About 78% of the total fish catch in both developing and developed countries is used as human food while the remainder is discharged as waste [1]. Fish processing waste such as skin, bones and fins is about 7.3 million tons/year [2]. There is considerable research that much of this waste, such as gelatin, collagen, fish oil and calcium, could have value as functional or bioactive materials [3–5]. However, some of the traditional methods for extracting these materials use organic solvents and acids that may lower the functionality and bioactivity of the native materials. These chemicals could also be harmful since they may accumulate in the environment and cause problems to humans, animals and the environment. On the other hand, enzyme hydrolysis of fish waste has also produced many bioactive compounds. If combined with innovative processing methods such as membrane technology, there could be a new type of bioactive materials and we could avoid some of these limitations. The combination and integration of a bioreactor with membranes is known as a “membrane bioreactor” and is widely used in waste water treatment. Membrane bioreactors could also effectively produce compounds with desirable molecular weights (MW) such as chitooligosaccharides (COS) from chitin wastes, peptides from proteins and polyunsaturated fatty acids (PUFAs) from fish oil [4,6]. Depending on the design of the membrane bioreactor, membranes can be selected and used to separate specific peptides from fish skin, scale gelatin and muscle [7,8].

Many of the compounds produced have shown biological activities including anticoagulation, antioxidant, calcium absorption, antihypertensive, antimicrobial activities [9–13]. This review focuses on the use of membrane bioreactor technology for the production of bioactive compounds from sea-food processing wastes and their biological activities in relation to their health benefits. The bioactive compounds include chitooligosaccharides (COS), bioactive peptides, bioactive lipids and enzymes. The use of membrane bioreactors for waste water treatment from fish-processing plants to reduce the large amount of organic matter [14] and for clarification of environmental effluents [15] is beyond the scope of this review and is not discussed.

Batch reactors are conventionally used for producing bioactive compounds from sea-food processing wastes using enzyme hydrolysis. However, batch reactors have some drawbacks, such as: low productivity because the enzyme is used only once, variability in product characteristics and quality due to batch-to-batch differences, the long times needed which also leads to low productivity, large space requirements and inability to obtain the final product instantly and continuously [16]. The development of the membrane bioreactor overcame many of these problems. A membrane bioreactor integrates a reaction vessel with a membrane separation system as shown in Figure 1. It is composed of two main systems: a bioreactor unit that is responsible for biodegradation of raw materials and the membrane unit that is responsible for the separation of desired bioactive molecules according to molecular size. Various enzymes can be used depending on the desired final products. Compounds with specific molecular weight distributions can be obtained by using membranes with the appropriate molecular weight cutoff (MWCO) or pore size distribution.

The membrane bioreactor also allows the simultaneous recycle of the enzyme and the separation, fractionation and/or concentration of bioactive product molecules. The substrate (sea-food waste) is fed continuously into the reaction vessel and product is continuously withdrawn through the membrane. The advantages of such a system include improved utilization of the enzyme since it is continuously recycled, the product stream (the permeate) is free of suspended matter, product properties (such as molecular weight distribution and clarity) are more consistent and uniform resulting in products of high quality, and the process is continuous, leading to higher productivity.

Membrane bioreactors may appear to be more expensive due to the cost of the membrane unit. However this can usually be justified by its higher productivity, better utilization of the enzyme, superior product quality and no additional downstream clarification is needed. To minimize costs, the membrane unit should be optimized in terms of operating parameters such as pressures, temperature, velocity and concentration. The membrane configuration should be properly selected (e.g., hollow fibers vs. spiral wound modules) as well as the separation properties of the membrane. Fouling and concentration polarization should also be minimized by using the appropriate membrane chemistry and operating conditions. Fouling is a result of blocking of membrane pores by feed and/or product compounds. If high-molecular weight compounds such as the substrate are a cause of fouling, this can be alleviated by introducing a pre-hydrolysis step before passing the substrate through the main bioreactor unit. Pre-hydrolysis may also reduce the viscosity of the mixture and enhance or stabilize the enzymatic reaction in membrane bioreactor unit.

The production of COS from chitosan is one of the premium applications of membrane bioreactors in sea-food applications. Crustacean shells and shellfish wastes from sea-food processing plants are an attractive source of bioactive compounds which could potentially be used in the food and pharmaceutical industries [23–27]. Shell wastes are currently utilized for commercial-scale production of chitin as well as for the production of chitosan and COS. There are two methods for production of COS: enzymatic hydrolysis using chitosanase and chemical hydrolysis using acids [28,29]. Chemical hydrolysis is relatively harsh and non-selective and thus the product contains relatively large amounts of impurities including D-glucosamine and chitosan monomers [30]. However, enzymatic hydrolysis in a membrane bioreactor has several advantages with regard to product specificity. Kuroiwa et al. [18] obtained the desired target biomolecules (pentamers and hexamers of COS) by proper control of the hydrolysis reaction and using optimal enzyme concentrations. Further, the size of the final products can be controlled by the pore size of the membranes use in the membrane bioreactor system. Their final product concentration of pentamaers and hexamers was 2.6 g/L. The membrane bioreactor was more effective over other systems for the production of COS by hydrolysis of chitosan. Jeon and Kim [31] introduced a new system using dual reactors for producing COS by combining a column reactor packed with immobilized enzymes with an ultrafiltration membrane bioreactor (Figure 2).

