A Review of the Benefits of the Sustainable Utilization of Shrimp Waste to Produce Novel Foods and the Impact on Human Health

Abstract

In recent years, there has been an increase in the industrial processing of shrimp, aiming to cover the increasing demand for shrimp products for human consumption, and, consequently, an increase in shrimp by-products as shrimp waste. This waste includes the cephalothoraxes, heads, shells, tails, pleopods, and exoskeleton appendages of processed shrimps. The appropriate method for the enzymatic hydrolysis of shrimp waste can recover its bioactive substances, including carotenoids. Thus, these xanthophylls and carotenes are of high financial interest and have high antioxidant, anti-inflammatory, and anti-cancer activities. Therefore, these substances can be incorporated into fish feed as ingredients that improve fish health and simultaneously lead to the production of aquaculture fishes similar in coloration to the wild ones. Thus, the consumption of such novel food acts as a preventive factor for human health. In this regard, β-carotene has antioxidant and fat-soluble activities owing to vitamin A sufficiency and has an anti-cancer effect, too. Canthaxanthin can be used as a product for personal care and as a natural tanning agent for human skin. Zeaxanthin and lutein have positive effects on various eye and heart diseases, neuronal damage, human skin diseases, and certain types of cancer. Astaxanthin also has anti-diabetic and anti-obesity properties. Therefore, the purpose of this review is to highlight the sustainable utilization of shrimp waste via enzymatic hydrolysis, the benefits of a fish diet enriched with astaxanthin, the consumption of fish enriched with carotenoids, and the effects of carotenoids on human health. The problem of shrimp waste disposal affects the environment, does not contribute to sustainable development, and is directly related to the phenomenon of environmental change.

1. Introduction

One of the fastest-growing industries in the world is the aquaculture industry. A balanced diet includes seafood to achieve the proper functioning of the human organism. Therefore, the consumer public is focusing its interest on aquaculture fish products with high nutritional value, especially those with similar organoleptic characteristics to those of wild fish. Although the industry of aquaculture is relatively sustainable, there is a need for further investigation of the object of valorization of shrimp waste. Nowadays, the lack of processing of waste shrimp does not contribute to the sustainability of aquaculture. Notably, crustacean shells contain 20–40% protein, 20–50% calcium carbonate, 15–40% chitin, and small amounts of lipids (0–14%) [1,2]. Shrimp waste does not have the expected added value because its total biomass is not fully utilized but rather disposed of in the environment. Annually, more than 6 to 8 million tons of crustacean waste is produced from shrimp, crab, and lobster shells [1]. In general, up to 45–60% of shrimps are by-products, consisting of heads, shells, tails, internal organs, and ovaries, and they are discharged as processing waste [2,3]. These large quantities of shrimp waste, instead of causing ecological problems [4], could be properly exploited to further improve the sustainability of the aquaculture industry. The deposition of shrimp waste into the environment increases the biochemical oxygen demand (BOD), and its degradation produces a strong odor [4]. Crustacean waste, consequently, and more specifically shrimp waste, contains recoverable bioactive ingredients, e.g., proteins, chitin, and carotenoids [5]. Enzymatic hydrolysis is an innovative method for releasing digestible peptides from processing by-products with various physiological functions. With the help of protease, peptide bonds are broken, releasing peptides as well as other bioactive substances [5,6,7]. Therefore, the fermentation conditions, as well as the type of enzyme, determine the composition and properties of the recovered bioactive substances [8]. To date, the attention of the research community has focused on the matters of the nutritional value and chemical composition of shrimps, but probably not with the same interest in shrimp by-products. Admittedly, this may be due to the reduced knowledge of the beneficial bioactive ingredients contained in shrimp by-products; however, such ingredients are beneficial to human health. In recent years, new processes have been developed for the recovery, isolation, and quantification of bioactive substances from crustacean waste, leading to the development of new innovative products, such as functional food ingredients, medicines, cosmetics, feeds, and fertilizers [9,10,11,12,13]. Shrimp heads, which contain proteins, amino acids, chitin, and polyunsaturated fatty acids (PUFAs), have a sodium content of less than 1.5% and are therefore suitable as food for human consumption. Boiled shrimp shells release pleasant volatile compounds due to the presence of pyrazines, thiazolines, and thiazoles, which can be used as flavor enhancers in food. The dried exoskeletons of shrimps contain chitin and chitosan. Chitosan helps to fight various pathogens in the human body and contributes to the preservation of food. Dried shrimp cephalothorax contains lipids, unsaturated fatty acids, astaxanthin, castaxanthin and carotenoids, palmitic acid, and oleic acid. All of the above-mentioned bioactive ingredients derived from shrimp by-products are raw materials for producing high-added-value products such as seasoning, bread, crackers, fortified drinks, pastes, detergents, and cheese [14]. In particular, cultured salmonids may be improved by enrichment with recovered carotenoproteins, such as β-carotene, canthaxanthin, zeaxanthin, and astaxanthin, from natural sources, e.g., yeast, bacteria, algae, higher plants, and crustaceans, in the development of innovative fish feed [15]. Cultured salmonids require an improvement in the redness of the flesh and skin through pigment diets so that they resemble those of wild fish. The degree of color is one of the most important criteria for the quality of commercial-value aquaculture fish [15]. Astaxanthin (3,3′–dihydroxy–β,β-carotene-4,4′-dione) is a derivative of keto-carotenoid. It is classified as a carotenoid compound, and it has oxygen-containing components and bioactive hydroxyl and ketone groups. It is an active metabolite of zeaxanthin and/or canthaxanthin [16]. Canthaxanthin (β-carotene-4,4′-dione) is a carotenone that consists of beta,beta-carotene bearing two oxo substituents at positions 4 and 4′. It has been used as a biological pigment and a food colorant and recovered as a fungal metabolite and an Escherichia coli metabolite. It is derived from a hydride of beta-carotene [17]. Zeaxanthin (β,β-carotene-3,3′-diol) is a natural xanthophyll carotenoid that is derived from plants, algae, and microorganisms. It is a dihydroxy derivative of β-carotene [18,19]. Lutein is also a carotenoid with the chemical structure β, ε-carotene-3,3′ diol, resulting from the hydroxylation process of α-carotene [19,20]. Lutein and zeaxanthin have the same number of double bonds in their chains, but they differ in the position of one of these double bonds in the ring, making zeaxanthin a better antioxidant than lutein [21]. β-Carotene is a tetraterpene with 11 conjugated double bonds with the chemical structure β,β-carotene and has pro-vitamin A activity [19,20]. The action of carotenoproteins is not limited to pigment improvement exclusively, as they also act as antioxidants, capable of protecting human health due to the bioactive substances contained in carotenoids. There are about 600 different carotenoid species, none of which can be synthesized by animal cells, so they must be obtained through the consumption of food. Animal cells can alter the chemical structures of carotenoids to be assimilated by the organism [22,23,24]. Eco-green waste recycling can contribute positively to the reduction in operating costs over time with appropriate investments in infrastructure and specialized staff.

