Mao, Yujie
(2020)
Understanding the extraction of pectin from food waste.
PhD thesis, University of Nottingham.
Abstract
Food waste of multi tonne scale every year is wasted throughout the food supply chain (FSC) from initial agricultural production down to final household consumption or food manufacturing industries. According to the Food and Agriculture Organization of the United Nations (FAO) data, approximately 1.3 billion tones of food (that is roughly one-third of total food produced for human comsuption) is wasted globally (Gustavsson et al., 2011); and in example of the European Union, just under 90 million tonnes of food is wasted each year, 38% of which is directly produced by the food manufacturing sector (Commission, 2010). Therefore, the need to both avoid waste and find new renewable resources has led to a new and promising research avenue: the use of food supply chain waste (FSCW) as a renewable biorefinery feedstock for valorised chemicals production (Pfaltzgraff et al., 2013).
Pectin is one good example that can be valorised, and the extraction of pectin was studied in the present thesis. Traditionlly in the food industry, pectins are usually manufactured by hot acid method, a conventional solvent extraction (CSE) process that uses a mineral acid at pH 1 - 3 and 80 - 90 oC (Ralet, Bonnin and Thibault, 2002). This process yields homogalacturonan (HG) pectin, but destroys rhamnogalacturonan-I (RG-I) pectin (Fraeye et al., 2007) and produces significant volumes of acidic waste. To enable the development of pectin products with different product requirements, many alternative extraction methods have been proposed and studied. They can be divided into two approches: 1). investigating the use of alternative solvents, for example chelator, alkali, water and the use of enzyme (enzyme-assisted extraction (EAE)); 2.) investigating alternative heating methods, for example ultrasound-assisted extraction (UAE), microwave-assisted extraction (MAE) and microwave-assisted hydrothermal extraction (MAHE) under pressure etc..
In the present thesis, the effects of different solvents on pectin extraction has been investigated. Using micronised sugar beet pulp, it is found that low pH acids and high pH alkali can both achieve high pectin yields but the structure of exracted pectin varies that acid pectin has high HG content while alkali pectin has high RG-I content. Chelator can also achieve high yields but due to chelator itself remaining in the extract, which suggested that chelator may not be suitable for pectin extraction. Water although only yields limited amount of pectin compared to other solvents, it helps to preserve both HG and RG-I structure. While the use of different solvents affects the quantity and quality of pectin extracted, it does not affect the optimum extraction time.
The use of CSE and MAE were also compared for pectin extraction in this thesis. A review of the literature suggests MAE can dramatically enhance and/or accelerate pectin extraction compared with CSE. However, much of this work was not performed using a direct comparison of heating methods; in other words one or more other parameters (such as reactor volume and heating rate) were varied, potentially confounding the results. The work presented in this thesis overcame this limitation, particularly with the control of heating rate in heating methods as emphasised in the work conducted by Galan et al. (2017). Using various biomasses, we found no difference in MAE and CSE in terms of optimum extraction time, extract yield and quality using micronised sugar beet pulp, banana peel and carrot pulp; however, using the similar experimental setting, MAE has been found better than CSE using of orange peel, mango peel and apple pomace. We hypothesise that this is because the use of those acidic biomass samples results in a different heating rates between MAE and CSE, which can be explained by the knowledge of dielectric properties and thermal conductivity of the studied solvent-biomass systems.
It is hoped that the work in this thesis can firstly help with the understanding of the effect of processing conditions on the yield and purity of pectin extracted, with a view to informing process selection for the different requirements and production of novel pectin-based products. Secondly, it can help to propose an explanation for the differences in extraction results as a function of feedstock type and to understand the mechanisms of microwave heating on biomass materials in a fundamental way that very little research has looked into to date. Finally, this thesis tested a preliminary scaled-up microwave-assisted extraction rig design for pectin extraction, which was one of the very few continuous systems in this research area and therefore has brought a big step forward in the development of novel based pectin products.
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