Valorisation of Animal By-Products: Comparison
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Please note this is a comparison between Version 2 by Beatrix Zheng and Version 1 by Rui Boavida-Dias.

The treatment and reduction of animal by-products has registered an increase in the awareness that this type of materials is underutilised and can represent a valuable resource if treated correctly. Consequently, it is no longer practical to dispose of animal by-products, especially when a significant amount of potential raw materials is produced, which can have a high economic potential through the production of new products with significant added value. The reuse and valorisation of animal by-products (ABPs) generated in the food retail sector can involve sending these by-products to another company/organisation or industry, where they will be processed in order to obtain added-value products. This type of valorisation originates an industrial symbiosis.

  • agri-food business
  • animal by-products
  • food retail sector
  • industrial symbiosis
  • circular economy

1. Industrial Symbiosis

Industrial symbiosis (IS) is the evolution of the concept of industrial ecosystems, first proposed in 1989 [54][1]. Therefore, IS is the approach of a more sustainable integrated industrial system that identifies business opportunities which leverage the synergistic exchange of underutilised resources, including water, energy, material, residues, waste and by-products [9][2]. IS involves organisations operating in different sectors of activity, and its main objective is not only to avoid the use of landfills, but also to maximise the reuse and recovery of surplus streams, preventing resources from becoming waste as a first option.
In 2008, the definition presented by Chertow [10][3] suggested a criterion of distinguishing IS from other exchanges, the 3-2 heuristic. According to this heuristic, IS has to involve at least three different entities and the exchange of at least two different resources without any of the entities having recycling as the main function.
Companies producing waste can implement two distinct IS strategies: internal IS, where the company uses the waste produced by a given production process in other production processes within the company’s boundaries, replacing inputs of virgin raw materials, or external IS, where a company sends their waste to other companies that will use it in their production processes [55][4].
The ultimate objective of IS is producing more without spending more resources or energy through cooperation between organisations, where companies use waste or by-products from other companies. This is an effective method of “locking” the matter cycle and, consequently, to obtain a zero level of waste. According to Neves et al. (2020) [56][5], the implementation of an IS project can have several beneficial impacts not only on an environmental level, but also on a social and economic level. The environmental benefits are mostly related to the reduction of the impacts associated with the processes and methods of waste disposal and the extraction and import of virgin raw materials, which lead to GHG emissions, scarcity of natural resources and waste that would stop at landfills and incinerators. The social benefits are due to the creation of jobs by new activities related to the transformation of residues and by-products and the valorisation of labour resources due to the decrease in costs of raw materials. Combined with social and environmental benefits, an IS project also leads to economic gains that are related to the reduction of raw material costs and waste treatment. In addition to these benefits, an IS project can also be a possible solution for organisations in order to meet environmental requirements, such as reducing GHG emissions.
These impacts verified at different levels demonstrate that creating synergies is not only about the exchange or sharing of resources; it is also a new value creation process for all parties involved. Therefore, the overall value created globally by the synergy will be greater than the sum of the value created by the organisations operating independently [57][6].