This review’s main purpose is to describe the enzymatic hydrolysis method for the waste of various types of commercial shrimp, the management techniques of the waste shrimps before enzymatic hydrolysis, the processes of performing enzymatic hydrolysis, and the investigation of produced extracts. Finally, this review focuses on the use of carotenoids (β-carotene, canthaxanthin, zeaxanthin and lutein, and astaxanthin) as enriching ingredients to improve the coloration of farmed fish and other organoleptic characteristics as a preventive or therapeutic action for human health.

2. Methodology

The data presented in this review were collected from published articles using rigorous and reliable selection criteria. The selected studies were examined for their official quality and originality. The articles were obtained by searching the Elsevier, Springer, Google Scholar, and Scopus databases with a combination of keywords: shrimp waste, shrimp by-product, astaxanthin, carotenoids, enzymatic hydrolysis, fish diet, human health. In total, 137 papers were examined, of which 133 were included in the present research and 4 were excluded. All articles retrieved were published during the period between 1983 and 2024. Articles that did not exclusively target the enzymatic hydrolysis method and shrimp by-products were excluded from the research. In some studies, additional extraction procedures were performed before and/or after the enzymatic hydrolysis process without providing basic information on the chemical composition of the products generated, emphasizing further utilization. Finally, in some studies, the experiments were not performed exclusively with shrimp waste but in combination with other sources of plant or animal origin. Finally, we grouped and described articles with the following issues: the method of enzymatic hydrolysis, the pigmentation of fish skin by astaxanthin, and the impact of carotenoids on human health. This research focuses on the enzymatic hydrolysis method using proteolytic enzymes exclusively in shrimp waste leading to the recovery of astaxanthin and other bioactive components. The influence of carotenoids, with an emphasis on astaxanthin, as a special diet for farmed fish was also examined. The positive effects of these special diets on the fish and the impact of the carotenoids on human health were a priority. The articles describe not only the recovery and isolation of carotenoids with an emphasis on astaxanthin but also other carotenoids that are useful for the development of aquaculture, as well as the impact of these carotenoids on human health. The prospect of the isolation of carotenoids and their use as auxiliary fish feed led to the production of commercial fish with high added value. Consequently, the supplementation of fish feed with carotenoids extracted from shrimp waste reduces the ecological footprint, promoting a circular economy, and simultaneously may improve human health.

3. General Techniques Reported for Processing Shrimp Waste

The method of enzymatic hydrolysis with the use of proteolytic enzymes can produce extracts, e.g., carotenoproteins and astaxanthin [25]. Treatment methods other than enzymatic hydrolysis for the extraction of bioactive compounds from shrimp waste include the following: solvents, e.g., ethanol or acetone, can be used on shrimp by-products that have been previously fermented with crude lactic acid, which can lead to the extraction of astaxanthin [26]. More sophisticated techniques with laboratory equipment requirements are the ultrasound technique to produce astaxanthin extracts [27], as well as supercritical CO₂ extraction (SCD) treatment, which can result in extracts in the form of lipids–astaxanthin [28]. Furthermore, ultrasound-assisted extraction (UAE), in combination with solvents, e.g., ethanol and cyclohexane, uses ultrasound to penetrate the solvents in contact with the solid matrix to extract the content from the sample solution [29]. Finally, the heat-drying technique results in a dried shrimp waste paste that leads to low-cost powdered products for further processing [30]. All waste treatment methods can extract bioactive components, e.g., recovered carotenoids and astaxanthin proteins. Therefore, processing methods differ in quality, quantity, time, cost, yields and concentrations of carotene-proteins, carotenoids, and astaxanthin extracts, and environmental impacts.

However, each method has advantages and disadvantages. In particular, the solvent extraction method with the use of acetone presents a risk of toxicity when the recovered products are processed for food applications, and therefore, they are unsuitable for consumption. Since the structure of acetone has similar carbonyl groups to astaxanthin, it increases the extraction rate of astaxanthin by 44 ± 1% compared to other organic solvents, such as methanol, ethanol, and acetonitrile [31]. The method of ultrasound-assisted extraction (UAE) with the use of organic solvents has short extraction times, requires a lower amount of solvent, and has a higher extraction efficiency. However, the drawbacks include high costs, many variables, and high effort. [32,33]. The supercritical fluid (SCF) method using CO₂ is an expensive method to apply due to the high temperatures and pressures prevailing at the critical point of extraction. Nevertheless, it is a modern and environmentally friendly extraction method using CO₂ and does not necessarily require organic solvents. However, its combination with ethanol leads to a higher astaxanthin extraction rate, up to 80–90%, from the microalgae species H. Pluvialis compared to the conventional method [31]. The drying processing method is a cheap alternative for producing dehydrated materials of organic origin from pastes, suspensions, and solutions, which, in turn, can be reused by spraying food. A major advantage of this method is the quality of astaxanthin, which includes other undesirable compounds [30].

3.1. Preparation of Raw Waste Shrimp Samples

The preparation that follows the collection of the non-edible parts of shrimps, i.e., shells, heads, tails, carapaces, exoskeletons, cephalothoraxes, and appendages, is described in

Table 1. Description of waste preparation before enzymatic hydrolysis derived from different species of shrimp.

