Extraction of valuable compounds from industrial apple pomace and birch bark residues under ultrasound and subcritical water

Abstract

The multidisciplinary Green Extraction approach has recently advanced by integrating the principles of Green Chemistry and the Circular Economy. In this framework, waste products are redefined as negative-cost raw materials for the recovery of high-value compounds with cross-sectoral applications, contributing to both economic and environmental sustainability. This study focuses on the valorization of apple pomace and exhausted birch bark, by-products from distinct production chains, using Subcritical Water Extraction (SWE) and Ultrasound- Assisted Extraction (UAE) as enabling technologies. The aim is to identify optimal extraction protocols to maximize yields, enhance polyphenol recovery, and improve antioxidant capacity in the final extracts. Carbohydrate and pectin content were also specifically evaluated for apple pomace. Furthermore, sequential extraction strategies and scale-up processes were investigated to refine the most effective protocols for each matrix and advance their Technology Readiness Level (TRL). For apple pomace, optimal results were achieved using SWE at 125 °C for 30 minutes, whereas birch bark required a higher temperature of 150 °C for 30 minutes to obtain the best outcomes. The achieved results highlights that green technologies can achieve performance comparable to, or even exceeding, that of conventional extraction methods, demonstrating strong potential for application in industrial settings.

Popular Abstract

Extraction of valuable compounds from food industrial residues without chemicals

How is it possible to extract the value compound to elaborate functional products or vitamin supplements from the waste from the food industry? The new technologies are the answer.

The food industry generates tons of residues every year. For example, around 25% of the total apple production (approximately 21 million metric tons annually) becomes a residue, also called a by-product, after juice and cider production. These by-products contain a large number of valuable compounds beneficial to human health, which can help reduce the risk of cancer, cardiovascular diseases, asthma, hyperglycemia, diabetes, and more. However, most of these by-products currently end up in incineration plants to produce energy or in anaerobic digestion to produce biogas or compost. When these valuable compounds, such as polyphenols, are extracted, the process is traditionally done using petroleum-based solvents, which are expensive, not renewable and can leave harmful residues in the final product. Another alternative is chemical synthesis, though it too comes with environmental and health economic concerns.
Fortunately, this is changing. Valuable compounds are now being extracted from food industry waste using green technologies such as ultrasound (a mechanical wave above 20 kHz, inaudible to the human ear) or subcritical water (liquid water heated up to 374°C under pressure). These methods enable the extraction of the same beneficial compounds using only water, making the process safer for humans and more environmentally sustainable. This approach gives waste a second life and contributes to a more circular economy.
In this study, titled Extraction of Valuable Compounds from Industrial Apple Pomace and Birch Bark Residues under Ultrasound and Subcritical Water, conducted by Carmen Doblas de León- Sotelo and supervised by Dr. Giorgio Capaldi in the University of Turin, residues from apple juice and cider production (apple pomace), as well as birch bark (after the extraction of its most valuable compound, betulin), were examined. For both samples, the best extraction results were

achieved using subcritical water at 125°C and 150°C for 30 minutes, yielding similar or even better outcomes compared to traditional methods using pure ethanol. Even though ultrasound wasn’t very effective for these two by-products, each by-products has unique characteristics and requires a tailored protocol using different technologies. Just like when you have many keys to open a door but don’t know which one is correct; you have to keep trying until you find the right key.
Furthermore, the experiments were successfully carried out at a semi-industrial scale, demonstrating the feasibility of green extraction technologies. These findings show that green extraction can effectively replace conventional methods, improving safety, reducing environmental impact, and shortening extraction time. However, further research is needed to refine the processes and determine the best protocols and technologies for each specific by- product.
To close the loop, these valuable compounds can be reintegrated into the food system in several ways. They can be added to products to create functional foods with health benefits, such as using apple pomace polyphenols in apple juice, used in packaging to extend shelf life, or applied as natural cleaners in the food industry, replacing chemical products.
Just as you can't judge a book by its cover, you can't judge a potential source by its appearance or origin, because the true value, or in this case, the important compounds, lies within, often imperceptible to the human eye.

• author: Doblas de León-Sotelo, Carmen
• supervisor: Giancarlo Cravotto; Javier Linares-Pastén
• organization: Food Technology and Nutrition (M.Sc.)
• course: KLTM02 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Technology and Engineering; Agriculture and Food Sciences
• keywords: apple pomace, birch bark, subcritrical water extraction, ultrasound, green extraction, polyphenols, pectins, food engineering nutrition and food chemistry
• language: English
• id: 9200596
• date added to LUP: 2025-06-18 14:24:33
• date last changed: 2025-06-18 14:24:33