2. Implemented Industrial Processes and Technologies

Concerning the ABPs of the retail sector (muscle, bones and fats), the industrial processes and technologies already applied to this type of organic by-products were assessed. ABPs have a high content of proteins and lipids, and consequently, many possible technologies for the valorisation of these materials are related to the extraction or recovery of these components due to the possibility of developing new products from them for commercial applications. However, the application of ABPs is challenging because by-products do not have a homogeneous composition, have low water solubility and also have a high risk of being contaminated with pathogens, which leads to the need for special operating conditions [58][7].
On an industrial scale, the most used process in the management of all ABPs is the rendering process, in which the stabilisation and sterilisation of these materials occur by digesting them under severe conditions of temperature and pressure (133 °C, 3 bar, 20 min). The sterilisation step consists of removing hazardous microorganisms, eliminating the risk of diseases, while the stabilisation step involves water removal to prevent product decomposition, making its storage safer for later use in other production processes. Its two main final products are animal meal and animal fat, which can be used as animal feed and for biodiesel production, respectively. This process includes an initial step of reducing the granulometry of the meat products (up to approximately 50 mm), followed by a heat treatment for sterilisation in a continuous or discontinuous system under the conditions of pressure and temperature mentioned above. After the digestion step, the product obtained is pressed in a screw press to separate the solid fraction from the liquid fraction. The solid stream is fed to another screw press, operating at higher pressure, to remove the residual water and fat. The main output of this screw press is a solid stream, rich in protein, that will be dried and ground, originating a solid product: animal by-product meal. The liquid stream obtained in the first screw press is decanted to remove entrained solid particles, and it is centrifuged to separate the organic phase from the aqueous phase. After this step, the products obtained are an aqueous stream, which will be treated before being discharged into the environment, and an organic stream, animal fat, that is stored. The water removal is very important in the rendering process, and the aqueous stream obtained represents about 65% of the initial mass of the raw materials [59,60][8][9].
The BioRefinex process also incorporates all ABPs generated in the retail sector, where the combination of thermal hydrolysis and anaerobic digestion allows for the production of organic fertilisers and biogas with a methane content between 55 and 75% [61][10].
Regarding bones, pyrolysis technology can be used for the production of charcoal, which in turn can be applied as an organic fertiliser due to its high concentration of phosphorus and calcium and low carbon content that give the charcoal great agronomic efficiency. This final solid product obtained is called bio-phosphate [62][11]. The application of this technology to animal bones reduces the use and exploitation of mineral phosphate (apatite), which is a compound widely used in the formulation of agricultural fertilisers, and according to European Commission (2017) [63][12], it is classified as a critical raw material due to its economic importance and high supply risk. In pyrolysis, the bones are crushed and sent to a pre-treatment of sterilisation, similar to the rendering process, whose operating conditions are 133 °C, 3 bar and residence time of 20 min. The solid stream obtained is fed to the 3R pyrolysis reactor (recycle–reduce–reuse), where the pyrolysis occurs at 850 °C at a relative pressure of −50 Pa for 20 min. During pyrolysis, the volatile substances and proteins are removed from the mineral part, and the products obtained are a solid stream (bio-char) and a gas stream. The effluent gas stream from the pyrolysis reactor is sent to cyclones in order to remove some solid particles that have been entrained with the gas. Then, it goes to a partial condenser, in which a part of the gaseous stream is condensed, obtaining a liquid stream composed of an aqueous and an organic phase. The non-condensed gases (pyrolysis gas) are sent to storage and can be used as syngas or can be directed to catalytic processes for the production of jet fuel or nitrogen recovery. The liquid stream obtained after the partial condensation of the gases is centrifuged or decanted to separate the existing phases, obtaining an aqueous phase and an organic phase (bio-oil), which can be used as a fuel. After drying, the final solid product obtained represents 46% of the initial mass of raw materials [60][9].
The gelatine production process uses the combination of an alkaline pre-treatment and thermal hydrolysis to produce gelatine from animal bones, usually from cattle. In this process, the bone particles are subjected to demineralisation by adding a hydrochloric acid solution for the removal of the inorganic content. Then, the alkaline pre-treatment with a supersaturated lime solution allows non-protein substances to dissolve and changes the structure of collagen, making it soluble in water. Finally, the thermal hydrolysis involves about 3–6 extractions in series at progressively higher temperatures, with 5–10 °C difference between steps. The first extraction takes place at a temperature of 50–60 °C, and the last usually takes place at a temperature close to the boiling point of water (100 °C). This procedural step involves the breakdown and solubilisation of collagen. After concentration of the solution of hydrolysed collagen, the gelatine obtained has a water content of 10% and less than 1% of impurities [64,65][13][14].
Animal bones can also be used in the production of chondroitin sulphate, which is one of the acids that make up the intercellular substance, responsible for repairingcells and giving firmness totissues. This industrial process uses the combination of enzymatic and alkaline hydrolysis to obtain a chondroitin sulphate powder with a degree of purity greater than 90% [66][15]. The final product has many applications, namely in the pharmaceutical industry, such as the production of medicines for osteoporosis problems, in the cosmetics industry, for the production of creams and products for hair and skin, and in the animal feed industry, where it can be used as a food supplement.
Muscle has thermal and enzymatic hydrolysis as its main destinations. In these two technologies, the main objective is to recover the protein content of the animal’s muscle in order for it to be incorporated into animal feed formulations or in the production of flavourings and protein ingredients [67,68][16][17].
Table 71 summarises the industrial processes and technologies described above, as well as the ABPs used as raw materials in each process, the final products and their possible uses.
Table 71.
Summary of the industrial processes and technologies for the valorisation of muscle, bones and fats.
Industrial Process Technology Raw Materials Final Products Possible Uses of the Final Products References
Rendering Digestion (133 °C; 3 bar; 20 min) for sterilisation and stabilisation of ABPs Muscle

