1. Introduction
Stingless bee honey (SLBH) is an emerging functional food due to its many health benefits. SLBH is rich in flavonoid and phenolic content, which contributes to its high antioxidant activity
[1]. The common phenolic compounds in SLBH are the same as
Apis mellifera, such as salicylic acid, p-coumaric acid, caffeic acid, chlorogenic acid, ferulic acid and quercetin
[2]. Despite its benefits, SLBH has generally higher moisture content than
Apis spp.
[3]. A previous study showed that the moisture content in SLBH is the highest with 33.24%, compared to
Apis spp. which is between 21.96–27.41%
[4]. SLBH of
Melipona spp. has a moisture content above 24.8% compared to
Apis spp. with 18.6%
[5]. A comprehensive review reported that SLBH contains more moisture (21.52–31%) than Tualang and Gelam honey (17.53–26.51%)
[6]. Furthermore, SLBH has a high water activity of 0.76 compared to a range between 0.60–0.67 in
Apis spp.
[4].
The high moisture content in SLBH makes it more susceptible to alcoholic fermentation contributing to honey acidity
[7]. The rapid fermentation by microorganism growth in SLBH leads to honey spoilage
[8]. Apart from high water content, SLBH has higher free acidity, electrical conductivity and lower diastase activity compared to
Apis spp.
[3]. Hence, it is difficult for SLBH to follow the honey standard. Several studies have proposed a different standard for SLBH given the difficulty for SLBH to follow the International Honey Commission (IHC) standard
[9].
Owing to the high water content, it is a challenge to maintain the quality of SLBH. Therefore, dehydration of SLBH upon collection is suggested to lower the moisture content. Microbial stability can be achieved through dehydration, thus, prolonging the shelf life of honey
[8]. Additionally, lowering the moisture content will help the SLBH adhere to the standard. However, previous studies have shown that the dehydration process can reduce the phenolic content
[10]. In addition, thermal treatment can increase hydroxymethylfurfural (HMF) content in honey
[11]. The phenolic content is an essential source of antioxidants in honey
[1]. Meanwhile, HMF is a potential carcinogenic and genotoxic agent
[12]. Therefore, a suitable dehydration method is needed to obtain the maximum benefit from SLBH by reducing its moisture content without compromising its phenolic content and ensuring a safe level of HMF.
Globally, almost 500 species of SLBH are distributed in South America, Africa, Australia and Southeast Asia
[13]. Despite the numerous species, the most commonly domesticated stingless bee honey
s by beekeepers worldwide are from
Melipona and
Trigona genera
[14]. To the best of the knowledge, there are limited publications on the dehydration of SLBH that provide information on the changes in its physicochemical properties.
RThis re
searchersview aims to provide an overview of the available information on the physicochemical properties of SLBH before and after the dehydration process. This will help to determine the most optimal setting and method of dehydration for SLBH without compromising its benefit. The physicochemical information retrieved includes moisture content, water activity, pH, free acidity, hydroxymethylfurfural (HMF), ash, electrical conductivity, diastase, sugar content, total soluble solids, total phenolic content and total flavonoid content.
2. Physicochemical Properties of Dehydrated Stingless Bee Honey
2.1. Moisture Content
Moisture content is the amount or percentage of water present in the honey
[15]. Water in honey is the key factor for honey quality as it determines the ability of honey to resist spoilage by microorganism fermentation
[16]. Previous studies presented in
Table 1 showed that the percentage of moisture content of raw SLBH was between 23.9 and 40%. However, another study showed that the moisture content of raw SLBH was between 13.26 and 45.8%
[9]. The wide range in the percentage of moisture content was due to environmental factors such as seasonal weather and humidity
[8]. Harvest and storage conditions also influenced the moisture content in SLBH
[9].
Table 1. Moisture content of raw and dehydrated stingless bee honey (SLBH).
Several studies summarized in
Table 1 showed that reduction in the water content of the SLBH after harvesting could be achieved either by increasing the temperature through various dehydration methods or via passive diffusion. The temperature used in the dehydration process was between 30 and 95 °C, while the temperature for the passive diffusion method was between 25 and 35 °C. As a result, the moisture content of SLBH was reduced between 29.6 and 5% after the dehydration process using these various methods. Moisture content below 17% could prevent the fermentation process by the microorganisms
[26].
As
a conclusion, according to the data presented in
Table 1, the dehydration process using thermal treatment will only cause less than a 10% reduction in water content. Meanwhile, another study showed that the thermosonication method of the dehydration process caused a 16.6% reduction in water content compared to 6.9% using the thermal method
[19]. These findings suggest that thermosonication is the better method for the dehydration process for SLBH compared to thermal treatment. However, both methods could not reduce the moisture content below 17% (25.9% for thermosonication and 28.8% for the thermal treatment method).
A study showed that the moisture content of the SLBH was reduced from 31.9 to 11 and 5% after the dehydration process using vacuum and freeze-drying methods
[20]. As presented in
Table 1, the vacuum drying and freeze-drying at 5% moisture setting could achieve an 84.3% reduction in water content. Meanwhile, a 65.5% reduction in water content of the SLBH was observed after dehydration using vacuum drying and evaporation at 11% moisture setting. These findings suggest that both vacuum treatment and freeze-drying are the best methods in reducing the moisture content of SLBH. In addition, both methods could achieve a safe level of moisture content below 17%.
A study by Yegge et al.