Discarded fish bones and cutoffs may contain considerable amounts of muscle proteins which are nutritionally valuable and easily digestible with a well-balanced amino acid composition [62]. Enzyme hydrolyzates of proteins from seafood wastes exhibit several physicochemical properties and biological activities [63]. The molecular weight of peptides has a large effect on their biological properties and activities and it is desirable to develop methods to produce and separate peptides with different molecular weights. Membrane bioreactors using ultrafiltration membranes of the appropriate molecular weight cutoffs have been effectively used to produce such peptides from seafood wastes [7]. Enzymes used typically are chymotrypsin, trypsin, Alcalase and Neutrase. Byun and Kim [8] used a three-step recycling membrane reactor (Figure 4) to produce peptides having several discrete molecular weights using consecutive proteolytic digestions using three different enzymes followed by fractionation of the hydrolyzate using various membranes. Many studies have confirmed the suitability of this method to obtain low molecular weight peptides that show potent bioactivities [64,65].

Peptides from seafood processing wastes have shown various biological activities including antihypertensive, antioxidative and antiradical activities (Table 2). The molecular weight of the peptide is the single most important factor for determining its biological activity.

Fish oils are readily available source of n-3 PUFA such as eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) which plays an important role in the prevention of a number of diseases, including atherosclerosis, coronary heart disease, hypertension, inflammation, hypotriglyceridemic effect, allergies and diabetes [86–88]. The use of membranes in lipid processing has attracted great interest in recent years since lipids that are heat labile can be preserved and concentrated with minimal changes to the lipid [18]. Most studies in this area have been on hydrolysis of oils and fats [89], the synthesis of acylglycerols with two phase membrane reactors [90] and epoxidation of fatty acids [91]. Sahashi et al. [92] combined solvent extraction with membranes to fish oil triglycerides from free fatty acids. An ethanol solvent (75% aqueous ethanol) was added to the fish oil which contained triglycerides with polyunsaturated fatty acids (PUFAs) and free fatty acids (FFA). Several flat-sheet membranes were tested for their ability to separate the oil from the solvent phase which contained the FFA. A hydrophilic polyimide ultrafiltration membrane with 20 kDa molecular cut-off was found to be the most suitable for the separation of the solvent phase as permeate. A rotating disk membrane was effective in a large-scale operation for getting PUFA-rich glycerides without oil quality deterioration. Xu et al. [93] used membrane bioreactor to separate medium chain triacylglycerols and n-3 PUFAs from fish oil. In their study lypozyme was used as the biocatalyst to accelerate the reaction in the bioreactor. The medium chain fatty acids were separated by 10 kDa and 2 kDa MWCO membranes.

Linder et al. [22] conducted the enzymatic hydrolysis using stereospecific sn-1 sn-3 hydrolytic lipase from Aspergillus oryzae followed by membrane filtration. The substrate was then partially hydrolyzed by the protease Neutrase to obtain most of the oil after centrifugation of the mixture. Immobilized 1,3-specific lipase IM60 (Lysozyme IM) was used for re-esterification. The incubation of the mixture was carried out at 37 °C, pH 7 for 24 h under stirring at 800 rpm. Lipase hydrolysis was carried out in a batch reactor blanketed with nitrogen. Membrane separation increased the PUFAs from 30.4% in crude oil to 43.3% in permeate. D'Arrigo et al. [90] have used a membrane bioreactor for hydrolysis as well as transphosphotidylation reactions. In many studies, lyposyme has been used as the catalyst to separate eicosapentaenoic acid (EPA) and docosahexaenoic acid (DHA) in fish oil [94].

Fish oil intervention resulted in a significant increase in platelet phospholipids EPA content, decrease in arachidonic and γ-linolenic acid levels and significant decrease in fasting and postprandial triacylglycerol levels [87]. Long chain PUFAs are considered as essential for the growth and development of infants and they have been included in food supplements in the form of concentrated fish oil. Hence, there is a marked demand for marine fatty acids as a health food and for use as a food supplement.

Proteases are widely used in the food industry for the production of protein hydrolysates, meat tenderization and fermentation [95]. Proteases such as trypsin and chymotrypsin from yellowfin tuna spleen extract (one of the best sources, according to Jantaro [96]) were separated by ultrafiltration using regenerated cellulose membranes with 30 kDa and 100 kDa MWCO membranes [97]. The limiting fluxes at cross-flow rate 120, 240 and 360 L/h for the 30 kDa membrane were 17.3, 43.9 and 54.7 L/m² h, respectively and the limiting fluxes at the same flow rate for 100 kDa membrane were 34.1, 51.1 and 68.4 L/m² h, respectively. In recent years, ultrafiltration has extensively been used for the separation of a wide range of compounds, including proteases from surimi wash water [98], proteases and protein hydrolysates from fish viscera [99].

Seafood processing wastes have great potential for the manufacture of bioactive substances. Even a few value-added compounds recovered from such wastes are economically more important than the target products themselves. However, their suitability as bioactive compounds and their pharmacological and nutraceutical values have not yet been properly evaluated and need to be studied more before considering them as nutraceuticals. Development of novel methods to produce and recover them is necessary. Membrane bioreactor technology can be used to separate and fractionate these bioactive compounds from marine wastes and will lead to the development of more profitable processes, thus giving rise to many great opportunities in the marine industry.

The increased focus of membrane manufacturers on membrane bioreactor applications is likely to continue in the future and result in more cost-effective systems. Better membranes and more efficient modules are also likely additional future developments. Introduction of low-cost, low-energy bioreactor technology is necessary to increase its utilization. A better understanding of membrane fouling is needed and new strategies need to be developed to concentrate the bioactive materials (involving novel reactor designs), and to increase the immobilization of enzymes in the reactor (use of carrier materials).