Scientific Names of Species/by-Products Used	Country/ State/ Company	Raw Waste Shrimp	General Description of Waste Preparation before Hydrolysis	References
Penaeus monodon, Penaeus indicus, Metapenaeus monocerous, Penaeus Monodon	India Company	head	Raw shrimps were maintained on ice and beheaded. Shrimp waste from different species was stored separately, packed in a polyethylene bag, and stored at −20 °C.	[5]
Metapenaeus monoceros	India, Visakhapatnam, factories	cephalothorax, shell, tail, appendages	Shrimp shell waste was uniformly ground to a smooth paste.	[34]
Pandalus Borealis	Factory BioHenk AS	head, scale	The shrimp waste was packed in plastic bags and stored at −20 °C before use.	[35]
Parapenaeus Longirostris	Tunisia, Sfax, processing plant	head, cephalothorax, shell, appendix	The shrimp waste was washed with distilled water, ground, and stored at −20 °C before use.	[36]
Penaeus braziliensis and Penaeus subtilis	France, Saint Malo, Company comapeche	processing discards	The shrimp waste was ground, packed in polyethylene vacuum bags, and kept frozen at −20 °C.	[37]
Penaeus Vannamei	Vietnam, Khanh Hoa, seafood processing companies	head	Shrimp waste was transported on ice, washed, and ground into pieces of 0.3 to 0.5 cm. The shrimp waste was packed into plastic bags and frozen at −20 °C until use.	[38]
Pandalopsis Dispar	Canada, Vancouver, Albion Fisheries Ltd.	shell, tail, head	Cooked shrimps were hand-peeled, thawed overnight at 4 °C, and allocated to packages for storage at −25 °C until use.	[39]
Penaeus Monodon	-	head, shell	The shrimp waste was washed with water, milled, and dried. The shrimp waste was packed in a low-density polyethylene bag and frozen at −20 °C until use.	[40]
Litopenaeus vannamei	Mexico, Huatabampo City, Sonora State, Company El Camaron Dorado	head	Frozen shrimp waste was defrosted in a microwave at 55 °C. The dried shrimp waste was ground and stored under vacuum at −0.2 bar at room temperature.	[41]
Penaeus Indicus	Local market	head, carapace	Chilled shrimp waste was transported to the laboratory, homogenized with an equal volume of distilled water, and stored at −20 °C.	[42]
Shrimps	China, Zhejiang, Marine Fishery Co., Ltd.	by-products	Chilled shrimp waste was transported to the laboratory, homogenized with an equal volume of distilled water, and stored at −20 °C.	[43]
Xiphopenaeus Kroyeri	Brazil, Guaruja, Alpha Pescados	cephalothorax, shell, tail	The shrimp waste was washed with water, ground, packed in plastic bags, and frozen at −20 °C.	[44]
Penaeus Kerathurus	Processing factory	head	Shrimp waste was packed in plastic bags and stored at −40 °C until use.	[45]
Penaeus Monodon	Madagascar, UnIMA processing factory	head	Shrimp waste was packed in plastic bags and stored at −20 °C until use.	[46]
Parapenaeus Longirostris	-	exoskeleton, cephalothorax	The shrimp waste was thawed, minced, and dried at 45 °C for 40 h.	[47]
Litopenaeus vannamei	Mexico	discards	The shrimp discards were stabilized by lactic acid fermentation using Pediococcus pentosaceus	[48]
Penaeus Semisulcatus	Iran, Bushehr, fisheries from Bushehr	head, carapace	Fresh shrimp were cut by hand, separated into heads and carapaces, and ground into a paste. The paste was stored in a polyethylene bag and kept at −20 °C.	[49]
Shrimps	China. Wenzhou City, Zhejiang, aquatic product market	by-products	Shrimp by-products were stored at −18 °C until their use.	[50]
Shrimps	Canada, Newfoundland, local fish market	head, shell, tail	The shrimp waste was ground and packed in plastic bags. The ground shrimp materials were stored in plastic bags and kept frozen at −18 °C until their use.	[51]
Pandalopsis Dispar	Canada Vancouver, Albion Fisheries Ltd.	shell, head, tail	Cooked shrimps were hand-peeled in frozen form. The shrimp waste was kept overnight at 4 °C, distributed into packages, and stored at −25 °C until use.	[52]

.

The supply of shrimps usually comes from factories [35,45,46] and goes to limited liability industrial fisheries and fishery companies [39,52] and to other companies and local markets [5,34,36,37,38,40,41,42,43,44,47,48,49,50,51]. The shrimp samples originated from different countries, such as India, Tunisia, France, Vietnam, China, Canada, Brazil, Madagascar, Mexico, and Iran [5,36,37,38,43,44,46,48,49,51]. The non-edible parts of shrimps can be processed with or without further manipulation. However, in most scientific reports related to enzymatic hydrolysis, the preparation of shrimp samples is a necessity. It is reported (Table 1) that the main parts of shrimp waste to be processed are the shell, head, tail, carapace, exoskeleton, cephalothorax, and appendages [34,39,47,49]. This requires beheading operations [5], the manual separation of heads and carapaces by hand [49], and hand peeling for frozen forms [52]. The samples are then washed with distilled water [36] and running water [38,40,44], as well as thawed at 4 °C overnight [39]. In some cases, the preparation of the samples is more complicated, so more advanced techniques are used, such as grinding [37,38,44,49,51], drying with a solar drier at 32 °C for 30 min [40], defrosting in a microwave oven operating at 2.45 MHz [41], drying in an industrial dryer, [41], homogenization with distilled water [43], and stabilization by lactic acid fermentation using the process described by Armenta et al. [48,53], and finally, some samples do not undergo any further preparation and are used as raw materials [35,45,46]. After the preparation of shrimp waste, the samples are packed in polyethylene bags [5,49], in vacuum polyethylene bags [37], or in plastic packages [35,38,44,45,46]. Then, they are stored and kept at temperatures between a minimum of −40 °C [45] and a maximum of −18 °C [43]. The preparation process of raw waste shrimp samples is summarized in Figure 1.

There are differences in the disposal of shrimp waste between the countries mentioned above. In Brazil, the heads of the shrimp are disposed of in the environment. According to Cavalheiro et al. (2007), a possible solution is to transform them into animal or fish feed [54]. In Tunisia, waste from the shrimp industry causes various environmental problems that are probably due to discharge into the environment [55]. In Quebec City in Canada, the seafood industry plays an important role in the economy. Shrimp waste disposal has high environmental and financial costs, and efforts are made to prepare marketable consumer products from such waste [56]. In China, high quantities of fish and shrimp waste, together with trash fish, anchovies, and skinnycheek lanternfish, are used to produce fish feed. The annual food waste in China is estimated to be around 195 million tons and is possibly disposed of in landfills [57]. In Mexico, the main problem in shrimp aquaculture is the production of organic matter and nutrients that are dumped into coastal waters, negatively affecting the profitability of these industries [58]. The shrimp fishery industry in Madagascar was developed after 1967, with a total of 10,000 to 12,000 MT caught between 1995 and 2004, representing about 73% of fisheries exports. Bycatch from the shrimp fishing industry is provided to local and national markets for human consumption and for animal feeds. According to a survey, integrated management in the fishery sector is a future target [59]. In India, significant concentrations of shrimp waste have been found in the coastal areas of Chennai and the landfills of Kellaria [60]. Consumption and the seafood processing industry produce high quantities of by-products (around 2 MMT in India) that end up as waste [61]. In the Bushehr province of Iran, municipal shrimp waste from households and fish stores is collected for recycling [62]. In the eastern region of France, industries process frozen shrimp and use the shrimp waste as raw material to derive shrimp paste for soups [63]. In Sóc Trăng City in Vietnam, 13 shrimp processing factories produce significant amounts of shrimp waste, and according to Vietnamese regulations, the shrimp heads must be further processed to be utilized as animal feed [64].

3.2. Description of the Enzymatic Hydrolysis Process

Regarding enzymatic hydrolysis, the articles were classified and evaluated based on the following common parameters: scientific species, proteolytic enzymes (pH, temperature, time of incubation), temperature/time of enzyme inhibition, and recovered extracts. The conditions of the applied enzymatic hydrolysis of shrimp waste are presented in

Table 2. Description of the conditions of enzymatic hydrolysis of different species of shrimp.

