Exploring the influence of sterilisation and storage on some physicochemical properties of coconut (Cocos nucifera L.) water

Coconut (Cocos nucifera L) water, also referred to as coconut juice, is a refreshing natural drink common and mostly consumed in the tropical regions of the world [1, 2]. It is a clear, colourless, sweet, naturally flavoured slightly acidic drink, with reported pH ranging between 4.2 and 6.0 [3, 4].

Over six decades of research has shown that coconut water contains proteins, fats, and is rich in carbohydrates and nutritionally important elements (potassium being the most abundant) [3–7]. It is also a rich source of essential amino acids (lysine, histidine, tyrosine and tryptophan), fatty acids, glucose, fructose, cellulose, sucrose, and organic acids such as tartaric, citric and malic acids [7–10].

Coconut water's rich enzyme systems include very effective and selective reductase [1], polyphenol oxidase (PPO) and peroxidase (POD). These are involved in its development of a brownish colour when it is exposed to air for a long time [11]. Based on its content and properties, coconut water has been used in the treatment of child and adult diarrhoea, and gastroenteritis as well as for urinary stone dissolution, short-term intravenous hydration and protecting against gastrointestinal tract infections [12].

While still in an undamaged fruit, coconut water remains sterile and stable, but it may become unstable when extracted from the fruit and stored for a few days at 4°C. In Ghana, it is commonly available in the coastal areas and mostly consumed fresh and directly from the fruit. However, the conduct of its vendors and consumers exposes it to a high risk of contamination with bacteria, specifically coliforms.

Assessing the risk of bacteria infections through the consumption of coconut water is made a difficulty since there are limited reports that show the survival and growth of pathogenic bacteria in coconut water. Considering the risk of bacteria contamination of coconut water in Ghana, the possibilities of survival and growth of bacteria in coconut water and the potential for the use of coconut water as a bacterial growth media in resource-limited countries/laboratory like ours, we initiated this study. We studied some characteristics of coconut water obtained from locally grown crop. We also investigated how these change with autoclaving, gamma irradiation and storage. We report here, changes in colour, turbidity, dry matter, pH and UV absorbance as well as the influence of temperature on the visible light absorbance of coconut (Cocos nucifera L) water after autoclaving, gamma irradiation and storage at 4°C. An assessment of the possibility of measuring bacterial growth by measuring the turbidity and dry biomass of bacteria cultures in fresh, stored or sterilised coconut water was also presented.

The observed sterility of the extracted, sterilised and stored coconut water imply the changes observed for the fresh, stored and sterilised coconut water were not due to contaminating microbes as was the case in the study by Chowdhury et al., [14] who also studied coconut fruits obtained from the local market, with no data on the age of maturity.

Although the maturity of the fruit could not have been determined accurately, the thickness of the endosperm were within the range (between 4 and 10 mm) reported for 12 month old matured coconut fruit by Santoso, et al., [8]. The volumes of coconut water (266.7 ± 16.7 mL) were lower than those reported (350 mL to 500 mL) by Campbell-Falck et al., [7] and Santoso, et al., [8]. These variation in volume of coconut water, as well as variations in enzyme activity, pH and nutritive values have been reported to be due to differences in plant variety, locality of cultivation and the age of maturity (before and after eighth month), [15, 16].

The observed differences in colour and turbidity (Figure 1) were confirmed in the visible spectra (Figures 2A-C). When exposed to oxygen, polyphenol oxidases catalyse the hydroxylation of monophenols, such as in tyrosine (present in coconut water), to O-diphenols; these are further oxidised to O-quinones and semiquinone, which polymerised to different extents, leading to the yellow, orange or brown colours [11, 16–18]. The added heating to 60°C or above enhances the activity of the enzymes (PPO and POD), which is reduced/lost at temperatures above 90°C [11]. Such colour changes (due to PPO and POD) have been reported for Mueller-Hinton broth after autoclaving [19].

The change in colour after gamma irradiation is explainable on similar bases, since irradiation with gamma rays (ionizing radiations) leads to the formation of hydroxyl phenal radicals (phenol side group of tyrosine and gallic acid) [20] which then serves as the precursor for the coloured O-quinones.

The visible light spectra after two weeks of storage (Figure 2B) also indicate changes in colour and turbidity of fresh, autoclaved and irradiated coconut water. Similar observations were made by Puchakawimol et al., [18] where fresh coconut water became orange-yellow in colour and increased in the saturation of colour on the later days of storage at room temperature. Also, Chowdhury et al., [14] showed that after one month of storage at 0°C, fresh coconut water changed in colour and flavour, increased in pH and turbidity with the formation of gas in addition to being contaminated with fungi.

These observations suggest that the reactions of polyphenol oxidase and peroxidase, and the formation of the insoluble matter continued during storage at 4°C. The decrease and subsequent increase in colour intensity of autoclaved coconut water after 2 weeks and one month respectively, suggests that polyphenol oxidase and peroxidase (which catalyse reversibly, both oxidation and reduction reactions [11, 18, 16, 21]) in response to the reduction and increase in oxygen (due to storage in an air tight container for two weeks, and after opening the air tight container respectively) converted the chromophore and its precursors reversibly. However, the continuous increase in the turbidity and absorbance above 540 nm (Figure 2B and 5), suggests that the reactions forming the insoluble matter and colour were still occurring and that the enzymes involved where also heat resistant.