Bones

Fats		 ABP meal Animal feed [59,60][8][9]
Animal fat Biodiesel production
BioRefinex Thermal hydrolysis (180 °C; 12 bar; 40 min) + anaerobic digestion

(50–60 °C; 10–35 days)		 Muscle

Bones

Fats		 Hydrolysed proteins Fertilisers [61][10]
Biogas
Figure 10.
Possible valorisations for the animal by-products generated in the retail sector.

3.1. Fat Valorisation Systems

Poultry, beef and pork fats have been involved in several biodiesel production processes, being seen as an alternative, cheaper and sustainable raw material for the production of a biodegradable, renewable and sulphur-free fuel [71][20]. The most used process for producing biodiesel from animal fats is transesterification. The difference in fat composition among the different animal species is seen in Table 82. Moreira et al. (2015) [72][21] tested the alkaline transesterification of poultry fat at 30 °C and obtained a biodiesel yield of 81%. The properties of the biodiesel produced fulfilled the European biodiesel quality standard EN 14214, and this experiment allowed for the conclusion that transesterification can occur at low temperatures (T < 70 °C), making it possible to reduce energy and raw material costs.
Table 82.
Fat composition of different species.
  H2O Proteins Lipids Ash Reference
Poultry 28.7 3.7 67.4 0.3 [73][22]
= 88.5%
Fuels
Bovine [76][25]Gelatine production Alkaline pre-treatment + thermal hydrolysis (50–100 °C; 10–36 h) Bones Gelatine powder Food products Pharmaceuticals Photographic products [64,65][13][14]
Chondroitin sulphate production Enzymatic hydrolysis (60 °C; 8 h; alcalase) + alkaline hydrolysis

(35 °C; 1 h; pH > 11)		 Bones Chondroitin sulphate powder Pharmaceuticals

Cosmetics

Pyrolysis gas
Thermal hydrolysis

(90–110 °C; 0.5–10 h)		 Muscle Meat extract Meat flavourings

Animal feed		 [67][16]
Meat powder
Enzymatic hydrolysis

(50–52 °C; 50 min; papain)		 Muscle Protein powder Animal feed [68][17]
.
There are several technologies which allow for the processing of animal by-products into animal feed. The rendering process and hydrolysis are the most utilised and use mainly muscle by-products. The extraction of the protein content allows the production of food for animals. Regarding fats, up-scaled technologies address the production of biodiesel and are exclusively focused on that. The enzymatic hydrolysis process can derive different final products depending on the initial by-product. For example, if feed is meat and muscle, it allows for the extraction of the protein content and the production of animal food. When bone feedstocks are considered, chondroitin powders are produced which have multiple applications across the pharmaceutical and cosmetic production industries. The flexibility of the hydrolysis process alongside the fine tuning of operational parameters constitutes the biggest benefits of this process.

3. Emerging Low-TRL Systems

Due to the greater concern with the management of resources and the reduction of environmental impacts caused by the production, deposition at landfills and incineration of residues and by-products, there are several research studies that address the use of ABPs in the production and extraction of added-value products [69,70][18][19]. Figure 10 synthesises the valorisation procedures for the ABPs generated in the food retail sector—fats, bones and muscle.
/media/item_content/202203/62392d60f1853sustainability-14-02830-g010.png
4
1.5
94.0 0.5 [74][23]
Rendering, mixing (methanol (6:1)), supercritical transesterification (400 °C; 41.1 MPa; 6 min) ηbiodiesel = 88% [77][26]
Beef Fat PHA production PHAs—polyhydroxyalkanoates

