Effect of drying on the kinetics, stability of active ingredients and non- destructive testing of Backhousia citriodora (lemon myrtle) dehydrated leaves

Abdul Kahar, Ainaa (2021) Effect of drying on the kinetics, stability of active ingredients and non- destructive testing of Backhousia citriodora (lemon myrtle) dehydrated leaves. PhD thesis, University of Nottingham Malaysia.

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The dehydration process of lemon myrtle leaves (LML) in Malaysia is still conducted by using conventional method of drying that takes long duration thus decreasing the quality of the dried LML. To prolong the shelf life, LML has to be dried to reduce its moisture content. The present research highlighted the drying kinetics, application of heat pump drying on LML and the application of non- destructive testing on moisture content analysis of dried LML. In the present study, LML were dried by OD (40 to 60°C), VD (40 to 60°C at 50 mbar) and HPD (45°C). Both engineering properties (drying kinetics, effective moisture diffusivity and activation energy) and quality properties (colour, biochemical content and volatile content) of all dried LML were assessed and evaluated.

All drying methods only exhibited falling rate period, indicating that the drying was governed by the movement of internal moisture to the surface for evaporation. The moisture diffusivity (Deff) was found to vary in range of 8.07 x 10-10 to 4.53 x 10-9 m2/s for all drying methods and conditions. The activation energy (Ea) was obtained and the values were 13.42 kJ/mol, 45.41 kJ/mol and 72.85 kJ/mol for HPD, VD and OD, respectively. It was found that the drying air velocity of 2.0 m/s (FR2.0) in the HPD was the suitable drying condition as it gave minimum colour changes and the highest retention of volatiles which was recorded at 89.5%. The total phenolic content (TPC) and antioxidant activities of samples drying at 2.0 m/s also showed high retention, recorded at 74%, 95% and 80% for TPC, 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity and ferric reducing antioxidant potential (FRAP), respectively. It was found that HPD sample gave the highest value of greenness (a*) with the lowest value of total colour difference and browning index. For biochemical content, VD samples showed high retention of TPC, DPPH and FRAP assay followed by HPD samples, while OD samples showed the lowest biochemical content for all drying conditions. The essential oil of the dehydrated LML subjected to OD50, VD50 and HPD was extracted by using simultaneous distillation and extraction (SDE) method and analysed for its volatile compounds by using Gas Chromatography-Mass Spectrometer (GC- MS). HPD samples showed the highest retention of volatiles compounds especially cis- and trans-citral with a total concentration of 89.5%.

Dehydrated LML were packed under four different conditions, normal and vacuum packaging (N and V), each of the packaging was placed at room temperature (25°C) or chilled condition (4°C) (RT and CH) stored for a period of 6 months. It was found that the vacuum packaging yielded higher retention of colour and biochemical content than the non-vacuum packaged LML. This approach combined with storage at lower temperature (4°C) resulted in a better retention of green colour and higher percentages of TPC, DPPH and FRAP (49%, 72% and 56% respectively) for HPD samples.

Partial least squares regression (PLSR) and cross-validation modelling were used to explore the feasibility of these spectroscopic techniques over three types of prediction models which were full spectral ranges of near infrared (NIR) and dielectric measurement (DM), selected spectral range of NIR and fusion of both non- destructive methods. NIR model gave the highest coefficient of determination, r value of more than 0.99 and ratio performance to deviation (RPD) value of 3.16 that indicated excellent prediction of moisture content in the dried LML. The selected spectra analysis of NIR spectra showed no improvement in PLSR results. Whereas, fusion of the non-destructive models showed improvement of the r, root mean square error and RPD values especially for DM method.

The effect of drying on the stability of active ingredients of Backhousia citridora (lemon myrtle) dehydrated leaves was investigated and discussed in this study. HPD was found to be a suitable method for LML dehydration, as it resulted in the highest retention of volatiles compound, greenness and antioxidant values after drying and after 6 months storage. The high quality of dehydrated LML subjected to HPD ensures the market acceptability as well as its functionality. Therefore, this study suggested that HPD with drying air velocity of 2.0 m/s could be used as a proper dehydration process for LML that preserve its functionality for the applications of nutraceuticals, cosmeceuticals and pharmaceuticals.

Item Type: Thesis (University of Nottingham only) (PhD)
Supervisors: Law, Chung Lim
Ong, Sze Pheng
Watson, Nicholas J.
Keywords: lemon myrtle leaves, heat pump drying, volatile content, citral, colour change, antioxidant, non-destructive techniques
Subjects: T Technology > TP Chemical technology
Faculties/Schools: University of Nottingham, Malaysia > Faculty of Science and Engineering — Engineering > Department of Chemical and Environmental Engineering
Item ID: 65684
Depositing User: Abdul Kahar, Ainaa
Date Deposited: 04 Aug 2021 04:43
Last Modified: 04 Aug 2021 04:43
URI: https://eprints.nottingham.ac.uk/id/eprint/65684

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