Effect of drying temperature on physicochemical properties of Moringa oleifera leaf

Nguyen Tuyet Giang^1,2 and Le Thi Thuy Hang^1,2

¹ An Giang University, An Giang, Vietnam
ntgiang@agu.edu.vn
² Vietnam National University Ho Chi Minh City, Vietnam

Abstract

The current study was conducted to investigate the effect of drying temperature (50, 55, 60 and 65◦C) on quality attributes and colour parameters of Moringa oleifera leaf. The results showed that increased oven temperature reduced drying time and the nutritive value of dried leaf. Temperature significantly increased the contents of protein, lipids, fibers, non-fiber carbohydrate, calcium, and iron, but reduced vitamin C level of fresh leaf. Compared with a fresh sample, the dried moringa leaf showed a 43.6-58.3% reduction in vitamin C content. During thermal treatment, color values L^* and b^* decreased, while a^* and ∆E^* increased. It can be found that the convective drying with the temperature of 55^oC was suitable to produce moringa leaf powder. At 55^oC, moringa leaf powder obtained 4.45% moisture, 29.2% protein, 10.5% ash, 6.64% lipids, 5.97% fiber, 43.2% non-fiber carbohydrate, 894 mg/100g calcium, 21.3 mg/100g iron, and 144 mg/100g vitamin C. A high correlation was observed between drying temperature and physicochemical properties of dried moringa leaf. Moringa leaf powder obtained from this experiment has potential for the development of new functional products not only in human nutrition but also in livestock feeding.

Key word: colour, drying temperature, drying time, moringa leaf, vitamin C

Introduction

Moringa (Moringa oleifera) is the most common woody plant in the Moringa genus, which belongs to the family of Moringaceae. Moringa is native to India but grows well in both tropical and subtropical countries because of outstanding characteristics such as rapid growth, drought resistance as well as excellent nutritive and phytochemical content (Falowo et al 2018; Modisaojang-Mojanaga et al 2019). Being cited as one of the world’s most useful plant, all parts of moringa tree, such as root, leaf, bark, flower, pod and seeds are found to possess various pharmacological functions, including antioxidant, diuretic, antipyretic, anti-inflammatory, anti-hypertensive, blood-clotting and anti-cancer properties (Saini et al 2016; Abd El-Hack et al 2018). Remarkably, moringa leaf are widely used because it contains a variety of essential phytochemicals to provide 9 times more protein than yoghurt, 10 times more vitamin A than carrots, 7 times more vitamin C than oranges, 17 times more calcium than milk, 25 times more iron than spinach and 15 times more potassium than bananas (Gopalakrishnan et al 2016). Moringa leaf also contains low dietary level of antinutrients such as lignin, oxalates, lectins, tannins, protease inhibitors, phytic acid and saponins. These substances can vary depending on the environment and methods of cultivation, collection, processing and storage (Bamishaiye et al 2011; Shih et al 2011; Saini et al 2014; Stevens et al 2015). The high nutritive value of moringa leaf has positioned it as a choice of feed ingredient or feed additive (Kakengi et al 2007; Martens et al 2012; Mukumbo et al 2014; Su and Chen 2020).

Moringa leaf is considered to be highly perishable ingredient due to the high moisture content (Ali et al 2017). It is therefore processed by drying to increase the shelf life as well as preserve the nutritional and sensorial quality of fresh leaf. Drying is the oldest and commonly technique used in food and feed preservation to prevent microbial growth and other unwanted biochemical reactions. Mass reduction also occurs after drying making products easier for processing, transportation and storage (Calín-Sánchez et al 2020). Convective hot air drying is still extensively used due to unquestionable advantages such as simple apparatus and a well-known drying mechanism. However, drying may cause thermal damage thus adversely affects in the physical and chemical properties including textural changes, discolouration and loss in nutrients (Zhang et al 2017). Previous studies have confirmed that drying temperature in conventional ovens or dehydrators vary considerably depending on the materials. High temperature drying shortens the drying time but may reduce the product quality, increase energy consumption and cause heat damage on the exposed surface. Lower temperature, on the other hand, may improve the product quality but increase the drying period (Premi et al 2010; Karim and Mohammad 2014; Kannan and Thahaaseen 2016; Suliman et al 2016; Clement et al 2017). In addition, improper heat control may affect on the nutrients and sensory attributes of dried moringa leaf. Hence, this study sought to investigate the effect of drying temperature on some phytochemicals and to ascertain how this affects colour indices of the dried moringa leaf.