Shrimp by-Product	Proteolytic Enzymes	Incubation (pH)	Incubation Temperature	Incubation Time	Enzyme Inhibition Temp./Time	Recovered Extracts	References
head		Single pH	Single temp.	Single time		Caroteinoids, astaxanthin	[5]
	Trypsin	7.6	45 °C	2 h	100 °C for 10 min
	Papain	6.2	55 °C	2 h	100 °C for 10 min
	Pepsin	4.0	45 °C	2 h	100 °C for 10 min
cephalothorax, shell, tail, appendages		Single pH	Single temp.	Single time.		Caroteinoids, proteins, carotenoproteins	[34]
	Pepsin	4.6	28 ± 2 °C	3–4 h
	Papain	6.2	28 ± 2 °C	3–4 h	Boiled for 10 min
	Trypsin	7.6	28 ± 2 °C	3–4 h
head, scale		-	Single temp.	Single time		Protein hydrolysate, astaxanthin, chitosan	[35]
	Alcalase
	2.4 l FG		40 °C	2 h	90 °C for 20 min
head, cephalothorax, shell, appendix	Trypsin	Single pH	Single temp.	Different periods:	Addition of acetic acid	Carotenoproteins	[36]
		10	25 °C	1 h, 3 h, 5 h, 7 h
processing discards	Alcalase	Single pH	Single temp.	-	Boiling water for 20 min	-	[37]
	2.4 L	6.0–10.0	50 °C–70 °C
head	Alcalase	-	Single temp.	Different periods:	-	Proteins, minerals, chitin, carotenoproteins, lipids	[38]
			55 °C	2 h, 4 h, 6 h, 8 h
shell, tail, head		-	Single temp.	Different periods	Boiling water for 10 min	Protein hydrolysate	[39]
	Alcalase		50 °C	1 h,
	Bromelain		50 °C	4 h,
	Flavourzyme		50 °C	8 h,
	Protamex		50 °C	24 h
head, shell		Single pH	Single temp.	Different periods:	90 °C for 5 min	Protein hydrolysate	[40]
	Alcalase	7.0–8.0	56 °C–60 °C 47 °C–50 °C 50 °C–52 °C 50 °C–55 °C	30 min, 45 min, 60 min, 75 min, 90 min
	Neutrase	6.3–6.5
	Protamex	7.2–8.0
	Flavourzyme	5.5–7.5
head	Alcalase Flavourzyme Lysozyme Inovapure 300 Papain Trypsin VI	-	-	-	-	Chitin	[41]
head, carapace	Alcalase	-	Different temp.:	Different periods:	-	Caroteinoids, proteins, carotenoproteins	[42]
			20 °C,	60 min,
			35 °C,	150 min,
			50 °C	240 min
nonspecified by-products	Alcalase	Single pH	Single temp.	-	Boiling water for 10 min	Iron-binding peptides	[43]
		7.8	55 °C.
cephalothorax, shell, tail	Alcalase	Single pH	Single temp.:	-		Proteins, astaxanthin, chitin	[44]
		8.5	60 °C		90 °C for 5 min
		8.5	60 °C
head	Trypsin	Single pH	Single temp.:	Single time		Proteins	[45]
		7.9–8.0	50 °C	1 h	90 °C for 5 min
head		Single pH	Single temp.	Single time	Addition of NaOH	Protein hydrolysate, lipids, chitin	[46]
	Pepsin	2.0	40 °C	24 h	------
	Novozyme	3.0	50 °C	24 h	85 °C for 25 min.
	Protex 6L	9.5	60 °C	24 h	85 °C for 20 min.
	Delvolase	10	60 °C	24 h	90 °C for 20 min.
exoskeleton, cephalothorax	Protamex Flavourzyme Alcalase	-	-	-	-	Astaxanthin	[47]
discards		pH	Single temp.	Single time	-	Carotenoproteins	[48]
	Savinase,	8.0	30 °C	24 h
	Lipase	8.0	30 °C	24 h
head, carapace	Alcalase	Single pH	Single temp.	Single time	-	Proteins	[49]
		8.0	50–60 °C	1 h
nonspecified by-products		Single pH	Single temp.	Single time	95 °C for 10 min	Proteins, calcium	[50]
	Flavourzyme	7.0	50 °C	6 h
	Protamex	6.5	50 °C	6 h
	Alcalase	8.0	55 °C	6 h
	Pepsin	2.0	37 °C	6 h
	Trypsin	8.2	40 °C	6 h
head, shell, tail		Single	Single	Single		-	[51]
		pH	temp.	time	90 °C for 20 min
	Alcalase 2.4 L	8.0	40 °C	1 h
shell, heads tail	Alcalase Bromelain Flavourzyme Protamex	-	-	-	-	-	[52]

.

A wide variety of industrial proteolytic enzyme preparation reagents are used to hydrolyze waste shrimp, leading to the recovery of extracts and other bioactive ingredients, which are of high research and financial interest. The proteolytic enzymes are Alcalase, Flavourzyme, trypsin, Protamex, pepsin, papain, and bromelain [5,50,52]. The above enzymes are frequently used in the enzymatic hydrolysis method, while others, such as neutrase, lysozyme inovapure 300, novozyme, protex 6L, delvolase, and savinase [40,41,44,46,48], are rarely used. A general description of the enzymatic hydrolysis process is described in Figure 2.

Depending on the method, the pretreated biomass of shrimp waste is usually placed in a container with at least equal (1/1) or greater quantities (1.5/1, 2/1, 2.5/1) of water (% w/v) in relation to biomass waste, and the mixture is then homogenized [35]. As shown in Table 2, a key factor is primarily the choice of the proteolytic enzymes for the specific waste, and secondary is the pH, the temperature, and the time of incubation of the mixture. Each proteolytic enzyme achieves its optimal action with defined physicochemical parameters, more specifically, with a combination of pH, temperature, and the time of incubation. Some enzymes require the substrate to dissolve in an acidic or alkaline environment, e.g., the pepsin enzyme at pH 2 as a typical case [46,50], while trypsin, Alcalase, and delvolase are used in alkaline environments, i.e., pH 7.8–10 [36,37,46]. The incubation temperature, in general, ranges from a minimum of 20 °C [42] to a maximum of 70 °C [37] in the case of Alcalase. Moreover, the incubation time can last from 30 min [40] to 24 h [39,46,48], either intermittently or continually. Each proteolytic enzyme has a corresponding time of action after which the enzymatic effect ends, and the enzyme must be inhibited by a sharp increase in temperature over a short period at 90 °C for 5 min [44,45] or by using additives such as NaOH or acetic acid [36,46]. The thermal inhibition of the proteolytic enzyme completes the enzymatic hydrolysis, resulting in a hydrolysate. Often, the temperature of the hydrolysate is gradually returned to ambient temperature without any other action [46]. The next phase involves the separation process by straining the supernatant and the shrimp cake biomass from the hydrolysate and the centrifugation of the supernatant, leading to the distinct separation of the protein hydrolysate from the supernatant [35]. Afterward, the supernatant and the sediment need further processing for the recovery of the bioactive ingredients [35]. The bioactive ingredients extracted by the enzymatic hydrolysis process of shrimp waste are usually recovered carotenoproteins, carotenoids, and astaxanthin [5,34]. There are other bioactive compounds that are beyond the scope of this review but are worth mentioning, such as proteins, lipids, chitin, chitosan, calcium, and minerals [35,38,46,50], and are summarized in Table 2 and Figure 2.