[21] showed that the dehydration process using microwave heating and dehumidification methods could reduce water content by up to 52% and 45%, respectively, as presented in
Table 1. In the study, both methods could reduce almost half of the water content in raw SLBH. The microwave heating method used a power level of energy (PL) of 20, 60 and 100. However, only the microwave heating method at 60 PL for 60 s could reduce the moisture content below 17% (from 31.47 to 15.04%). Meanwhile, the dehumidification process was performed for 1 to 2 days. Therefore, microwave heating at 60 PL for 1 min was the best method for achieving the recommended moisture content level below 17%. In addition, this method was more practical because it takes less time to prepare the dehydrated SLBH.
From the data provided in
Table 1, a previous study has also shown that the dehydration process of SLBH using a food dehydrator could reduce the water content of SLBH up to 80–100%
[22]. The food dehydrator could achieve 80% water reduction at 40 °C for 36 h or at 55 °C and 70 °C for 18 h. Complete water reduction was achieved at 55 °C and 70 °C by prolonging the duration of the dehydration process to 36 h
[22]. Another study showed that a dehydrator developed by the Malaysian Agricultural Research and Development Institute (MARDI) could reduce 35% of the water content
[23]. However, the MARDI dehydrator set at 30 °C for 8 h was unable to reduce the moisture content below 17%
[23]. Meanwhile, the conventional food dehydrator set between 40 and 70 °C for the duration of 18 to 36 h could achieve recommended moisture content level below 17%
[22]. These findings suggest that a higher temperature would result in a higher reduction in moisture content.
Several studies summarized in
Table 1 showed that the dehydration process of the SLBH can be performed via passive diffusion by storage in a clay pot. A study by Ghazali et al.
[24] showed that the reduction in the moisture content was significant in the clay pot compared to the glass container. In addition, the storage in a clay pot with a larger surface area resulted in a 10.9% reduction in water content compared to a smaller clay pot with only 7.21%. This finding suggests that the larger the surface area of the container, the more effective the passive diffusion process will occur. On the other hand, the storage of SLBH at 35 °C for three days could reduce up to 24.2% of water
[25]. Meanwhile, the storage of SLBH at room temperature (25 °C) for 21 days could reduce water content by up to 29.8% content
[25]. These findings suggest a higher temperature would expedite the passive diffusion process. However, the dehydration process via passive diffusion requires a long duration to reduce the moisture content of the SLBH. Furthermore, the moisture content after storage in the clay pot was between 18.13 and 25.13%, which was still above the recommended moisture content level at 17%.
Various dehydration methods of SLBH can reduce moisture content depending on the temperature and duration of the dehydration process. Researchers concluded that the higher the temperature setting, the more reduction in water content. For that, researchers suggest the dehydration method of SLBH at a high temperature setting to achieve at least less than 17% moisture content to retard the fermentation process. The methods that yield low moisture contents are vacuum treatment, freeze-drying, food dehydrator and microwave heating at 60 PL. In conclusion, researchers observed that the food dehydrator is the best method because it could remove up to 80 to 100% water content, resulting in moisture content of less than 17%. However, it takes up to 18 to 36 h in duration. Therefore, microwave heating at 60 PL is the method of choice due to the short duration of 60 s with the moisture content of less than 17%. Although the vacuum treatment could reduce the moisture content to 5 and 11%, the duration of the dehydration process was not mentioned by the authors.
2.2. Water Activity
Water activity is a measurement of free unbound water that can be utilized by microorganisms for growth
[27]. Water activity gives a better prediction of the likelihood of the fermentation process occurring compared to moisture content
[16]. Therefore, water activity is used as an indicator of food stability, which is important for the determination of honey spoilage due to microbial growth
[4]. Water activity (aw) is expressed in decimals and calculated from equilibrium relative humidity (ERH) divided by 100 (aw = ERH (%)/100)
[28]. ERH is the equilibrium of humidity of the food product with its environment.
Microorganisms will not grow below a particular water activity level, which is 0.90 for bacteria and 0.70 for molds. A water activity of less than 0.6 will halt all types of microbial growth
[27]. Hence, it is crucial to maintain the water activity of SLBH below 0.6. Water activity is strongly correlated with moisture content
[16]. Therefore, the dehydration process of SLBH is needed to reduce moisture content and water activity in SLBH. Subsequently, the dehydration process will help to prevent the likelihood of fermentation due to the inability of the microorganism to grow in the SLBH. A previous study reported that SLBH has the highest water activity of 0.76 compared to
Apis spp. and commercialized honey with water activity ranges between 0.54–0.67
[4]. Several studies compiled in
Table 2 showed that the water activity of raw SLBH was between 0.79 and 0.807. After the dehydration process, the water activity was reduced between 0.28 and 0.785 as presented in
Table 2.
Table 2. Water activity of raw and dehydrated stingless bee honey (SLBH).
|
- |
|
- |
- | - | ↑ | ↑ | ↑ |
Freeze-drying
(5% moisture) | −54 °C | 24 h | <17 | 84.3 | <0.6 | 9.29 | NC | NC | - | - | - | - | ↓ | ↑ | ↑ |
Microwave heating | 60 PL | 60 s | <17 | 52 | - | - | NC | - | - | - | - | ↑ | - | ↑ | - |
Dehumidification | 35 °C | 2 days | >17 | 45 | - | - | NC | - | - | - | - | ↑ | - | ↑ | - |
Food dehydrator | 55 °C | 18 h | <17 | 80 | <0.6 | <5.81 | - | - | - | - | NC | - | - | ↑ | - |
MARDI dehydrator | 30 °C | 8 h | >17 | 35 | - | 2.39 | - | - | - | - | - | - | ↑ | ↑ | - |
Clay pot storage | Large surface area | 25 °C | 10 days | >17 | 10.9 | >0.6 | - | NC | NC | ↑ | - | - | ↑ | ↑ | - | - |
| 35 °C | 3 days | >17 | 24.2 | >0.6 | - | ↑ | NC | NC | ↑ | - | ↑ | - | - | - |