Materials and methods

Sample collection and preparation

Fresh moringa (M. oleifera) leaf was harvested from the trees at around 5 years of age in An Giang province, Vietnam and and immediately brought to the laboratory. The sample was then divided into four portions and stored at 4°C. Each portion was dried under four different drying temperatures. The initial moisture content of the fresh moringa leaf was determined using the oven method (AOAC 2005). The leaf was then thoroughly rinsed in running water to remove foreign materials, and spread on a stainless steel tray for draining. The leaf was manually separated from the stalk and stem for subsequent drying treatments.

Drying process

Drying experiments were performed in an electric convection oven (Universal oven UF750, Memmert, Germany) which could be regulated to desired drying temperature between 20 and 300°C with ± 0.1°C accuracy. Samples of fresh moringa leaf, weighing about 300 g (sample thickness: 5 mm), were spread uniformly on metal wire mesh tray with the cavity dimension of 1,000 × 600 × 20 mm (w×d×h). The drying temperatures were 50, 55, 60 and 65°C which served as treatments. The air velocity was maintained at 1.0 m/s. Initial moisture content of the examples was determined before drying process. Moisture loss was obtained evenly at 30-min interval during drying until moisture content of below 7% was reached. For moringa leaf, a moisture content less than 7% is considered satisfactory to prevent microbial growth for long term storage, as recommended by Wickramasinghe et al (2015).

Three replications (3 trays per each) were conducted for each drying condition. The dried leaf was then ground and passed through a 300 μm sieve to produce a powder with uniform colour. The powder samples were kept in air-tight containers and stored at 4ºC for further analysis.

Proximate analysis

The proximate composition of the fresh and dried samples (moisture, ash, crude protein, crude lipids, crude fibers, calcium, iron and vitamin C) was determined according to the AOAC (2005) official standards. Each test was carried out in triplicate. The non-fiber carbohydrate (NFC) was also calculated using equation 1.

NFC = 100 - (moisture + ash + crude protein + crude lipids + crude fibers) (1)

Colour measurement

A portable colourimeter (CR-20 Chromometer, Konica Minolta, Japan) was used to measure the CIE (Commission Internationale de l'Éclairage) coordinates. The colourimeter was calibrated against a standard calibration plate of a white standard plate (CIE Standard Illuminant D65). Four readings were taken randomly from different locations of each sample and following colour parameters were determined: ­the L∗ measures the lightness and darkness which ranging from black at 0 to white at 100; th­e a∗ value depicts greenness when negative and redness when positive and the b∗ value measures blueness when negative and yellowness when positive (CIE, 1978). The colour differences (∆E^*), the magnitude of total colour differences, were also calculated assuming fresh leaf as a reference. It was calculated from L^*, a^* and b^* values according to using equation 2.

where, L^*₀, a^*₀ and b^*₀ and refers to the colour reading of fresh leaf.

Statistical analysis

Data obtained was analyzed by one-way analysis of variance (ANOVA) using a General linear model (GLM) of Minitab ver. 16.0. Pearson correlation coefficient (r) was used to assess the linear relationship between drying temperature and quality variables of moringa leaf powder. Statistical significance was considered at the 5% level of probability.

Results and discussion

Drying time

The total drying time of moringa leaf at selected temperatures are shown in Table 1 and Figure 1. It is obvious that increasing the drying temperature caused a decrease in the moisture content, therefore reduction in the drying time. The drying times to reach the moisture content of below 7% for the fresh sample were 300, 270, 180 and 150 min at 50, 55, 60 and 65˚C, respectively. As the temperature increased by difference of 5˚C, from 50˚C to 55˚C, 55˚C to 60˚C and 60˚C to 65˚C, the drying time decreased by 10.0, 33.3 and 16.7%, correspondingly. Maximum reduction of drying time (50%) was obtained when drying temperature increased from 50˚C to 65˚C as compared from 50˚C to 60˚C and 55˚C to 65˚C (Table 1).

The time series graph shows that the initial moisture content of fresh moringa leaf (72.2 ± 1.62%) steadily decreased during drying process but the time taken to reach the disired moisture of below 7% is different for drying conditions. T50 took longest time to dry the leaf compared to other treatments (Figure 1). Of all the conditions, T65 was the most effective treatment in moisture removal. This in turn suggests that convection oven operated in the range of 50-65^oC was effective to dry moringa leaf. These results are in good agreement as compared to the earlier studies on moringa and other leaves (Premi et al 2010; Ali et al 2014; Razak et al 2016).