Based on the collected articles [35,44,51] in this review, a precise methodology for the application of enzymatic hydrolysis to shrimp waste to obtain astaxanthin is formulated as follows: the pretreated biomass of shrimp waste is added to a container with a ratio of 1:1 w/v biomass waste/distilled water, and the mixture is then homogenized. The proteolytic enzyme used for enzymatic hydrolysis is Alcalase, while fermentation is accelerated under the conditions of pH 8 and a temperature of 40 °C, and the experimental duration is 1 h, while the inhibition of the enzyme takes place at 90 °C for 20 min. The insoluble fraction in the supernatant liquid is separated by centrifugation at 16,000× g for 15 min at 4 °C. After the centrifugation of the supernatant liquid, a precipitate containing astaxanthin is obtained, while the quantification of the extracted astaxanthin is achieved by the equation of Kelley and Harmon [35,44].

3.3. Recovered Bioactive Substances from Shrimp Waste and Applications

Shrimp waste contains valuable bioactive components, such as proteins/peptides, chitin/chitosan, pigments, enzymes, lipids, minerals, and vitamins [65]. Shrimp exoskeleton waste (Crangon crangon) contains almost 17.8% chitin content [66]. Chitin is a linear poly-β-(1,4)-Ν-acetyl-D-glycosamine and is found in three polymorphic forms: α-chitin; β-chitin; and γ-chitin [2]. α-Chitin is the most stable among the three types and is present in crustacean shells, insect cuticles, and fungi. Chitin is highly hydrophobic and thus insoluble in water and many organic solvents, except for formic, dichloroacetic, and trichloroacetic acids. Mixtures of chitin with LiCL are soluble in dimethylacetamide and in N-methyl-2-pyrrolidone [67]. The extraction of chitin from shrimp waste is achieved by either chemical or biological means. The first one uses an acidic or alkali treatment, and the second one, bacteria [66]. Chitosan is a linear polysaccharide that contains copolymers of D-glucosamine and N-acetyl-D-glycosamine linked by β-(1,4)-glycosidic bonds. It is obtained by the partial deacetylation of chitin through alkaline or enzymatic methods [66]. Its solubility depends on the pH, the ionic concentration, the degree of acetylation, the nature of the acid for protonation, the distribution of acetyl groups along the chain, and the conditions of the isolation and drying of the polysaccharide. It has a hydrophilic character and is pH sensitive [67]. Chito-oligosaccharides are the depolymerized products of chitosan [68]. Hydrolyzed products of chitosan can be obtained through chemical or enzymatic methods (chitosanases, cellulases, lipases, papain, lysozyme, hemicellulases, pectinases, pepsin, and pronase) [66]. These products have lower molecular weights and viscosities than chitosan and are ideal for industrial applications. Such oligosaccharides are O-and N-carboxymethlchitosans, chitosan 6-O-sulfate, N-methylene phosphonic chitosans, trimethylchitosan ammonium, carbohydrate branched chitosans, chitosan-grafted copolymers, alkylated chitosans, cyclodextrin-linked chitosans, etc. [67]. Shrimp shells contain significant amounts of protein and calcium carbonate, CaCO₃, which is quite biocompatible and bioavailable compared with the mineral calcium carbonate. Dietary supplements from crustacean waste can be used to increase calcium intake, supporting human metabolism and bone health [68]. For instance, marine calcium supplements such as oyster shells and coral calcium are commercially available. Moreover, other marine-derived calcium supplements, such as Aquamin, Hake fishbone (HBF), a fishbone powder (Phoscalim), a ray cartilage hydrolysate (Glycollagene), and calcium tablets from haddock bones, are high-content sources of calcium with benefits to human health [68]. Shrimp waste is rich in proteins. Nitrogen sources are amino acids, peptides, and nucleic acids. Shrimp waste is rich in arginine, glutamic acid, glycine, and alanine, and their extraction is important, since they can be used as dietary supplements for humans or animals. The isolation of proteins from crustaceans is carried out through chemical extraction, the fermentation process, enzymatic extraction, and isoelectric solubilization/precipitation [69]. Minerals are also basic components of crustacean waste, such as phosphorus, sodium, magnesium, and zinc; however, in the demineralization process, the specific metals are recovered [69,70]. Finally, they are abundant in lipids and pigments. In the edible parts of shrimps, the average lipid content is approximately 1%. Lipid composition is classified as 65–70% phospholipids, 15–20% cholesterol, and 10–20% total acyl glycerols [71]. Omega fatty acids, polyunsaturated fatty acids, and lipid-soluble vitamins (A, D, E) vary according to species, gender, weather, and the environment. The amounts of fat-soluble vitamins (A, D, E) are equal to 180 IU, 2IU, and 1.32 μg, respectively, in 100 g of shrimp [71]. Recent studies have shown the presence of bioactive long-chain omega-3 polyunsaturated fatty acids (n–3 PUFAs), such as all-cis-5,8,11,14,17-eicosapentaenoic (EPA, C20:5 n–3), all-cis-7,10,13,16,19-docosapentaenoic (DPA, 22:5 n–3), and all-cis-docosa-4,7,10,13, 16,19-hexaenoic (DHA, 22:6 n–3) acids, and minor fatty acids, such as furan fatty acids (F-acids), in fishes and their benefits for fish oil [72]. Moreover, the appropriate pigments are presented, such as astaxanthin, lycopene, zeaxanthin, β-carotene, and lutein [69]. Shrimp contains 2% carotenoids, and the astaxanthin levels (which corresponds to 86–98% of total carotenoids in crustacean species) in wild shrimps vary from 740 to 1400 μg/100 g of edible meat portions [71].

Chitin is used in the health industry as a wound dressing and bone-filling material. Also, such materials containing chitin have adsorbent properties [67]. Hydrolyzed shrimp proteins are used as raw materials for nutritional supplements in baby food and sport drinks, as flavor enhancers in fish feed in aquaculture, and as a source of nitrogen in nutrient substrates for microbial growth and enzyme production [73]. Shrimp cephalothoraxes contain lipids such as polyunsaturated fatty acids (PUFAs), eicosapentaenoic acid (EPA), and docosahexaenoic acid (DHA), which are used as drugs in the health industry but also as fish feed additives in aquaculture [74,75]. Specifically, shrimp heads are converted through appropriate processes into edible aromatic flavors for food enrichment [76].

