Ultrasonic Pectin Extraction from Fruit and Bio-Waste

• Pectins are a very frequently used food additive, mainly added for its gelling effects.
• Ultrasonic extraction increases the yield and quality of pectin extracts significantly.
• Sonication is known for its process intensifying effects, which are already used in manifold industrial processes.

Pectins and Pectin Extraction

Pectin is a natural complex polysaccharide (heteropolysaccharide) found in particular in the cell walls of fruits, especially in citrus fruits and apple pomace. High pectin contents are found in the fruit peels of both apple and citrus fruits. Apple pomace contains 10-15% of pectin on a dry matter basis while citrus peel contains 20-30%. Pectins are biocompatible, biodegradable, and renewable polysaccharides with excellent gelling and thickening functionality, making them highly valued additives. They are widely used in food, cosmetic, and pharmaceutical products as rheology modifiers, functioning as gelling, glazing, stabilizing, and thickening agents and, in some formulations, as emulsifiers. Ultrasonic extraction is an efficient method to isolate high-quality pectins from fruit peels and pomace, increasing yield while reducing processing time and overall cost.

Conventional pectin extraction for industrial applications is performed using an acid-catalyzed processes (using nitric, hydrochloric or sulphuric acid). Acid-catalysed extraction is the most frequented process in industrial pectin production, since the other extraction techniques such as direct boiling (60ºC-100ºC) for up to 24hrs and low pH (1.0-3.0) are slow and low in yield and can cause thermal degradation of the fiber extracted and the pectin yield is sometimes limited by the process conditions. However, the acid-catalyzed extraction comes with its disadvantages, too: The harsh acidic treatment causes depolymerization and deesterification of the pectin chains, which affects the pectin quality negatively. The production of large volumes of acidic effluent requires post-processing and expensive recycling treatment, which makes the process an environmental burden.

Ultrasonic Pectin Extraction

Ultrasonic extraction is a mild, non-thermal treatment, which is applied to manifold food processes. In regards to the extraction of pectins from fruits and vegetables, sonication produces pectin of high quality. Ultrasonically extracted pectins excel by their anhydrouronic acid, methoxyl and calcium pectate contents as well as its degree of esterification. The mild conditions of the ultrasonic extraction prevent a thermal degradation of the heat-sensitive pectins.
Pectin quality and purity can vary depending on anhydrogalacturonic acid, degree of esterification, ash content of extracted pectin. The pectin with high molecular weight and low ash (below 10%) content with high anhydrogalacturonic acid (above 65%) are known as good quality pectin. Since the intensity of the ultrasonic treatment can be very precisely controlled, the properties of the pectin extract can be influenced by adjusting amplitude, extraction temperature, pressure, retention time and solvent.

 

 
Find the protocol for ultrasonic pectin extraction from grapefruit peel demonstrated in the video above here!
 

Ultrasonic extraction can be run using various solvents such as water, citric acid, nitric acid solution (HNO₃, pH 2.0), or ammonium oxalate/oxalic acid, which makes it also possible to integrate sonication into existing extraction lines (retro-fitting).

Fruit waste as source: High-performance ultrasound has already been successfully applied to isolate pectins from apple pomace, citrus fruit peels (such as orange, lemon, grapefruit), grape pomace, pomegranate, sugar beet pulp, dragon fruit peel, prickly pear cladodes, passion fruit peel, and mango peels.

Pectin Precipitation after Ultrasonic Extraction

Adding ethanol to an extract solution can help to separate pectin through a process called precipitation. Pectin, a complex polysaccharide found in the cell walls of plants, is soluble in water under normal conditions. However, by altering the solvent environment with the addition of ethanol, the solubility of pectin can be reduced, leading to its precipitation from the solution.

The chemistry behind pectin precipitation using ethanol can be explained by three reactions:

• Disruption of Hydrogen Bonds: Pectin molecules are held together by hydrogen bonds, which contribute to their solubility in water. Ethanol disrupts these hydrogen bonds by competing with water molecules for binding sites on the pectin molecules. As ethanol molecules replace water molecules around the pectin molecules, the hydrogen bonds between pectin molecules weaken, reducing their solubility in the solvent.
• Decreased Solvent Polarity: Ethanol is less polar than water, meaning it has a lower ability to dissolve polar substances like pectin. As ethanol is added to the extract solution, the overall polarity of the solvent decreases, making it less favorable for pectin molecules to remain in solution. This leads to the precipitation of pectin out of the solution as it becomes less soluble in the ethanol-water mixture.
• Increased Pectin Concentration: As pectin molecules precipitate out of the solution, the concentration of pectin in the remaining solution increases. This allows for easier separation of the pectin from the liquid phase through filtration or centrifugation.

Precipitating pectin using ethanol is a simple and effective method to isolate pectins from the extract solution, which is a process step that can be easily run after ultrasonic pectin extraction. The addition of ethanol to the extract solution alters the solvent environment in a way that reduces the solubility of pectin, leading to its precipitation and subsequent separation from the solution. This technique is commonly used in the extraction and purification of pectin from plant materials for various industrial and food applications.
 
 
Interested in the valorization of pomace, peel and pulp? – Read more about polyphenol extraction from fruit waste!

