Melatonin content in fruits and development of functional fruit tablets

Abstract

Melatonin is a hormone that controls the circadian rhythm in humans and also protects plants against severe stress conditions. Recently, melatonin has been used for immunomodulatory, antioxidant, and anti-cancer health purposes. More importantly, melatonin in plants enters humans through direct consumption and increases circulating levels in the blood. Thus, there exists a great interest in extracting and detecting melatonin present in fifteen Thai fruits. Serotonin and tryptophan contents were also evaluated because of the strong association between their levels and the melatonin biosynthesis pathway. Solid-phase extraction techniques and liquid chromatography-tandem mass spectrometry (LC-MS/MS) analysis were used as detection methods for melatonin and its derivatives in fruits. After that, fruit tablets were produced using three fruit powders with high melatonin, serotonin, and tryptophan. Simplex centroid mixture design was used to prepared mixtures for fruit tablets. Additionally, the physicochemical stability of fruit tablets was studied during six-month of the storage under controlled in either polyethylene or laminated plastic bags with relative humidity of 32 and 75% at temperatures of 4, 25, and 35 °C. The degradation of melatonin in aqueous solutions was also studied to gain preliminary information regarding melatonin pH (pH 1, 4, 7, 10, and 13) and temperatures (60, 70, 80, and 90 °C) stability using liquid chromatography-tandem mass spectrometry (LC-MS/MS).

The results showed that the highest levels of melatonin, serotonin, and tryptophan were detected in mango (2.40 ng/g dry weight), pineapple (68.09 ng/g dry weight) and mulberry (175.53 ng/g dry weight). Increasing pineapple and mulberry percentage significantly raised tryptophan level in tablets, whereas sensory attributes including odor, taste, texture, and overall liking were maximized at high mango percentage. Highest overall desirability of 0.808 was achieved for melatonin (2.16 ng/g dry weight), serotonin (19.02 ng/g dry weight), and tryptophan (21.25 ng/g dry weight) with the liking score of 6.47 for color, 5.97 for odor, 5.91 for taste, 6.63 for texture, and 6.49 for overall liking. Investigation of the physical change of tablets during the storage period showed that both storage conditions and storage time influenced mango tablet properties. A combination of high temperature (35 °C), high relative humidity (75%) and packaging in polyethylene plastic bags significantly increased moisture content and water activity of mango tablets. This led to changes in other properties including decreased diameter, thickness, L*, b* and hardness, and also increased weight, a*, and fracturability. No significant changes were observed in tablets packed in laminated plastic bags (non-permeable to oxygen and humidity) irrespective of temperature and relative humidity. Study on degradation, indicating that the highest remaining melatonin (CR) was observed in the lowest pH solution (pH=1, CR > 65%). Melatonin concentrations decreased with rising pH levels from pH 4 to 13 during storage time. The thermal degradation rate constant of melatonin (k) values obtained followed the order: k90 °C (0.175 hours-1) >k80 °C (0.123 hours-1) >k70 °C (0.082 hours-1) >k60 °C (0.027 hours-1). Thermal degradation kinetics followed the first-order reaction model with a high range of coefficients of determination (0.9744 < R2 < 0.995). Half-life value and D values at 60°C was about 6 times longer than that at 90 °C. z value and activation energy of melatonin in a temperature range of 60 to 90 °C were found to be 3.77 °C and 51.8 kcal/mole respectively. While the highest Q10 value was observed within the heating temperature range of 70-80 °C.

Our findings demonstrated that the fruits containing high melatonin (mango), serotonin (pineapple), and tryptophan (mulberry) are a potential source for functional food products. The developed tablet presented high active compounds with good sensory acceptability using only mango powder. The packaging was a key factor as material properties related to water vapor transmission rates and water absorption of the tablets that directly affected other physical and chemical qualities. Moreover, melatonin was most stable at low pH (pH1) and low temperature (60 °C).

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