4. The Application of Astaxanthin as a Complementary Feed Colorant in Fish Diets

The proper and balanced feeding of farmed fish is essential for producing high-quality food products. Feeding is an important factor that semiotically affects the viability factor of the aquaculture industry. Nutrition is therefore a series of processes where farmed fish digest food to survive and grow to the desired commercial size [77]. These farmed fish have more nutritional needs due to some qualitative characteristics that differ from the corresponding wild species, e.g., different body, skin, and flesh color between farmed and wild species [78]. Carotenoids are of remarkable academic and practical interest because of their abundance in the natural environment, as well as their added commercial value as a complementary feed colorant [79]. Many studies have shown that enhancing fish feed with bioactive ingredients, such as astaxanthin extracts, can improve skin color, increasing the added commercial value of the final fish product. Pigmentation of the skin and muscle, the rate of growth, and the rate of survival of farmed fish are the main characteristics that determine the quality and cost of catches [80,81,82]. Carotenoids, especially astaxanthin and canthaxanthin, are responsible for the pigmentation of the fish muscle [81,83]. Fish cannot synthesize carotenoids on their own, because these are obtained exclusively through the diet [81,83]. Astaxanthin, canthaxanthin, lutein, and zeaxanthin enhance fish skin color cells, i.e., chromatophores, e.g., melanophores, xanthophores, erythrophores, iridophores, leucophores, and cyanophores [81]. Usually, the carotenoid pigments that are assimilated by fish are tunaxanthein (yellow), lutein (green-yellow), β-carotene (orange), doradexanthin (yellow), zeaxanthin (yellow-orange), red canthaxanthin (orange), eichinonone (red), and taraxanthin (yellow) [81]. There are several types of research and reports related to the key issue of fish nutrition, focusing primarily on high-commercial-value fishes and secondarily on decorative fishes.

Studies have investigated (

Table 3. The results of a fish diet enriched with different doses of astaxanthin.

Scientific Names of Species	Fish Diet A: Initial (g) Weight B: Final (g) Weight	Dose of Astaxanthin mg/kg	Beneficial Effects on Fishes	References
Oncorhynchus mykiss	A: 800 g–900 g B: Not mentioned (g)	40–100 mg/kg	Pigmentation improvement	[93]
Salmo salar L.	A: 580 g B: Doubled weight (g)	75 mg/kg	Pigmentation improvement of the flesh	[94]
Pagrus pagrus	A: 44 g B: Not mentioned (g)	20–40 mg/kg	Pigmentation improvement of the skin and growth	[80]
Carassius auratus	A: 10 g B: Not mentioned (g)	0–100 mg/kg	Pigmentation improvement of the skin	[83]
Yellowtail	A: 6100 ± 900 g B: Not mentioned (g)	0–40 mg/kg	Improvement of egg quality	[84]
Oncorhynchus mykiss	A: 111 ± 6 g B: Not mentioned (g)	0–50 mg/kg	Pigmentation improvement of the skin	[95]
Salmo gairdneri	A: 63 g B: 96 and 123 g	200 mg/kg	Pigmentation improvement of the skin and growth	[87]
Salmo salar	A: 510 g B: Not mentioned (g)	41.4 mg/kg	Pigmentation improvement	[96]
Salmo salar	A: 309 g B: Not mentioned (g)	84.2 mg/kg	Pigmentation improvement of the muscle	[97]
Salmo salar L.	A: 23 g B: Not mentioned (g)	100 mg/kg	Growth and survival improvement	[98]
Sparus aurata	A: 97 ± 2 g and 150 ± 5 g B: Not mentioned (g)	40 mg/kg and 40 mg/kg	Pigmentation improvement of the skin	[99]
Oncorhynchus mykiss	A: 6.5 ± 0.5 g, 25 ± 2 g, and 120 ± 5 g B: 400 g	5.5 ± 0.3 mg/kg	Pigmentation improvement	[85]
Salrno salar L.	A: 115 ± 30 g B: 3275 ± 837 g	0–200 mg/kg	Pigmentation improvement	[100]
Salmo salar	A: 408 g B: 1200 g	60 mg/kg	Pigmentation improvement of flesh	[101]
Salmo salar	A: 2000 g B: Not mentioned (g)	47 mg/kg	Investigation of carotenoids as plasma bioavailability indicator	[102]
Oncorhynchus mykiss	A: 178 ± 23 g B: Not mentioned (g)	49.8 mg/kg	Improvement of growth	[103]
Symphysodon spp.	A: 10.3 ± 0.8 g B: Not mentioned (g)	0–400 mg/kg	Improvements in pigmentation of body and skin and antioxidant properties	[86]
Salmo salar	A: 569 g B: Not mentioned (g)	40 mg/kg	-	[104]
Salmo salar L. & Hippoglossus hippoglossus L.	A: 144 ± 2 g and 445 ± 16 g B: Not mentioned (g)	66 mg/kg	-	[105]
Salmo gairdneri Richardson	A: 135 ± 5 g B: Not mentioned (g)	0–200 mg/kg	Pigmentation improvement	[106]

and Figure 3) the use of astaxanthin as an enhancer of fish feed at all stages of fish life, from the larval stage to full growth and reproduction. According to specific references, astaxanthin addition to the fish diet occurs not only in fully reproductive mature fish, such as yellowtail with an initial weight of 6100 ± 900 g [84], but also in young fish, such as Oncorhynchus mykiss with an initial weight of 6.5 ± 0.5 g [85]. The amount of astaxanthin in diet experiments ranges from 5.5 ± 0.3 mg/kg [85] to 400 mg/kg [86]. Although many researchers have examined the effect of pigment diets on different-weight farmed fishes, it is necessary to associate the above diets with issues related to the sustainability of aquaculture. Performing these studies is also key to pigmentation improvement. In the rainbow trout fish, astaxanthin in the diet has better results for skin pigmentation compared to a canthaxanthin diet [87]. The desired skin color of farmed salmonids can be achieved by two main carotenoids, astaxanthin and canthaxanthin [88]. There are multiple other benefits to the robustness of the farmed fish itself. Specifically, fishes of the Paramisgurnus dabryanus species that were fed diets enriched in astaxanthin presented significant improvements in innate immunity and antioxidant activity in the intestine, muscles, and skin [89]. Similar research supported that fish of the species Salmo salar L. that were fed astaxanthin-enriched diets contained 2 to 20 times more antioxidant vitamins in the tissues, e.g., retinol and α-tocopherol, and in the liver, e.g., retinol, α-tocopherol, and ascorbic acid, compared to those fed astaxanthin-free diets. Immune parameters and hemoglobin tend to increase in fish on diets with low quantities of astaxanthin [90]. Most likely, astaxanthin effectively acts as an antioxidant in fish when administered through the diet; however, further investigation of this issue is required. Synthetic fish foods rich in astaxanthin can improve fish egg quality and increase larval production. According to researchers, the use of astaxanthin as a dietary supplement in the fish Gadus morhua L. demonstrates significant benefits, such as increasing the number of eggs produced by 20%, the number of floating eggs by 37%, and finally, the number of fertilized eggs by 47% [91]. A similar study reported that fish of the Hippocampus guttulatus species that were in captivity were fed frozen Palaemonetes varians as an artificial diet with elevated astaxanthin levels. These foods were effective in improving egg quality and the growth and survival of juvenile Hippocampus guttulatus [92]. However, other research indicates that yellowtail fish were intentionally fed astaxanthin to elucidate the effect of this carotenoid on egg quality and spawning performance. Astaxanthin was effective in improving egg quality and increasing the survival rate of yellowtail fish larvae [84]. Multiple other benefits, such as fish growth and survival, are reported in more detail for the Salmo salar L. fish. The fish were bred with a compound diet with astaxanthin, and the results showed that astaxanthin positively affected fish growth and survival [82]. Other research indicates that farmed Pagrus pagrus fishes in captivity are dark gray, while wild fishes of the same species are red, pink, and silver. These distinct differences between wild and farmed Pagrus pagrus fish result in differences in quality in the organoleptic characteristics of farmed fish, leading to lower consumer acceptance [80]. Such problems can be resolved with the application of a diet enriched with carotenoids such as astaxanthin. All of these factors are criteria that determine the quality and cost of catches, criteria that determine the consumer response to farmed aquaculture fish, and criteria that enhance the viability of the aquaculture industry.

