Investigating Polyphenol Extracts Using Microwave Technology

Abstract

The objective of this thesis was to investigate phenolic extraction from grape skins using microwave technology and to add to the active research on phenolics in grapes. A research scholarship was awarded by the Grape and Wine Research and Development (known as Wine Australia) during 2010 – 2015. Phenolics play a dual, friend or enemy, role in the wine and grape industry. Due to their antioxidant capacity, phenolics present in foodstuffs are understood to protect from chronic diseases caused by free radicals and lipid peroxidation and are widely used for nutritional and medicinal applications. On the other hand, that same antioxidant capacity is known to prevent efficient degradation in landfill of wine management waste (pomace/marc). Finally, phenolics affect the flavour, mouthfeel and colour of wines, and the modern wine industry is experimenting freely with the incorporation of phenolics in red and white wines to develop novel wines. Grapes are a rich source of phenolics including those responsible for the skin colour. Grape extracts can be used for manufacturing nutraceuticals, including natural colourants, and functional foods, skincare and textile dyes as well as incorporated into Cleaner Production and Circular Economy (as opposed to the linear economy, where raw materials are used, products made, and wastes dumped) in waste management. The extracts can be obtained from primary biological material (in particular, when grapes are in over-supply) as well as from the postwinemaking waste (non-fermented pomace/post-fermentation marc), the latter helping in reducing the amount going to landfill. There has been growing interest recently in finding more time and energy efficient and environmentally friendly methods of phenolic extraction. Microwave-assisted technologies offer such an opportunity. The extraction process can be optimized to achieve the maximal phenolic extraction for a particular cultivar. This study focusses on Australian commercial grape cultivars. The thesis is structured as follows. Chapter 1 presents an extensive literature review on different extraction methods, including conventional and microwave ones. Chapter 2 describes the microwave-assisted extraction (MAE) method, employing an industrial type of microwave reactor, which was compared with the conventional thermal extraction (CTE) method under the parameters producing the maximal phenolic extraction by each method. Multifactorial models were built to find optimal conditions to extract maximum total phenolics (TP) and colour (CIELAB chroma C*) from separate white and red skin mixtures, each of six commercial cultivars collected at veraison and harvest. A high throughput Folin- Ciocalteu assay was developed for this study to measure TP. Adding novelty, and contrasting with the studies reviewed in Chapter 1, the microwave-assisted and conventional extraction methods were parallel-optimised by response surface methodology. The MAE and CTE protocols were set to minimise the method-to-method experimental error. It was found that the efficiency of the two TP extraction methods was comparable regardless of skin colour. The CTE provided more colour than MAE for white skins, and the same amount of colour for red skins. It was shown that the MAE method was 2.7-fold more energy efficient and 15-fold faster for a single extraction than the CTE method. The chapter discusses why the MAE methodology may be relevant for future trials at laboratory, pilot and production scales on the processing of winemaking wastes (typically, up to 30% of the mass of crushed grapes) to reduce the landfill component. This approach would follow the Circular Economy tenets. This work has been published in Volume 251 online since 5th of Dec. 2019 by the Journal of Cleaner Production. Chapter 3 compares the performance of the MAE and CTE methods in extracting TP and C* from the skins of the same individual cultivars as were used in the mixture extractions in the study in Chapter 2. The extractions were made under the optimal practical conditions for the skin mixtures. This comparison also included the spectral absorbance profiles at 280-600 nm. The variation in the phenolic extraction from the grape skins of individual cultivars collected at veraison and harvest were quantified for the first time in the literature (to the best of our knowledge). The average performance of extraction from individual skin collections was in agreement with the extraction expected for the mixture under the conditions set. Interestingly, it was noticed for the red skins only under the CTE method that the average TP extracted from individual cultivars (veraison and harvest combined) was 5% less than the TP obtained for the same red skins mixture, suggesting a potential synergy effect. The chapter presents a quantification of possible varietal and seasonal phenolic extraction. The results may be useful in oenology and industrial waste management. The manuscript was submitted on 23rd of Feb. 2020 to the Journal of Cleaner Production as a sequel to the previous study. Chapter 4 investigates the applicability of a fast and simple attenuated total reflectance midinfrared (ATR-MIR) spectroscopy method for the evaluation of TP and C* of the white and red skin mixture extracts analysed in Chapter 2. This study was motivated by the observation that, although the Folin-Ciocalteu assay using 96-well plates and a liquid delivering robot was a high throughput method, it was still laborious and not likely to be attractive for the industry. Partial least square (PLS1) regression models were built between the reference TP as well as C*of the extracts and their raw and second derivative spectra. The full range of 4000-400 cm- 1 and sub-ranges of 4000-1100 cm-1, 1500-400 cm-1, 1500-1100 cm-1 and 1457-1168 cm-1 were examined. (The last two ranges are used as phenolic fingerprint ranges.) It was found that the PLS1 models based on the second derivative of the 4000-1100 cm-1 sub-range provided the best results (fair classification) for both TP and C*; hence these models are recommended for screening purposes. Moreover, some spectra ranges showed noticeable differences in absorption of phenolics and the compounds responsible for the colour of white and red skins. The study states that the ATR-MIR is worth considering for future trials in food industry and waste management. The manuscript has been accepted for publication on 6th of Jan. 2020 by Food Analytical Methods. Chapter 5 presents the executive summary of the key research findings for this thesis, emphasising the novelty and potential benefits to manufacturing of nutraceuticals (including natural colourants), dietary supplements and non-alcoholic products, skincare, textile dyes, as well as to the wine industry and waste management. Future research directions are identified as including ATR-MIR assessment for quick optimization of the extraction process in industrial settings.