The increase in colour and turbidity of irradiated coconut water for two weeks with no significant increase after an additional two weeks needs further investigation for a clearer understanding, since it is generally accepted that the activity of POD and PPO are not significantly affected by low dose (about 5 kGy) gamma radiations [22]. However, Frylinck et al., [23] detected a partial inactivation of these enzymes in mango fruit after low dose gamma irradiation. Therefore, we ask, is it that in this study of coconut water only small quantities of phenal radicals were produced after irradiation? Or is it that the activities of these enzymes were greatly reduced or lost after irradiation? A study that will measure the activity of these enzymes and estimate the changes in the concentration of radical in fresh and irradiated coconut water before and at regular time intervals during storage may help to throw more light on this.

The results of the dry matter (Figure 3 and table 1) correlated with the changes in the absorption spectra between the wavelength 560 nm and 600 nm (Figure 2A-C), as an indication of turbidity. Autoclaving resulted in a decrease in dry matter by 1.1%, seen as a lower absorbance of autoclaved coconut water compared to that of fresh coconut water (Figure 2A). Conversely, irradiation resulted in an increase in dry matter (Figure 2A) and an associated increase in absorbance in that same range. Similar correlations were observed after storage. (Figure 2B and 2C; table 1).

The results presented as Figure 4 demonstrate the changes in UV absorption of coconut water due to a combination of sterilisation and storage. Since coconut water is a mixture and/or a solution of various biomolecules, including aromatic amino acids (tyrosine), gallic acids, flavour giving aromatic hydrocarbons, biphenal products of the enzymes POD and PPO and others as reported by Yong et al., 2009 [24], the significance of the changes in UV absorption in relation to its biochemical and nutritional components cannot be evaluated with this data. However, these changes provide evidence for the occurrence of both physical and biochemical reactions (to different extents) after sterilisation and during storage at 4°C. The interactions between biomolecules have been shown as one of the physical processes by which the UV spectra of a molecule may be changed [25]. On the other hand, the reformation of aromatic system via proton and electron rearrangement or perturbations as a result of exposure to radiations, are some of the chemical process that influence and change absorption in the near UV region [26].

The reduction in pH, as was observed after autoclaving, irradiation and storage (table 2) was a result of the formation of the phenol radicals and oxides, leading to the formation of the yellow colour. Stored fresh coconut water experienced the greatest change in pH probably because its POD and PPO activities were not reduced (as was expected after autoclaving and gamma irradiation) and therefore contributed to a higher release of protons.

The temperature profile (Figure 5A and 5B) indicates that the changes in absorbance at 385 nm (indicative of colour) and 540 nm were linearly related to an increase in the temperature of stored coconut water (R ² of 0.87 and 0.93 respectively, p < 0.05). These data indicate that the reactions leading to the formation of the yellow colour were increasingly activated by the increase in temperature up to 99.8°C.

The reduction in the total carbohydrate of fresh coconut water during storage (Figure 6) was the result of reactions that use sugars and other simple carbohydrates in the formation of the insoluble matter, (the base substance of the coconut endosperm) which resulted in the increase in turbidity (Figure 1). Autoclaving, with its associated high temperature and pressure, is known to reduce nutritional value of media through the activation of a myriad of reaction involving amino acids, sugars, proteins and other carbohydrates [27, 28]. This, most likely resulted in the reduction of its total carbohydrate (Figure 6). Gamma irradiation leads to the formation of reactive radicals with little potential of being involved in reactions such as those associated with sugars, as such, a marginal reduction in total carbohydrate was observed for Irradiated coconut water.

Considering the changes observed thus far, it would be convenient to measure the increase in bacteria cells (growth) by measuring the dry biomass or turbidity of bacteria culture in fresh, autoclaved or gamma irradiated coconut water with out storage. This is because prior to storage, all of the three coconut water samples recorded low turbidity, low dry matter (< 1.5 mg/mL) and a wavelength of maximum absorbance of less than 600 nm. These would not obscure small changes in absorbance/turbidity of a growing bacteria culture, measured at wavelengths equal to or greater than 600 nm. However, for both stored fresh and gamma irradiated coconut water, the high and increasing turbidity and dry matter content (> 1.5 mg/mL) during storage imply possible difficulties in recording small changes during the measurements of turbidity or dry biomass (particularly at the lag phase) of a growing bacteria culture. The same possibility of inconsistencies and non-reproducibility is expected with autoclaved coconut water because of the decrease and subsequent increase in its turbidity and dry matter despite that these were low.

Sterilization of fresh coconut water by either autoclaving or low dose gamma irradiation led to the development of an intense or pale yellow colour respectively. Also associated with these, were different extents of variations in turbidity, dry matter and pH. The storage of fresh coconut water at 4°C for both two and four weeks resulted in the development of an intense yellow colour and a striking change in turbidity, dry matter and pH.

The use of turbidity measurement for measuring the growth of bacteria in fresh, autoclaved and gamma irradiated coconut water is practicable without any possibility of interference by the very low turbidity and bright colours of these different coconut waters. However, this may result in consistencies and, or non-reproducible data if stored fresh, stored autoclaved or stored gamma irradiated coconut water were used.