Easy Peasy Lemon Squeezy – An Eco-Friendly Process for Pectin and Essential Oil Extraction From Lemon Peels

By Alex Waked, Member-at-Large for the GCI

Industrial scale chemistry is not typically given much thought by most chemists in academia. But if the end goal is to produce our products for eventual commercial use, then why not design our syntheses and processes at the beginning to ensure that the scaling up will be smooth?

Fidalgo et. al. recently published a paper that caught my eye, in which they describe a scalable eco-friendly process for the simultaneous extraction of pectin and the essential oil d-limonene.¹ Pectin is a heteropolysaccharide that has found use in a wide variety of products. It can be used as a thickening agent in jams and shampoos,² in the medicinal field in wound-healing preparations, and has been shown to reduce blood cholesterol levels.³ In 2013, the global market for pectin reached $850 million.⁴ In a few words, it’s a valued, versatile product.

Pectin is contained in plant cell walls, and is extracted from citrus peel (such as lemons and oranges) traditionally by a water extraction method. This method involves heating the citrus peel for several hours under acidic conditions, filtering off the solid residue, concentrating the filtrate, and finally precipitating the pectin by addition of alcohol. A couple drawbacks include the large amount of acid waste and the excessive heating of the peel, which degrades the pectin as well as being energy intensive.

Figure 1. Microwave hydrodiffusion and gravity apparatus [5]

In this paper, the authors used two innovative methods to obtain pectin from lemon peels (the pectin obtained from both methods have slightly different properties which I won’t go into, but if you’re curious I encourage you to take a look at the paper!). The first method includes adding water to lemon peels, doing a microwave hydrodistillation (which is simply a distillation using microwave heating), separating the essential oil from the residual water, and finally freeze-drying the water to obtain pure pectin. The second method involves a technique called microwave hydrodiffusion and gravity,⁵ where the lemon peels and water are heated using a microwave source and the residual liquid that is expelled by the heating is passed through a filter and condenser to be collected (Figure 1). The collected aqueous solution is then freeze-dried to obtain pure pectin.

The first method was employed to test whether this process would be compatible with kilograms of material. It turns out that 20 kg of waste lemon peels produces 3 kg of pectin and 10 mL of essential oil, where 36 L of water was used (Figure 2). To put these numbers in perspective, common yields for pectin from the more conventional extraction methods are only roughly 3% of the peel weight – so 20 kg of lemon peels would produce 0.6 kg of pectin.

Figure 2. The semi-industrial scale extraction process presented in the paper [1]

So let’s take a look at some of the positive takeaways from this paper: 1) Significantly better yields of pectin were obtained compared to the current conventional processes; 2) Microwave heating (which is the only energy source in the processes) requires less time than normal heating, meaning less degradation of pectin and lower energy usage; 3) Water was the only solvent used, and; 4) This was the first reported simultaneous extraction of pectin and essential oil by an environmentally clean process.

References:

(1) Fidalgo, A.; Ciriminna, R.; Carnaroglio, D.; Tamburino, A.; Cravotto, G.; Grillo, G.; Ilharco, L. M.; Pagliaro, M. ACS Sustainable Chem. Eng. 2016, 4, 2243–2251.

(2) Willats, W. G. T; Knox, J. P.; Mikkelsen, J. D. Trends Food Sci. Technol. 2006, 17, 97−104.

(3) Wicker, L.; Kim, Y.; Kim, M.-J.; Thirkield, B.; Lin, Z.; Jung, J. Food Hydrocolloids 2014, 42, 251−259.

(4) Bomgardner, M. M. Chem. Eng. News 2013, 91, 20.

(5) Viana, M. A.; Fernandez, X.; Visinoni, F.; Chemat, F. J. Chromatogr. A 2008, 1190, 14–17.