5. Carotenoids and Their Impact on Human Health

The consumption of fish with a high level of carotenoids can be a strategy for the possible prevention and treatment of certain human diseases. It is, therefore, necessary to investigate the bioactivity of these substances in the face of possible preventive and therapeutic effects on human health. However, the assumption that the consumption of these fish will have the expected preventive and therapeutic effects on human health is not scientifically proven. This is an issue that needs further investigation. However, the modern way of life increases stressors that usually come from air pollution, tobacco, exposure to chemicals, or exposure to ultraviolet (UV) light, having the negative impact of an increase in free radicals (e.g., hydroxyl and peroxyl radicals). Free radicals can damage DNA, proteins, and lipid membranes. Oxidative damage leads to aging, macular degeneration, and carcinogenesis [107]. Therefore, by highlighting the antioxidant activity of carotenoids, the demand for these aquaculture fish may increase even more due to the multiple benefits offered by consumption. This section aims to analyze the major carotenoids used as colorants in applied diets with an emphasis on β-carotene, canthaxanthin, zeaxanthin and lutein, and astaxanthin in farmed aquaculture fish, as well as the impact of these carotenoids on human health.

Much shrimp waste can be used for the extraction of valuable substances that can help human health. The fermentation of shrimp heads with the fungus of Paenibacillus spp. contributes to the production of natural α-glucosidase inhibitors for the treatment of diabetes. Also, properly processed Pandalus borealis shrimp by-products produce hydrolyzed enzymes that help reduce high blood pressure and oxidative stress. The chitin extracted from shrimp by-products serves as a raw material to produce functional foods. Shrimp by-products in powder form contribute to a reduction in cholesterol and triglycerides. Cooked shrimp by-products that have undergone enzymatic hydrolysis yield recovered astaxanthin, which, in turn, is included in functional foods that provide various benefits for human health. Another food containing astaxanthin is carotenoid-enriched farmed fish, which provides benefits to human health [1,81].

According to Figure 4, β-carotene acts as a fat-soluble natural antioxidant [108]. β-Carotene is a dietary source of vitamin A [109,110]. The combination of all of these benefits could prevent the development of obesity [111]. Obesity is a disease directly linked to oxidative stress. β-Carotene as an ingredient has a high antioxidant capacity that prevents and controls oxidative stress. In addition, β-carotene as an ingredient regulates lipogenesis by suppressing peroxisome proliferator-activated receptor (PPAR)-γ; therefore, the consumption of β-carotene is effective against obesity [112]. On the other hand, the use of β-carotene in dietary supplements also has anti-cancer effects, while at the same time, people consuming higher amounts of β-carotene have a lower rate of lung cancer compared to those who do not consume β-carotene supplements [113]. Moreover, the action of canthaxanthin is beneficial to human health because it removes free radicals from the body and also has immunomodulatory activity [114]. In addition, canthaxanthin can be a personal care product as a natural tanning agent for human skin [115]. On the other hand, canthaxanthin has anti-cancer activity since it satisfactorily inhibits the malignant transformation of C3H/10T1/2 cells [116]. It also inhibits the growth of malignant human cell lines, such as WiDr cells from colorectal adenocarcinoma and SK-MEL-2 melanoma [117]. However, other researchers suggest that canthaxanthin consumption has negative effects on human health, causing diseases such as retinal dystrophy or aplastic anemia [115]. In conclusion, the use of canthaxanthin under specific conditions may provide benefits to human health. According to Figure 4, the combined action of the bioactive substances zeaxanthin and lutein has a positive effect on human health for various eye and heart diseases, as well as leads to further improvements in neuronal damage and human skin [118,119,120,121]. The combined action of zeaxanthin and lutein has a positive effect on ocular diseases such as cataracts [121,122] and on certain types of cancer [121]. The action of astaxanthin has high antioxidant, anti-inflammatory, and anti-cancer effects on human health [19,123,124,125]. In addition, astaxanthin protects against the negative effects of photo-oxidation caused by ultraviolet sunlight [124] and supports the health of human skin due to its photoprotective properties [19]. In addition, astaxanthin has anti-diabetic and anti-obesity properties. Astaxanthin inhibits adipogenesis at the intracellular level by antagonizing PPAR-γ in adipocytes. Thus, the consumption of astaxanthin has an anti-obesity effect, as it regulates adipogenesis [126]. Diabetes is associated with oxidative stress. Oxidative stress caused by hyperglycemia damages the β-cells of the pancreas. Astaxanthin reduces the oxidative stress in pancreatic β-cells caused by hyperglycemia, regulates blood glucose levels, and increases insulin secretion. Therefore, the consumption of astaxanthin is effective against diabetes [127]. It improves the health of the eyes, cells, and nervous system, as well as the liver [19,123]. Specifically, astaxanthin has preventive and therapeutic actions against fibrosis, tumors, and ischemia of the liver [128]. All of the carotenoids mentioned in this review have antioxidant activity and can thus scavenge free radicals produced by stressors. The use of these carotenoids has a preventive and sometimes therapeutic effect against specific human diseases. In some cases, certain carotenoids, such as canthaxanthin, have side effects. Therefore, carotenoids with side effects must be administered with a sense of personalization to be considered a safe food for human consumption. Three organizations, i.e., the Food Advisory Committee, the Scientific Committee on Animal Nutrition (SCAN), and the Scientific Committee on Food (SCF), emphasize that the daily consumption of canthaxanthin should not exceed 0.03 mg/kg, which is equivalent to approximately 1.5 kg of salmon per week [129]. The information provided by the recommendations and restrictions against certain carotenoids that have or do not have side effects has an advisory role in the regulation of the amounts of carotenoids given as dietary supplements to farmed fish, the amounts of carotenoids included in the consumed fish, and the desired daily or weekly amount of fish consumption by human beings.

