Non-linear regression was performed to determine goodness of fit of the selected models shown in Table 3.1 to the experimental moisture condition. Hayaloglu et al.(2007) MR (dimensionless) is the predicted moisture ratio, k (hr-1) is the drying constant, t (hr) is the drying time and a, b, n are drying model coefficients.
The slope of the lnMR versus time plot was used to estimate the effective moisture diffusivity.
Results and Discussions
- Ambient air conditions during drying
- Climatic conditions inside MVD
- The effect of pre-drying treatments and slice thickness on drying time
- The effect of pre-drying treatments and slice thickness on drying rate
- Mathematical modelling
- Estimation of the effective moisture diffusivity
- Conclusions
Lemon juice-treated and control samples of all mango thicknesses dried in OVD showed no significant difference (P>0.05) in the drying rate. 3mm-Lemon Juice Treatment 6mm-Control 6mm-Lemon Juice Treatment 9mm-Control 9mm-Lemon Juice Treatment 3mm-Control. 3mm-Lemon Juice Treatment 3mm-Control 6mm-Lemon Juice Treatment 6mm-Control 9mm-Lemon Juice Treatment 9mm-Control.
It was found that treatment with lemon juice did not affect the drying properties.
The thickness of mango slices significantly affected the drying characteristics such as drying time and drying speed. Experimental research on outdoor drying of hot red pepper; under the greenhouse and in the solar dryer. Application of exponential, Page and diffusion models to simulate drying kinetics of kiwifruit.
The influence of the combined microwave power and hot air fan on the drying kinetics and color quality of tomato slices.
Introduction
Studies on convective oven drying showed that a lower drying temperature of around 60°C and longer drying periods result in higher total color changes (ΔE) and a lower dehydration ratio for mangoes (Akoy indicates that convective oven drying is widely used in industrial drying and research studies have reported quality loss in terms of taste, color and hardening due to the use of hot air.Convective oven drying at 70-90°C is one of the drying methods that causes the degradation of important flavor substances and color changes ( Antal et al., 2015) .As a result, massive quality losses due to solar drying in open air, combined with relatively higher energy costs of convective oven drying methods, have resulted in consideration of other drying methods, such as the use of a greenhouse solar dryer, with modified ventilation.
Limited studies have been conducted on the quality parameters of Tommy Atkin mangoes dried by solar drying, uncontrolled open-air solar drying, and convective oven dryer.
Materials and Methods
- Fruit firmness
- Total soluble solids
- Colour
- Rehydration
- Sensory evaluation
- Microbial analysis
- Microstructure changes
A group of 55 untrained panelists, consisting of students and staff from the University of KwaZulu-Natal, carried out a sensory evaluation of the mango samples. The sensory evaluation was done at the Food Science Laboratory of the university in groups of 10 panelists at a time and they were separated by booths. The average of individual scores represents the sensory score of the dried mango sets evaluated.
An amount of 1 ml of the homogenate was aseptically pipetted, with a sterilized pipette, into the 10-0 dilution and from this serial dilutions from 10-1 to 10-4 were prepared.
Results and Discussions
- Fresh mango quality properties
- Colour changes
- Rehydration characteristics
- Sensory quality results
- Microbial load and safety results
- Changes in the microstructure of mango slices
- Conclusions
- References
However, lemon juice treatment had no significant (P≥0.05) effect on the lightness of the dried mango. However, lemon juice treatment had no significant influence (P>0.05) on the redness of the mangoes. There was a highly significant (P<0.001) difference in the consumer acceptability of the color of the treated lemon juice and the control mango slices.
The difference in the similarity of the taste of the dried mango was highly significant (P<0.001) as shown in Table 4.3.
CONCLUSIONS AND RECOMMENDATIONS
Research studies clearly show that the drying mechanisms are capillary and gravitational forces, as during a period of constant drying speed, damage to the surface of the product will result. It dries mango fruit in an acceptable drying period compared to OAD and drying kinetics are almost similar to convection oven dryer (OVD). MVD needs to be further developed by developing methods such as using solar thermal collectors and air recirculation to increase the temperature to reduce the drying time.
This study recommends further investigation into pre-treatments, which will increase drying speed, to further reduce drying time.
APPENDICES
- Appendix 3.1: Experimental Design
- Appendix 3.2: Effective Moisture Diffusivity Linear Curves
- Appendix 4.1: Table of Total Colour Change
- Appendix 4.2: Sensory Evaluation sheet
- Appendix 4.3: Ethics Approval Certificate
Effective linear curves of moisture diffusion for processed dried mango with (a) 3 mm, (b) 6 mm and (c) 9 mm thickness. 1 Dislike Extremely Bad Extremely Bad Extremely Bad 2 Dislike Very Bad Very Bad Very Bad.