Industrial Sonicators for Pectin Extraction

Hielscher Ultrasonics is your partner for extraction processes from plant material such as pomace, peel and seeds. Whether you want extract small amounts for research and analysis or process large volumes for commercial production, we have the suitable ultrasonic extractor for you. Our ultrasonic lab homogenizers as well as our bench-top and industrial sonicators are robust, easy-to-use and built for 24/7 operation under full load. A broad range of accessories such as sonotrodes (ultrasonic probes / horns) with different sizes and shapes, flow cells and reactors and boosters allow for the optimal setup for you specific extraction process.
All digital ultrasonic machines are equipped with a colored touch display, integrated SD Card for automatic data protocolling, and browser remote control for comprehensive process monitoring. With Hielscher’s sophisticated ultrasonic systems, a high process standardization and quality control is made simple.
Contact us today to discuss the requirements of your pectin extraction process! We will be glad to assist you with our long-term experience in ultrasonic extraction and help you to achieve highest process efficiency and optimum pectin quality!

The table below gives you an indication of the approximate processing capacity of our ultrasonicators:

Research Results of Ultrasonic Pectin Extraction

Tomato waste: To avoid long extraction times (12–24 h) in refluxing procedure, ultrasonication was used for intensification of extraction process in terms of time (15, 30, 45, 60 and 90 min). Depending on extraction times, the obtained pectin yields for first ultrasonic extraction step, at temperatures of 60°C and 80°C are 15.2–17.2% and 16.3–18.5%, respectively. when a second ultrasonic extraction step was applied, the yield of pectins from tomato waste was increased to 34–36%, depending on temperatures and times). Obviously, ultrasonic extraction increases rupture of tomato cell wall matrix, leading to better interactions between solvent and extracted material.
The ultrasonically extracted pectins can be categorized as high methoxyl pectins (HM-pectin) with rapid set gelling properties (DE > 70%) and a esterification degree of 73.3–85.4%. n. The calcium pectate content in ultrasonically extracted pectin was measured between 41.4% to 97.5%, depending on extraction parameters (temperature and time). At higher temperature of ultrasonic extraction, the calcium pectate contents are higher (91–97%) and as such present important parameter of pectin gelling ability compared to conventional extraction.
Conventional solvent extraction for a duration of 24hr gives similar pectin yields in comparison with 15 min of ultrasonic extraction treatment. With regard to obtained results it can be concluded that ultrasonic treatment decreases the extraction time remarkably. The NMR and FTIR spectroscopy confirm the existence of predominantly esterified pectin in all investigated samples. [Grassino et al. 2016]

Passion fruit peel: The extraction yield, galacturonic acid and esterification degree were considered as the indicators of the extraction efficiency. highest yield of pectin obtained by ultrasound-assisted extraction was 12.67% (extraction conditions 85ºC, 664 W/cm2, pH 2.0 and 10 min). For these same conditions, a conventional heating extraction was performed and the result was 7.95%. These results are in accordance with other studies, which report the short time for effective extraction of polysaccharides, including pectin, hemicelluloses and other water-soluble polysaccharides, assisted by ultrasound. It was also observed that the extraction yield increased 1.6 fold when the extraction was assisted by ultrasound. The results obtained demonstrated that ultrasound was an efficient and time saving technique for extraction of pectin from passion fruit peel. [Freitas de Oliveira et al. 2016]

Prickly Pear Cladodes: Ultrasonic assisted extraction (UAE) of pectin from Opuntia ficus indica (OFI) cladodes after mucilage removal was attempted using the response surface methodology. The process variables were optimized by the isovariant central composite design in order to improve the pectin extraction yield. The optimum condition obtained was: sonication time 70 min, temperature 70, pH 1.5 and the water-material ratio 30 ml/g. This condition was validated and the performance of experimental extraction was 18.14% ± 1.41%, which was closely linked to the predicted value (19.06%). Thus, ultrasonic extraction present a promising alternative to conventional extraction process thanks to its high efficiency which was achieved in less time and at lower temperatures. The pectin extracted by ultrasonic extraction from OFI cladodes (UAEPC) has a low degree of esterification, high uronic acid content, important functional properties and good anti-radical activity. These results are in favor of the use of UAEPC as potential additive in food industry. [Bayar et al. 2017]

Grape Pomace: In the research paper “Ultrasound-assisted extraction of pectins from grape pomace using citric acid: A response surface methodology approach”, sonication is used to extract pectins from grape pomace with citric acid as the extracting agent. According to the Response Surface Methodology, the highest pectin yield (∼32.3%) can be achieved when the ultrasonic extraction process is carried out at 75ºC for 60 min using a citric acid solution of pH 2.0. These pectic polysaccharides, composed mainly by galacturonic acid units (∼97% of total sugars), have an average molecular weight of 163.9kDa and a degree of esterification (DE) of 55.2%.
The surface morphology of sonicated grape pomace shows that sonication plays an important role in breaking up the vegetal tissue and enhancing extraction yields. The yield obtained after ultrasonic extraction of pectins using the optimal conditions (75°C, 60 min, pH 2.0) was 20% higher than the yield obtained when the extraction was carried out applying the same conditions of temperature, time and pH, but without ultrasonic assistance. In addition, pectins from ultrasonic extraction also exhibited a higher average molecular weight. [Minjares-Fuentes et al. 2014]