Rendering, fermentation (30 °C; pH = 6.8;

aeration = 0.5 vvm; C		O2 = 40%; 300–1200 rpm) Purity >99%

Production =

0.4 g PHA/g fat

Productivity =

0.36 g PHA/L.h		 [79][28]
Biodiesel production Heating, filtration, transesterification (50 °C; KOH + methanol), decanting, washing (hot water + acetic acid), mixing (methanol), vacuum distillation, dehydration (ethylene glycol) ηbiodiesel = 73% [78][27]
Pork Fat Food formulations Winterisation process Decrease of 28% in the saturated fatty acid content [80][29]

3.2. Bone Valorisation Systems

Regarding the possible uses of animal bones, they have been used in several processes, such as the extraction of gelatine and hydroxyapatite, the production of flavourings, fertilisers and adsorbents (bone char) and even the production of composites [81,82][30][31]. The difference in bone composition among the different animal species is seen in Table 104.
Table 104.
Bone composition of different species.
  H2O Proteins Lipids Ash Others Reference
Poultry 51.0 19.0 9.0 15.0 6.0 [83][32]
Bovine 46.0 19.0 15.0 20.0
Pig 7.7 2.9 88.7 0.7 [75][24]
Animal feed
[66][
Emiroğlu et al. (2018) [76][25] used turkey rendering fat in the production of biodiesel through a two-step reaction (esterification and transesterification), obtaining a final product with an ester content of 96.7% and meeting the EN 14214 and ASTM D6751 standards. The final yield obtained was 88.5%. Marulanda et al. (2010) [77][26] also tested the production of biodiesel through supercritical transesterification of chicken fat, obtaining an overall yield of 88%. This experiment showed that the transesterification of low-cost lipid feedstocks with low excess of methanol and without generation of glycerol is technically feasible, and it is likely to be used at an industrial scale. It was also concluded that the thermal decomposition of chicken fat is an important factor; however, it was found that this factor was not significant if heated up to 350 °C.
Souissi et al. (2018) [78][27] tested beef fats as raw material for biodiesel production. In this experiment, enzymatic and chemical transesterification were used, and it was concluded that although the biological method allowed them to obtain a FAME-richer biodiesel, by the chemical method, a biodiesel with better physicochemical properties was obtained. The FAME yield for the biological and chemical methods were 94 and 73%, respectively.
Beef fats were also tested as carbon feedstock in the production of polyhydroxyalkanoates (PHAs), which are biodegradable polyesters considered to be a possible alternative to petroleum-derived plastics. According to Riedel et al. (2015) [79][28], the production of PHAs involves very high production costs compared to traditional plastics, so using this cheaper raw material would reduce these costs. The final product obtained had high purity (>99%), and a product of 0.4 g PHA/g fat was obtained, showing that it is a process with the potential for industrial application.
According to Amorim et al. (2015) [80][29], pork fat can be used in food formulations through the winterisation process. This process is a method used to modify the characteristics of oils and fats to provide added value by concentrating the unsaturated fatty acids of the raw materials. The final product obtained in this experiment showed a decrease of approximately 28% in the saturated fatty acid content, and this process improved the quality of the pork fats, reducing the peroxide value and concentrating more than 70% of the unsaturated fatty acids. Table 93 provides technical descriptions of the experimental procedures and main results.
Table 93.
Low-TRL systems for animal fat valorisation.
ABP Final Use Experimental Procedure Results Reference
Poultry Fat Biodiesel production Heating (110 °C), filtration (30 °C), transesterification (methanol (6:1) and NaOH (1%); 30 °C; 90 min), decanting (1 h), evaporation (low pressure), mixing (50% (v/v) HCl (0.2%)), mixing (50% (v/v) H2O), dehydration (Na2SO4 (25%); 30 rpm), filtration ηextraction = 40%