Figure 1. Total drying time of moringa leaf at different temperatures (Error bars represent standard deviation)

Chemical composition

The proximate and micronutrient compositions determined in moringa leaf samples were higher than those reported by Ali et al (2017) (for fresh leaf) and Mbah et al (2012) (for dried leaf). Differences in geographical origin, cropping season, maturation stage, cultivation method and drying technique might affect the accumulation of nutrients by the plant (Bamishaiye et al 2011; Stevens et al 2015; Nobosse et al 2017). The remarkable nutritional profile of M. oleifera leaf could further enhance its nutritional competence and expectedly furnish a lot of health benefits, not only in human nutrition but also in livestock feeding.

During drying, subsequent removal of moisture took place; as expected, the moisture content of the leaf decreased with increased drying temperature, causing a number of chemical and physical changes in the product. Table 2 shows significant increase (p<0.001) in most of the chemical compounds of moringa leaf due to drying, except for the vitamin C. Remarkably, protein and non-fiber carbohydrate of dried moringa leaf increased about 3-5 times as compared to fresh leaf. The result shows that dried moringa leaf is better source of macronutrients than the fresh leaf. However, the vitamin C content of fresh moringa leaf was higher compared to dried leaf as it reduced with the increased drying temperature.

The oven 50^oC preserved the highest amount of nutrient content, except for non-fiber carbohydrate, and further increased drying temperature resulted in a decrease of protein, lipids and fibers. The same trend was observed for calcium, iron and vitamin C. Oven conditions did not affect the ash content (p>0.05), probably because this component was not completely burned. As regards the non-fiber carbohydrate, sample dried at 50^oC shows lowest content compared to the others. It can be speculated that the low molecular weight carbohydrates are caramelized and burned during long time heating (Danso-Boateng, 2013). Another significant effect was observed for the reduction of vitamin C which increased with rising temperature in the oven. Zhang et al (2017) stated that drying of vegetables leads to break down of nutrients, particularly protein and vitamin C. Protein are highly susceptible to denaturation which commonly caused by heat, through reversible or irreversible change of the ternary structure, followed by the release of amino acids from the protein molecules.

The reduction of vitamin C from 274 mg/100 g in fresh leaf to the range of 114-154 mg/100 g in dried samples (Table 2) indicated the negative effect of temperature on the vitamin C content and was in agreement with Alakali et al (2015). Ali et al (2017) explained that vitamin C is a heat-sensitive component therefore heating during drying may increase the degree of vitamin C degradation. The increase in drying temperature resulted in a higher loss of vitamin C in moringa leaf (Figure 2). The loss of vitamin C during drying was probably due to the oxidation of hydroxyl groups in its structure to dehydroascorbic acid at high temperature. In accordance with our finding, other reports of Olabode et al (2015); Kannan and Thahaaseen (2016) have observed greater losses of vitamin C caused by high temperature during drying.

Figure 2. Effect of drying temperature on vitamin C content of moringa leaf and the loss of vitamin C in dried leaf powder. T50: 50 oC, T55: 55 oC; T60: 60 oC; T65: 60 oC. A-C Means of vitamin C content with similar indices are different at p<0.05. a-d Means of the loss of vitamin C content with similar indices are different at p<0.05. Error bars represent standard deviation

Colour analysis

Colour is one of the most important quality attributes reflecting the quality of the product which directly influences consumer acceptance and preferences. The main objective of leaf drying is to improve the colour of the dried products and minimize the colour changes during processing and storage. For green leafy vegetables, colour depends mainly on the presence of chlorophyll, a natural plan pigments, which is easily degraded during drying. Colour of the dried products are therefore significant driven by temperature during heat exposure process, particularly in air drying oven which may cause intensive colour deterioration (Pathare et al 2013; Zhang et al 2017).

Moringa leaf contains relatively high level of chlorophyll which is easily degraded during thermal treament (Abdulkadir et al 2015). However, the dried leaf powder generally retained the colour characteristic of the fresh material, as illustrated in Figure 3. Table 3 shows the changes in colour parameters due to the effect of temperature during oven-drying. The fresh moringa leaf had values 48.1, -15.6 and 11.2 for L^*, a ^* and b^*, respectively. The drying temperature had a significant impact (p< 0.001) on the lightness (L^*), redness (a^*), yellowness (b^*), and colour difference (∆E^*) compared to fresh leaf. There were also statistically significant differences among the drying treatments (p< 0.001). The dried samples had lower L^∗ values compared to fresh sample, meaning that dried product that was significantly darker than the fresh material. The decrease of L^* values was probably caused by the degradation of chlorophyll during the drying phase. These findings are close to those found by Premi et al (2010) for parameter L^* with values from 43.9 to 49.0 for the moringa leaf powder after conventional oven-drying at between 50-80^oC. The result was also congruent with Pathare et al (2013), who indicated that the higher the degree of browning, the lower the L^* value of the material.