6. Trends and Future Challenges of Shrimp Waste Utilization

In the future, the enzymatic hydrolysis method could be a very promising and effective solution for the recovery of carotenoids and other bioactive substances, such as proteins and chitin, from suitably pretreated shrimp waste [5]. The enzymatic hydrolysis method can be easily applied at a low cost and with greater safety compared to other methods, such as the acid hydrolysis method. However, the cost of commercial enzymes is high, and in some cases, the yield of enzymes and the productivity obtained are low due to some secondary metabolites produced by the catalytic process of the enzyme–substrate complex [130,131]. One solution to such problems is the immobilization of the enzyme in a two-phase system, where one phase contains only the enzyme, and the other phase contains only the product. The immobilization of enzymes is an economical solution that increases productivity during the hydrolysis process because it facilitates the separation of the enzyme from the generated product, thus reducing the possible risks of contamination of the generated product while allowing the reuse of the enzyme [130,131,132]. The demand for carotenoids is increasing due to their multiple benefits for the development of new innovative products, such as functional ingredients in food, pharmaceuticals, cosmetics, animal feed, and fertilizers [9,10,11,12,13]. Although shrimp waste contains considerable amounts of recovered carotenoids, they do not seem to be fully utilized on an industrial scale. On the other hand, there is a trend to extract carotenoids from algae. However, this method of carotenoid production does not appear to yield the expected results, as it is not considered a fully financially viable solution [133]. The production of carotenoids from algae involves more processes and possibly higher costs than the enzymatic hydrolysis method because it requires the production of algae, the harvesting of algae, and carotenoid extraction processes [133]. Additionally, the production of carotenoids from algae is likely to burden the environment due to the new waste generated over time, while the lack of shrimp waste valorization will continue to degrade the environment. The potential future challenges of shrimp waste utilization for the recovery of carotenoids raise questions that need to be investigated and addressed. The important questions to be addressed are as follows: How can we change the trend in carotenoid production from algae toward shrimp waste? How can an integrated management and utilization system of shrimp waste be developed with the view of recovering carotenoids on an industrial scale? Could the production of carotenoids through the utilization of shrimp waste and the production of algae be combined? Further research is needed to answer these questions. Therefore, new and environmentally friendly technologies for shrimp waste collection, separation by category, and the pretreatment of biomass are needed to apply enzymatic hydrolysis on an industrial scale. This will have a positive impact on the extraction of larger quantities of recovered carotenoids. In this way, sustainable development will be promoted, and the problem of environmental change will be mitigated by reducing shrimp waste discharged into the environment and recycling shrimp waste. This future challenge is likely to change the trend in carotenoid production, with a focus on shrimp by-products.

7. Discussion

The problem of shrimp waste disposal affects the environment, does not contribute to sustainable development, and is directly related to the phenomenon of environmental change. Therefore, this review is important in analyzing such problems and providing an impetus for further research. A trend is observed in the extraction of carotenoids from the produced algal biomass rather than shrimp waste biomass without knowing whether the former is environmentally friendly and cost-effective or whether it contributes to sustainable development. The use of industrial shrimp waste, such as shells, heads, tails, carapaces, exoskeletons, cephalothoraxes, and appendages, has recently attracted the attention of researchers. Shrimp waste contains valuable bioactive substances such as proteins, carotenoids, chitin, and high-added-value substances that are extracted by appropriate techniques from low-value shrimp waste. The extraction of astaxanthin can be accomplished through several techniques, including enzymatic hydrolysis using proteolytic enzymes; the use of solvents, e.g., ethanol or acetone, with ultrasound-assisted extraction (UAE); the use of supercritical CO₂ extraction; and drying by heat. Each method has advantages and disadvantages, such as reducing the process time and energy consumption while at the same time increasing the cost of implementation capital, as well as the expected future upgrades over time. Obviously, the process of enzymatic hydrolysis with the use of shrimp waste can offer an effective, inexpensive, and environmentally friendly solution to recover bioactive ingredients as additives to improve the organoleptic characteristics of farmed fish. The commercial enzymes Alcalase, Protamex, and Flavourzyme are used to obtain astaxanthin from seafood waste, with the enzyme Alcalase being used more frequently, under the fermentation conditions of pH = 8.5, temp. = 40–60 °C, and an incubation time equal to 2 h, while enzyme inhibition is achieved by thermal inhibition at 90 °C for 20 min. On the other hand, enzymatic hydrolysis with the use of proteolytic enzymes does not present a weakness in the reuse of proteolytic enzymes. On the contrary, the trend for the production of algae and the extraction of carotenoids from algae does not seem to yield the expected results because it is not considered a financial solution. How can an integrated system of shrimp waste utilization be developed, with the view of recovering carotenoids on an industrial scale, so that sustainable development will be promoted? Consequently, further research is needed to answer these questions. Carotenoids are used in the aquaculture sector to improve certain organoleptic characteristics of farmed fish. Fish feed enriched with astaxanthin is administered to farmed fish with an initial weight of 6.5 ± 0.5 g up to a final weight of 6100 ± 900 g, and improvements in the pigmentation and antioxidant properties of the skin, flesh, and muscle, as well as improvements in the egg quality, growth, and survival of fish, were achieved by feeding a diet enriched with astaxanthin 5.5 ± 0.3 mg/kg at 400 mg/kg. The consumption of fishes enriched with astaxanthin can improve human health; e.g., the antioxidant action of carotenoids can neutralize free radicals often caused by stressors. Furthermore, carotenoids such as β-carotene, canthaxanthin, zeaxanthin, and astaxanthin have preventive or therapeutic effects against serious human diseases, such as cancer, heart disease, and diabetes. Canthaxanthin is harmful to human health when the daily intake exceeds 0.03 mg/kg, which corresponds to 1.5 ± 0.3 kg of weekly salmon consumption. Finally, the extraction of astaxanthin from shrimp waste can be applied to industries to increase their profits; however, a future challenge will be the development of sustainable technology so that the enzymatic hydrolysis method, with the use of proteolytic enzymes, can be applied in industries.

8. Conclusions

The commercial enzymes Alcalase, Protamex, and Flavourzyme are used to obtain astaxanthin from seafood waste, with the enzyme Alcalase being used more frequently, under the fermentation conditions of a pH equal to 8.5, a temperature equal to 40–60 °C, and an incubation time of 2 h, while the enzyme inhibition process is achieved by thermal inhibition at 90 °C for 20 min.

Fish feed enriched with astaxanthin is administered to farmed fish with an initial weight of 6.5 ± 0.5 g up to a final weight of 6100 ± 900 g.

Improvements in the pigmentation and antioxidant properties of the skin, flesh, and muscle, as well as improvements in the egg quality, growth, and survival of fish, were achieved by enriching feed with astaxanthin 5.5 ± 0.3 mg/kg at 400 mg/kg.

The consumption of fish enriched with carotenoids such as astaxanthin, zeaxanthin-lutein, and β-carotene has a positive effect on the prevention and treatment of human diseases.