η		biodiesel = 87% [
- [65][14]
Pig 36.6 21.8 17.5 24.1 - [84][33]
In the extraction of chondroitin sulphate, Wang et al. (2019) [83][32] tested two different methods: heat-resin static adsorption extraction and enzymatic extraction. The second method led to better results, obtaining a yield of chondroitin sulphate of 4.3%, while in the first method, it was only 0.14%. Therefore, it was concluded that heat-resin static adsorption extraction is a promising method to produce chondroitin sulphate; however, more investigation is needed in order to increase the process yield.
In the extraction of hydroxyapatite, the most used method is calcination at a temperature of 700 °C or above. Khoo et al. (2015) [85][34] concluded that these calcination conditions allow for the production of an organic free, crystalline and natural hydroxyapatite from cattle bones. Bee et al. (2019) [86][35] concluded that the optimal calcination temperature is 700 °C since it allows for the total removal of the organic content while conserving the CO32− content of the chicken bones, making the final products liable to be used in bone engineering applications. Azzallou et al. (2022) [87][36] also used waste bovine bones for synthesizing 1-amidoalkyl-2-naphthols derivatives. For this synthesis, the first step was the extraction of hydroxyapatite from the bones by thermal decomposition at 800 °C for 2 h. Then, the resulting product from the thermal decomposition was loaded with an aqueous solution of zinc chloride (ZnCl2), which was used as a catalyst for synthesizing 1-amidoalkyl-2-naphthols. After optimizing the reaction conditions, it was concluded that with a small amount of catalyst (50 mg ZnCl2/bovine-bone-derived hydroxyapatite), high yields (86–96%) can be obtained with residence times between 25–40 min at a temperature of 80 °C.
Erge et al. (2018) [88][37] tested the use of chicken bones in the production of gelatine. Therefore, the regular steps of the industrial process of producing gelatine from bovine bones were used at the laboratory scale, and it was concluded that chicken bones can be a good alternative raw material, since the obtained gelatine had properties similar to the commercial one. Hosseini-Parvar et al. (2009) [89][38] used an enzymatic treatment of cattle bones with neutrase before the extraction of gelatine. After optimizing the operating conditions of this treatment, an overall yield of 13.9% was obtained. Etxabide et al. (2017) [90][39] also highlighted the use of pig and bovine bones as raw material for gelatine production in order to develop active gelatine films that can be further used in the packaging industry [91][40].
Animal bones can also be used in the production of adsorbents, namely bio-char, and the most used procedure is pyrolysis. Shahid et al. (2019) [92][41] and Patel et al. (2015) [93][42] used the pyrolysis process in cattle bones, and the final products obtained showed an adsorption capacity of 10.6 mg F/g adsorbent and a percentage of 17-β oestradiol removal from water of 41.4%. These two procedures showed interesting results for possible environmental applications.
A similar process was used by Deydier et al. (2005) [94][43], in which chicken bones were subjected to double calcination in order to be further used as fertilisers. The coal produced had 56.4% phosphates and 30.7% calcium in its composition, which makes it a compound with high agro-economic efficiency for use as agricultural fertiliser.
Pig bones were also used in the production of bio-char through a three-step process, which included pre-charring under mild conditions, acid treatment with H2SO4 or H3PO4 and thermal activation (pyrolysis). In this process, the maximum conversion yield obtained was 68.3%, and the final product was tested on the adsorption of methylene blue in order to determine the impregnation ratios of the acid treatment [95][44].
Harish et al. (2018) [96][45] also used bovine bones in particulate-reinforced epoxy composite, which is widely used in industrial applications (aerospace, automotive, biomedical) due to its high strength with lower weight. The carbonised bone particles were incorporated into the reinforcement at different mass fractions (5 to 25%). It was concluded that the tensile and flexural strength increase up to 15%, and the use of carbonised bone particles allows for better strength properties than those of the reinforcement with non-carbonised bone particles.
Wang et al. (2016) [97][46] tested the use of chicken bones in the production of flavourings through hot pressure extraction (HPE). Regarding the percentage of protein and amino acid recovery, it was concluded that the HPE procedure is a promising process for the production of flavourings from bones. However, it is an inefficient process in the extraction of calcium since the calcium content in the flavouring produced was 4.8 mg/100 g, whereas in bones it is 1078 mg/100 g. Table 115 provides technical descriptions of the experimental procedures for the valorisation of animal bones.
Table 115.
Low-TRL systems for animal bone valorisation.
ABP Final Use Experimental Procedure Results Reference
Poultry Bones Chondroitin sulphate extraction Washing (H2O; 30 min), mixing (H2O (1.5:1)), heating (120 °C; 0.1 MPa; 120 min), filtration (100-mesh sieve), centrifugation, heat-resin static adsorption extraction, mixing (trichloroacetic acid (7% w/v); 4°C; 24 h), centrifugation (15,000×
]. Table 126 contains the composition of muscle of the different animal species. The main method used at the laboratory scale is hydrolysis, which can be acidic, alkaline or enzymatic.
Table 126.
Muscle composition of different species.
  H2O Proteins Lipids Ash Reference
g; 20 min), mixing (ethanol (70% v/v); 24 h), centrifugation (5000× g; 5 min), drying (60 °C), mixing (H2O), ultrafiltration, freeze drying 22.8ηCS = 0.14%

