Valorization of Bioactive Compounds from Shrimp Shells: Comparison between Ultrasound-Assisted and Subcritical-Water Extractions ^†

Abstract

In recent years, shrimp consumption has increased, resulting in massive amounts of waste. In this study, different extraction techniques, namely, ultrasound-assisted extraction (UAE), subcritical-water extraction (SWE) and conventional extraction (CE), were tested to evaluate their efficiency in recovering bioactive compounds from shrimp shells. SWE performed at 200 °C was the extraction technique that allowed for the highest recovery of polyphenolic and carotenoid compounds (5.43 ± 0.17 mgGAE/g dw and 59.9 ± 1.0 μg carotenoids/g dw) as well as the highest antioxidant activity in ABTS and FRAP assays (7.93 ± 0.01 and 4.35 ± 0.31 mgAAE/g dw). These results demonstrate the potential of shrimp shells, which can be further incorporated into other products.

1. Introduction

Shrimp shells are an abundant byproduct due to the increase in shrimp consumption on a global scale, and they are mostly disposed into landfills or back into the ocean, being associated with several environmental issues [1]; therefore, it is urgent to find them a profitable use. Due to their chemical composition, shrimp shells can represent as an interesting source of antioxidant compounds, which can be used in high-value products [2].

Usually, the recovery of bioactive compounds from shrimp shells is performed using organic solvents, which can represent a limitation for use in food industries. In recent years, several environmentally friendly extraction techniques, such as such as subcritical-water extraction (SWE) and ultrasound-assisted extraction (UAE), have been widely applied to recover bioactive compounds from different food wastes [3]. However, information is still limited on the effects of different solvents and extraction techniques on the extractability of bioactive compound from shrimp shells.

The main goal of the present work was to efficiently extract antioxidant compounds from shrimp shell waste. To this end, two environmentally friendly extraction techniques, namely, ultrasound-assisted extraction (UAE) and subcritical-water extraction (SWE), were optimized and compared to conventional extraction (CE). Then, to determine which one of tested extraction techniques was more efficient, different colorimetric methods, namely, total phenolic (TPC) and carotenoid content (TCC), ABTS radical scavenging activity and ferric reducing/antioxidant power, were applied.

2. Materials and Methods

2.1. Samples

Shrimp shells, kindly provided by Marcabo Company (Matosinhos, Portugal), were dried at 40 °C for 48 h, milled and stored at room temperature in plastic bags until further use.

2.2. Extraction Techniques

Samples were submitted to different extraction techniques:

• UAE: 3.0 g of shrimp shells were mixed with 60 mL of solvent (citric acid, water, ethanol, 50% aqueous ethanol or methanol) for 15 min at 25 °C [4];
• SWE: 7.5 g of shrimp shells were mixed with 150 mL of solvent (water or 50% aqueous ethanol) for 20 min at three temperatures (100, 150 and 200 °C) [5];
• CE: 1.5 g of shrimp shells were mixed with 30 mL of solvent (water, ethanol or 50% aqueous ethanol) for 20 min at 40 °C [6].

2.3. Extract Characterization

TPC and antioxidant activity, evaluated via FRAP and ABTS assays, were performed as previously described [7,8]. Results are expressed as milligrams of gallic acid equivalents (GAE) and ascorbic acid equivalents (AAE) per gram of dry weight (dw). TCC was also assessed via a colorimetric method and is expressed in µg/g dw [9].

3. Results and Discussion

3.1. Total Phenolic Content

Figure 1 presents the TPC obtained for the analyzed samples subjected to the different extraction techniques.

For all the analyzed extracts, the TPC ranged from 0.51 ± 0.02 to 5.43 ± 0.17 mg GAE/g dw for UAE performed with citric acid and SWE performed with 50% aqueous ethanol at 200 °C, respectively. From the tested extraction techniques, SWE was demonstrated to be the most effective, with TPC at least 5-fold higher than the values reported for CE (1.18 ± 0.27 mg GAE/g dw).

Despite the obtained results being lower than the ones reported in the literature [9], it must be highlighted that the differences in shrimp varieties as well as the extraction conditions applied, such as solvents, extraction time and temperature, may exert a huge influence on the amount of phenolic compounds recovered.

3.2. Total Carotenoid Content

Figure 2 presents the TCC obtained for the analyzed samples subjected to the different extraction techniques.

As previously observed for TPC, the highest amounts of carotenoids were reported for the extracts obtained via the SWE technique (59.9 ± 1.0 μg carotenoids/g dw). On the other hand, TCC was below 10 μg carotenoids/g dw for UAE and CE, except for UAE performed with methanol. These values agree with the ones reported by Maia et al. [9], which demonstrated that TCC levels are influenced by shrimp variety, season and sample location.

3.3. Antioxidant Activity

Figure 3 and Figure 4 show the obtained results for antioxidant activity assessed via the ABTS and FRAP assays.

The highest antioxidant activity, evaluated via ABTS and FRAP assays (Figure 3 and Figure 4), was registered for shrimp extracts obtained via SWE at 200 °C (7.93 ± 0.01 and 4.35 ± 0.31 mg AAE/g dw, respectively). On the contrary, extracts prepared via CE and UAE presented the lowest antioxidant activity, and were demonstrated to be less efficient. The same correlation was observed for the TPC and TTC results, demonstrating the close relationship between the different spectrophotometric assays.

In general, the extracts obtained through the application of the SWE technique, namely, at 200 °C either with water or 50% aqueous ethanol, presented the highest amount of bioactive compounds, as well as the highest antioxidant activity. However, additional characterization of the obtained extracts, namely, via high-performance liquid chromatography with diode array detection, is necessary to determine which individual phenolic compounds could be contributing to the described antioxidant properties.

Overall, the presented results demonstrated that SWE can be an efficient and green extraction technique for obtaining phenolic compounds from shrimp shells, which can be further incorporated in biofilms, creating added value for this residue.