Gelatin is a traditional functional protein with water-soluble properties and has the potential of forming transparent gels under certain conditions. The major source of gelatin is pigskin and is using in processed food and medicinal products. Though the use of food products adulterated with porcine-derived gelatin create concerns in the mind of Muslim communities, as in Islam; it is not acceptable or literally, it is called Haram in Islam Religion. However, in recent times initiatives have been taken in producing gelatin from Halal sources, such as fish, chicken and bovine slaughtered according to Islamic teachings. Therefore, we highlighted different porcine alternative derived gelatin sources and also methods to detect edible product contents pork or other haram stuff. This review could be useful in providing information to a large number of readers and food processing companies to minimize or if possible eradicate the use of porcine-derived gelatin in commercial food and medicinal products.

Keywords: gelatin, muslim, halal, fish, chicken, islam, religion

Gelatin is a fibrous protein with high molecular weight, derived from collagen which comprises about 25 to 35% of total body protein, through thermal hydrolysis.^1,2 It is the main protein connective tissue and widely found in mammals, birds and fishes.^3,4 In general, gelatin plays a vital role in food processing and formulation (i.e., gelling process and some respond to the surface behavior of gelatin).⁵ The other functional properties of gelatin are of foaming, emulsifying, setting index and water holding capacity.⁴ Gelatin has commercial use in food, pharmaceutics, cosmetics, and photographic application.^6,7

The Muslim population share 23.4% of the total world population (1.6 billion), which has been reported by Jamaluddin et al.⁸ The demand for halal food is on the rise.⁹ Regarding the increase in demand of halal food, an issue is raised by many scholars and scientists that, gelatin derived from the pig skin¹⁰ is using in almost every processed food products. However, Muslims do not approve gelatin derived from prohibited sources like porcine gelatin except the extreme situation where there is no other alternative.¹¹ In contrast to that, pork derived gelatin can be replace by using gelatin derived from halal sources. Such as fish,¹² cow,¹³ chicken and turkey¹⁴  derived gelatin.

In the recent past, a handsome amount of review articles has published.^5,15,16 However, based on our exploration capability, no comprehensive study is available to clarify between porcine, and it’s alternative (Halal) derived gelatin. The need of this study is to shed light on the issue of Halal and Haram gelatin-based products consumption. Further, showcased its importance in improving antioxidant activities and also outline some methodology to detect porcine adulteration and market data of major Islamic countries. This review might be helpful in the future regarding knowledge and significance about porcine, and its Halal alternative derived gelatin.

The primary source of gelatin is pigskin, but some other sources are also contributing to fulfill requirements of gelatin production.^10,17 Porcine content is 46% to the total production of gelatin, bovine hide (29.4%) pork, and cattle bones (23.1%).¹⁸ Therefore, more attention has been given to the alternatives of porcine gelatin.¹⁹ Further talking about porcine-derived gelatin alternatives, the fish gelatin market share is still low comparing to bovine and porcine gelatin.²⁰ But there is a sizeable amount of available scientific studies reported that aquatic source derived gelatin exhibit better film-forming properties than that of mammalians.²¹ In addition to that, fish gelatin with a low melting point was reported for having good release properties of food sensory attributes.²² Therefore, it became an important issue to provide gelatin derived from Halal Sources. In contrast to which, gelatin derived from poultry, animals (considered as Halal and slaughter according to Islamic rules) and especially marine sources²³ can be accepted as Halal and might be a potential alternative for porcine gelatin (Table 1).

Commercial uses of gelatin

The unique gelling, stabilizing, healing, ointment, capsule and coating properties of gelatin made it as the most widely used biodegradable compound in commercial food production, pharmaceutical and photographic industries.^42‒46 The clear and transparent structure of gelatin accounts for its significance, especially in the food and pharmaceutical industries.⁴² Further, it has been reported that, annually, tons of gelatin has been used in candies, desert, meat, ice cream and bakery products.^42,47 Moreover, the gelatin also inhibits the recrystallization of lactose sugar during cold storage.⁴⁸ While in the pharmaceutical industry, the making of hard and soft capsule shells, tablets, granulation and syrups, all requires gelatin because it serves as a natural coating material and is also highly digestible. According to a report, pharmaceutical industry is using approximately 6% of the total gelatin production.⁴⁹ For the sports industry, gelatin plays an important role in energy drinks production for athletes and is a necessary component of energy drink.⁵⁰ In photography, first it was used in 1871 after coating the sensitizing agent on a glass plate in gelatin. The use of gelatin in the cosmetic industry is of high importance, as it is commonly using in shampoo, lipstick, conditioner, cream and fingernail formulas (link available in the reference section). Furthermore, gelatin derived from aquatic sources may be more applicable in the halal/kosher market than that of mammalian and porcine gelatin.