% recovered = 67.4%

M		CS = 35.81 kDa72][21
15
]
]
[83][32] 1.0 1.2 [ Rendering, filtration, heating (110 °C; 1 h), esterification (methanol (40%) + H2SO4 (2.5%); 63 °C; 1 h), decanting, mixing (H2O; 65 °C), heating (110 °C), transesterification (methanol (20%) + KOH (1%)), decanting, mixing (H2O; 65 °C), heating (110 °C)
Crushing, washing (acetone), filtration, drying

(60 °C; 24 h), mixing (H		2O (1.5:1) + trypsin), extraction (47 °C; 6 h), heating (10 min), filtration (100-mesh sieve), centrifugation (12,000× g; 10 min), mixing (ethanol (70%); 4 °C; 24 h), centrifugation (5000× g; 5 min), drying (60 °C)ηbiodiesel ηCS = 4.25%

M
Gelatine extraction
Crushing (1–3 mm), washing (H
2
O), demineralisation (HCl (50 g/L); 8 °C; 2 h), washing (H2O), enzymatic treatment (neutrase; pH = 9; 50 °C), heating (100 °C), mixing (pH = 7), gelatine extraction (T; 3 h), centrifugation (30 °C; 900×
Poultry 75.0 99][48]
CS = 37.18 kDa
Bovine 75.1 19.2 4.4 1.3 [100][49] Hydroxyapatite extraction Washing, drying (oven), crushing, calcination

(electric furnace; P		atm; 700 °C) % lost mass = 28.72% [86][35]
g
Pig 75.1 22.8 Flavouring production Crushing, washing (H2O (1.5:1); 10 min), hot pressure extraction (H2O; 135 °C; 120 min), filtration (200-mesh sieve), centrifugation (16,000× g), evaporation (0.08–0.1 MPa; until 30% solids) % recovery:

Proteins = 83.51%

Collagen = 96.81%

Amino acids = 31.03–47.73%

C		Ca = 4.2–4.8 mg/g [97][46]
Gelatine and collagen extraction Crushing (1–2 mm), mixing (H2O (1 g:2 mL)), heating

(35 °C; 1 h), washing (H		2O), filtration, acid treatment (HCl (1 g:2 mL); 10 °C; 24 h), washing (H2O), filtration, alkaline treatment (NaOH (1 g: 4 mL; Troom; 48 h), mixing (phosphoric acid until pH = 4), washing (H2O), filtration, mixing (H2O (1 g: 3 mL); 76–82 °C; 105–183 min), centrifugation (5000 g; 30 °C; 30 min), drying (oven; 42 °C) Gel strength = 1175.8 g



T		melting = 33.71 °C

T		gelling = 25.15 °C [88][37]
Fertiliser production Dehydration (110 °C; 4–5 h), rendering (133 °C; 3 bar;

20 min), double calcination (electric furnace; 550 °C)		 Coal represents 24% of initial poultry meal mass

Coal:

56.33% phosphate

30.7% calcium		 [94][43] Pyrolysis

(850 °C; 20 min)		 Bones Bio-char Fertilisers and adsorbents [62][11]
Pig Bones Bio-char production Crushing (2–5 cm), precarbonisation (450 °C; N2 atmosphere; 10 °C/min), crushing (0.25–0.35 mm), pyrolysis (800 °C), washing (H2O) H = 68.3% [95][44] Bio-oil
Bovine BonesFuels Bio-char production Washing (H2O, 90 °C; 24 h), pyrolysis (350 °C; 2 h), cooling (Troom
), vacuum filtration, mixing (Ca(OH)
2
until pH = 9), flocculation, centrifugation, ion exchange H = 13.9%

