5.1.1. Conventional Heat Extraction

One of the oldest extraction methods is conventional heating extraction (CHE). CHE involves direct contact with a thermal source either by conduction or convection as the heating mechanism. The CHE thermal source usually is an used oven, hot plate, or water bath, while acidic solutions are used as the agents. Recent research on the FVW and FVB extraction methods showed that CHE was not favored as compared to the new emerging extraction technologies, such as MAE [130–132] and ultrasonic-assisted extraction (UAE) [133–135]. Although the CHE method produced more bioactive compounds than the MAE method, such as pectin extracted from black carrot pomace [132] and lime peel [136], the CHE method has several drawbacks, including high energy requirements and long processing time, making it inefficient in industrial applications; however, the combination of CHE and other techniques has shown tremendous potential in FVW and FVB processing. High PLE [137] and UAE [133] are two technologies that can be integrated with CHE, which can increase the extracted yield by over 50%. Table 4 shows the optimal yields of bioactive compounds from FVW that have been achieved in recent years.


**Table 4.** Progress studies of FVW using conventional heating extraction.

#### 5.1.2. Soxhlet Extraction

Another traditional thermal extraction method is Soxhlet extraction (SE), which involves high temperatures, long extraction times, and a large amount of solvent, meaning this method triggers environmental concerns. Similar to conventional heating extraction, the SE method also results in highly extracted bioactive compounds from FVW and FVB, including papaya [138], grape [139], and soapberry seed oil [140]. This classical extraction method has been compared to MAE [141–143] and UAE [144,145]. Table 5 shows the findings from the latest studies on SE methods using FVW. Throughout the experiments, Fernandez-Pastor et al. [142] showed that olive peel took 30–90 min to yield around 19 to 23% of bioactive compounds as compared to other studies that required longer times for the extraction technique (4 to 8 h).



Sukatta et al. [146] studied the bioactivity levels and characterization of rambutan peel extract (RPE) and the feasibility of RPE as a bioactive compound for antimicrobial and antioxidant applications in whey protein isolate (WPI)/cellulose nanocrystal film. The RPE was extracted for 16 h in a Soxhlet extractor with 95% ethanol. Using the HPLC chromatogram, the main components of RPE were corilagin, ellagic acid, geraniin, and gallic acid. The main bioactive components were classified as phenolic compounds, which exhibited antioxidant and antimicrobial properties.

As an alternative to existing synthetic packaging films, Jridi et al. [147] developed grey triggerfish skin gelatin films containing phenolic extracts from blood orange (Citrus sinensis) peel. The dried orange peel extract (DOPE) was obtained by extracting the dried orange peels via Soxhlet extraction using 300 mL of ethanol for 6 h. Similarly, fresh orange peel extract (FOPE) was obtained by extracting the fresh orange peels. It was found that the extraction yield of the DOPE was 31.2% (*w*/*w*), which was significantly higher than that of FOPE (25.3%).