Figure 3. Fresh moringa leaf (A) and samples of moringa leaf powder produced at 50 oC (B), 55 oC (C), 60oC (D) and 65 oC (E)

Samples dried in all treatments showed an increased in a^* value (-14.3, -14.3, -12.8, and -11.7 at 50, 55, 60 and 65°C respectively) compared to the fresh leaf, meaning that the green colour of leaf decreased at higher temperature. Our results were consistent with Premi et al (2010), who indicated that the a^* value became more positive as drying temperature increased, thereby giving a brighter green colour. The values of coordinate b^* indicated that drying changed gradual colour of the leaf from yellow into bluish. It was in good agreement with Razak et al (2016), who assumed that the decrease in the value b^* (yellowness) of leaves was probably due to the carotenoid decomposition. However, in this study, the effect of drying treatment on the b^* value of moringa leaf was not very noticeable (Table 3). The b^* values of samples dried at 50 and 55°C were closer to fresh sample. In the case of the total colour difference, the lowest value of ΔE^* was observed in T50 sample (2.54), indicating that 50^oC was better to preserve the original colour of moringa leaf, while the highest ΔE^* value was recorded at 65^oC (8.75).

According to Wickramasinghe et al (2020) the natural green colour of leaf is due to a mixture of chlorophylls in which chlorophyll a (responsible for blue-green colour) is easier to be degraded to pyropheophytin and pheophytin than chlorophyll b (giving yellow-green colour) during drying. Therefore, the ratio of chlorophyll a to chlorophyll b decreases giving a rise of visually dull yellow-green colour. Colour changes in moringa leaf is due to chemical and biochemical transformations in the presence of oxygen during heat treatment. The overall colour variations may be caused not only by the non-enzymatic browning reaction but also by the destruction of plant pigments present in the leaf (Pathare et al 2013; Zhang et al 2017). In addition, the activity of enzyme polyphenoloxidase is responsible for the darkening of the product. Colour observed in dehydrated materials may be due to a non-enzymatic browning reaction which is also known as Maillard reaction and caramelization. These reactions result to the reduction of amino acids and carbohydrates in the raw materials. In this study, the browning reaction took place mainly due to the relatively high protein and carbohydrate content of moringa (Table 1).

Correlation analysis between drying temperature and physicochemical properties

Table 4 shows Pearson correlation analysis between drying temperature and physicochemical properties of moringa leaf. Given that correlation coefficient values (r) higher than 0.6 indicate a correlation, the significance levels are varied depending on the parameters. The results show that drying temperature were significantly negatively correlated (p<0.05) with drying time (r = -0.98), lipids (r = -0.97), calcium (r = -0.98), iron (r = -0.99), vitamin C (r = -0.99), and the loss of vitamin C (r = -0.99) but proportional (p<0.05) to the content non-fiber carbohydrate (r = 0.96). There is a high correlation (above 0.9) between drying temperature and colour but the correlation was positive for a^* value (r = 0.95) and ΔE^* value (r = 1.00) but negative for L^* value (r = -1.00), as an effect of heat treatment. Independently, it was observed that drying time showed inverse relationship (p<0.05) with a^* value (r = -0.97) and ΔE^* value (r = -0.98) but significantly positively correlated (p<0.05) with the lipids (r = 0.97), calcium (r = 0.99), vitamin C (r = 1.00), the loss of vitamin C (r = 1.00), L ^* value (r = 0.98) and b^* value (r = 0.98). It is suggested that varied drying temperature in convection oven are responsible for differences in physicochemical properties which contribute to the quality of of moringa leaf powder.

Conclusions

• In this study, it was observed that Moringa oleifera leaf had a high proximate and micronutrient content. Conventional drying of moringa leaf at 50, 55, 60 and 65^oC significantly influenced descriptors as:
1. Drying air temperature is a significant factor in drying of moringa leaf.
2. Higher drying temperature resulted in a shorter drying time.
3. Different values of physicochemical properties were obtained among the treatments.
   
   
• For practical application, the order of preference of the drying temperature which ensures adequate retention of nutrients of moringa leaf as observed in this study is 50^oC > 55^oC > 60^oC > 65^oC.
  
  
• Further studies are required to determine the appropriate drying techniques as well as pre-treatments for optimizing energy consumption and product quality.

Acknowledgments

The authors gratefully acknowledge the laboratory facility provided by An Giang University. We also acknowledge Mr. Dung, Ms. Loan, Mr. Tho and group of DH18CN students for their technical assistance.

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