Gel strength = 243.22 g

µ = 4.915 cP		 [89][38]

3.3. Muscle Valorisation Systems

The muscle, which corresponds to the edible part of the animal, is mostly used to recover its protein content, which can be further used in food formulations, including flavourings, protein supplements and animal feed [98][47
1.2
1
[75][24]
Regarding the enzymatic hydrolysis method, Nchienzia et al. (2010) [101][50] used poultry meal and concluded that the use of a combination of endopeptidase (alcalase) and exopeptidase (flavourzyme) allows for better hydrolysis results than its separate operation, obtaining a degree of hydrolysis of 11.13% and 58% recovery of hydrolysed material. Therefore, this method allows for the production of inexpensive hydrolysed poultry meal, which can be used in animal food products. Kurozawa et al. (2008) [102][51] also performed the enzymatic hydrolysis of chicken muscle, obtaining a fraction of hydrolysed proteins of 31% and a recovery of 91% of the proteins. The final product obtained showed good application as a protein supplement.
The procedure followed by Stiborova et al. (2020) [103][52], in addition to laboratory scale, was scaled up, and similar results were obtained. The final product presented the following composition: 77% proteins, 9% chondroitin sulphate, 7% hyaluronic acid and 4% amino acids. It has a commercial value of 88 USD/kg (approximately 73 EUR/kg). Saiga et al. (2003) [104][53] also studied the inhibitory effect of enzymes in chicken breast after double enzymatic hydrolysis with aspergillus and trypsin. The hydrolysed extract showed stronger inhibitory activity than the chicken extract without hydrolysis (1.1 mg% and 1060 mg%, respectively), and when applied to rats, it allowed for the reduction of their blood pressure by 50 mm Hg.
According to Wang et al. (2018) [105][54], enzymatic hydrolysis was also performed on turkey muscle, and it was concluded that flavourzyme is an effective enzyme for the preparation of antioxidant hydrolysate from turkey meat, which can be used as a functional ingredient in food formulations.
To improve the enzymatic hydrolysis method, Thoresen et al. (2020) [106][55] studied the effect of pre-treatments in enhancing the properties of the hydrolysate product. On the one hand, it was concluded that the microwave pre-treatment, by affecting the protein structure, promoted its solubility, and the ultrasound pre-treatment promoted the antioxidant properties of the hydrolysate proteins. On the other hand, the high-pressure pre-treatment induced not only the antioxidant properties but also the protein solubility when a pressure between 100 and 200 MPa was applied.
Selmane et al. (2008) [107][56] used the thermal hydrolysis method to recover the protein content of poultry muscle, and the obtained hydrolysate was purified and concentrated by successive microfiltration and ultrafiltration. An extraction yield of 83% was obtained; however, the overall yield of the process was 55%. This method was shown to be a good alternative for the extraction of proteins from animal by-products since it allowed for the maintenance of the functional properties of the extracted proteins.
The isoelectric solubilisation/precipitation (ISP) method was performed by Tahergorabi et al. (2012) [108][57] to recover proteins from poultry meat. This method induces structural changes in some proteins, namely actin; however, the addition of TiO2 allows for restructuring the products based on the proteins recovered by this method, leading to the formation of a potential new food product, which has to be further subjected to several studies such as sensory and storage stability tests.
Chicken muscle was also used at the laboratory scale for the production of adhesives/glues. According to Wang et al. (2012) [109][58], after several alkaline and acidic treatments for protein extraction, the product obtained was mixed with a solvent in order to form the adhesive. Sodium dodecyl sulphate (3 M) and urea (3%) were the solvents whose produced adhesives had the best performance.
Regarding the beef and pork muscles, there are not many research studies on the valorisation and use of these animal by-products. However, it is mentioned in the literature that the main destination of these materials is indeed the rendering process [5,50][59][60]. According to this, the Lifevalporc project [110][61] uses pig carcasses; after rendering sterilisation, the pork fat is sent to the biodiesel production process, and the remaining material is sent to the anaerobic digestion process for the production of organic fertilisers. Table 137 provides technical descriptions of the experimental procedures for the valorisation of animal muscle.
Table 137.
Low-TRL systems for animal muscle valorisation.
ABP Final Use Experimental Procedure Results Reference
Poultry Muscle Protein recovery (hydrolysis) Rendering, mixing (H2O), enzymatic hydrolysis

