# Effect of Temperatures on Polyphenols during Extraction
## 1 Introduction  
Polyphenols are naturally occurring substances found primarily in fruits, vegetables and most plant materials. Polyphenols is an umbrella term, so phenolic acids, flavonoids, stilbenes and lignans are all types of polyphenols. These substances show high antioxidant activity and are correlated to many positive health benefits [1, 2]. Polyphenols are attributed with having protective effects against diabetes, cancer and cardiovascular diseases (CVD) [3]. The most well-studied subclass of polyphenols are flavonoids [4]. Some of the most important flavonoids are catechins, hesperidin, naringenin and quercetin. While the exact mechanism behind how these substances provide these health benefits is not well understood, studies show a clear positive correlation. Many of these positive benefits have been attributed to their antioxidant capacity, and the global market for antioxidants has grown to about US$3000 million [5]. With such a rise, there has recently been a growing number of studies investigating various extraction techniques. The extraction techniques can be broadly divided into traditional (conventional) and novel (emerging) technologies. The traditional extractions include solvent extraction, Soxllet extraction and maceration. The disadvantages of these extractions are the long extraction times, high energy consumption, the need for expensive, high-quality organic solvents, evaporation of the solvents during treatment and the use of these potentially toxicsolvents, which then poses problems with disposal and possible loss of functionality [6]. The novel technologies include ultrasound-assisted extraction (UAE), enzymatic extraction, microwave-assisted extraction (MAE) and pulsed electric field (PEF) extraction. These emerging technologies are non-thermal, do not use any hazardous chemicals, provide a shorter extraction time and have high energy and extraction efficiency. Most of these novel extraction technologies are considered \"green\", as they fit the criteria set by the Environmental Protection Agency (EPA) [7; 8]. Supercritical/subcritical fluid extraction is another prominent technique in the field. This technology is often categorised as both a novel and traditional technology depending on the individual user. However, unlike other emerging technologies, this process does sometimes include treatment at high temperatures. The fluid chosen is also of importance for categorisation, as the critical temperature of the different fluids varies widely. In the case of supercritical CO\({}_{2}\) extraction, the treatment may be considered non-thermal (around 40-60 \({}^{\circ}\)C); for example, the critical point for CO\({}_{2}\) is around 31.2 \({}^{\circ}\)C [9]. As this technique may also use propane, methanol and other chemicals as solvents, it is sometimes categorised as a traditional extraction technology. On the other hand, subcritical water extraction is known by many names--pressurised liquid extraction (PLE), pressurised hot water extraction (PHWE) and superheated water extraction (SHWE). As water is a low-cost, non-toxic and environmentally friendly solvent, this extraction technique is very beneficial [10]. However, pressurised liquid extraction (hereafter referred to as PLE) often requires high temperatures between 100 \({}^{\circ}\)C and 300 \({}^{\circ}\)C and, hence, a very high-pressure vessel for treatment. In traditional extractions, the highest polyphenol yield is observed around 60-80 \({}^{\circ}\)C, and the most common solvents used for these extractions are ethanol, methanol and acetone [10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. As there is solvent loss at temperatures above 60-80 \({}^{\circ}\)C, studies investigating the impact of extraction temperatures did not include higher treatment temperatures (>80-90 \({}^{\circ}\)C). Studies that investigated the impact of drying temperature on polyphenolic yields have provided insight into the behaviour of polyphenols at these temperatures.