(7 h; 50 °C; pH adjustment with NaOH (5.4 M)), heating (85 °C; 15 min), centrifugation (1000× 		g;

4 °C; 30 min), freeze drying (0.045 mbar; −44 °C)

Enzymes: alcalase (pH = 8) and flavourzyme (pH = 7)		 % hydrolysed = 11.13%

% recovered = 58.1%		 [101][50]
Sterilisation (121 °C; 15 min), enzymatic hydrolysis (phosphate buffer (50 mM);

50–56 °C; 18 h), filtration, centrifugation (15000 rpm; 30 min), filtration, spray drying (67 °C; 4 h)

Enzyme: papain Cproteins = 768 mg/g

C		CS = 89.6 mg/g

C		HA = 73.9 mg/g

C		amino acids = 44.2 mg/g [103][52]
Crushing, mixing (H2O (3:1) + NaOH), heating, enzymatic hydrolysis (52.5 °C; pH = 8 with addition of NaOH), heating (85 °C; 20 min), centrifugation (3500rpm; 20 min)

Enzyme: alcalase (4.2%) % hydrolysed protein = 31%

% recovered protein = 91%		 [102][51]
Crushing (3000 rpm; 3 min), mixing (H2O; 1100 rpm; 5 min), hydrolysis (40 °C; pH = 9; 60 min), centrifugation (10,000× g; 15 min), microfiltration (2 bar), ultrafiltration (2 bar), isoelectric precipitation (HCl (37%) until pH = 4), centrifugation (5000× g; 5 min), mixing (hexane + isopropanol (3:2 v/v); 1 h; 20 °C), evaporation ηextraction = 83%

η		process = 55% [107][56]
Crushing (2.3 mm), ISP (H2O + TiO2 (6:1);

32–34 °C), mixing (NaOH until pH = 11.5; 10 min), centrifugation (10,000× 		g; 10 min), mixing (HCl until pH = 5.5; 10 min), centrifugation Addition of TiO2 to the ISP-recovered proteins resulted in increased gel strength [108][57]
), crushing (75–300 µm)
Adsorption capacity =

10.56 mg F/g [92][41]
Washing, drying (110 °C), crushing (1–2 mm), washing (acetone + H2
Mixing (H2O; pH = 4–4.5; 3.5 h), filtration, centrifugation, evaporation (until 25° Brix), enzymatic hydrolysis (aspergillus (0.06%); 50 °C; pH = 7; 1 h), heating (10 min), enzymatic hydrolysis (trypsin/chymotrypsin (1%); 37 °C;

pH = 7; 1 h), heating (10 min), centrifugation		 Inhibitory activity:

Chicken extract = 1060 mg%

Hydrolysed extract = 1.1 mg%		 [104][53]
Adhesive and glue production Mixing (H2O (1:4 w/v); 10 min), filtration (200-mesh sieve), centrifugation (10,000× g; 4 °C; 25 min), mixing (NaOH (2 M) until pH = 11), centrifugation (10,000× g; 4 °C; 25 min), mixing (HCl (2 M) until pH = 5), centrifugation, washing (H2O), freeze drying, mixing (sodium dodecyl sulphate (3 M) or urea (3%) and NaOH (10%) until pH = 10) Urea (3%)/SDS (3 M):

Dry strength = 7.99/9.35 MPa

Wet strength = 3.35/2.9 MPa

Soaked strength = 5.21/8.89 MPa		 [109][58] O), filtration, drying, pyrolysis (400 °C; 2 h; 10.2 °C/min) Removal of 41.4% of 17-β oestradiol from water [93][42]
Composite production Crushing, washing (H2O), drying, carbonisation (550 °C; 1 h), crushing (100 µm) Composite strength increases with bone carbonisation [96][45]
Hydroxyapatite extraction Washing (H2O; 1 h), washing (acetone; 2 h), drying, crushing (45–125 µm), calcination (T; 3 h; 10 °C/min) Optimal calcination temperature

≥700 °C		 [85][34]

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