The increase in the production of biodegradable polymer like gelatin getting worldwide attention, one of the reasons why producer finds it as an attractive option is due to its antioxidant properties.^51,52 There are many sources of gelatin, however, there is a notable growing interest in producing gelatin from fish waste because of the outbreak of mad cow disease and the unacceptability of bovine and porcine-derived gelatin by Muslims, Jews and Hindus community.⁵³ Additionally, fish gelatin possess biologically active peptide, and such peptides have the potential to act as an antioxidant against the like of linoleic acid.^54,55 Further, the fish gelatin hydrolyzes with papain to produce antioxidant peptides, which exhibit high radical scavenging properties.⁵⁶ In contrast to that, hydrolysate derived from fish gelatin can be used as a functional food material that induces immunity against ultraviolet A in the skin and also protects food and others biological system from oxidation.^17,57 Moreover, the gelatin derived from the Pacific cod skin was hydrolyzed with pepsin and produced two bioactive peptides namely GASSGMPG (662 Da) and LAYA (436 Da),⁵⁸ it shows the strong inhibitory effect of Angiotensin-I-converting enzyme (ACE), an important enzyme in the control of hypertension and type-2 diabetes.⁵⁸ Further, they suggested using it in functional food preparation to lower the blood pressure and cardiovascular diseases (CVD).⁵⁹ In another study, gelatin from cuttlefish is reported for stopping the β-carotene bleaching by donating an atom to peroxyl radicals of linoleic acid. Which further demonstrated its importance in protecting food from drying and exposure to light.⁵⁸ Meanwhile, gelatin derived from the poultry waste also exhibit metal chelating and radical scavenging properties and can be considered as a Halal alternative of porcine gelatin.⁶⁰ Some other valuable peptides are also reported for its beneficial activity by many researchers confirmed the broad and wide range of available and functional peptides from porcine alternative source.^54,55,61,62 Thereby, according to Jridi et al.⁵⁹ all gelatins in all probability contained peptides which are electron or hydrogen donors that converts the free radical to the more stable product by reacting with them and dismiss the radical chain reaction.⁵⁹ However, we will suggest after studying the recent research that fish gelatin (advantage of having odorless properties) has the edge over poultry gelatin due to the complication in managing poultry wastes.⁶³ Therefore fish gelatin can be utilized as a substitute antioxidant driver for porcine and bovine-derived gelatin.

In recent times, about 50,000 tons of beef meat has been found adulterated with horse meat in Europe,⁶⁴ and it cannot be an accident but a fraudulent act to mix the meat of different species such as horse or pork and blend it into the cattle beef, which creates concerns in the mind of ethnical groups (Muslims and Jews).⁶⁵ As the 1.5 billion Muslims shares around 20% of the world population and for them, the use of porcine-derived food products is strictly prohibited according to the teaching of Islam.^65,66 Such is the case with the production of gelatin as well. According to a report, about 80% of gelatin produced from the pigskin in Europe.¹⁰ In addition to that, most of the food manufacturers use porcine-derived gelatin rather than its alternatives.^63,67 Due to the vast use of porcine gelatin, it is necessary for the Muslims to test the processed food for the detection of porcine-derived gelatin adulteration.⁶⁸ Because for the Muslim, the tolerance level becomes 0% when it comes to porcine and porcine-derived gelatin contamination in processed food.⁶⁵ Therefore we tabulated some of the advanced techniques regarding the detection of porcine gelatin in food products, from the previously published research articles (Table 2).

The contribution from the waste of livestock, fisheries and poultry industry is important for a country GDP growth.⁶³ The main reason is that animal byproducts have the capability of decreasing the level of protein malnutrition and food insecurity.⁷⁵ According to the available online data, Muslim countries produce heaves of animal waste and do have the potential of producing a large amount of Halal gelatin (Table 3).^76‒79 However, the exported values (Table 3) showing huge gap between the production of gelatin and the other variables (Meat and fisheries). This showcasing the poor management of animal by products in all the major Islamic countries. The reason might be lack of production facility and less knowledge about managing animal waste products. Therefore more attention is needed particularly in the area of managing waste from farm animals, aquatic sources and the poultry industry. As it is contributing in the local market economy, decrease concerns regarding the use of Halal and Haram gelatin and also increase the country economy by reducing the percentage of imported gelatin.

The need for this review is to highlight the issue regarding Halal and Haram gelatin. As the major gelatin producing source in the international market is pigskin but it is always controversial for ethnical groups, such as Muslims and Jews. Therefore we summarized different porcine alternative gelatin sources, which provide better gelling, antioxidant and functional properties than that of porcine-derived gelatin. In addition to that, we tabulated some market data of major Islamic countries, which suggests that all those stated countries can produce a handsome amount of Halal gelatin and can make themselves gelatin sufficient. However, apart from Turkey, all other countries are producing gelatin less than their average requirement. The reasons must be varied such as the lack of waste management practices and industrial technology. Therefore, more work is required in the sector of waste management and adapt the state of art industrial technology to produce Halal gelatin inside the country.

None.

Author declares that there is none of the conflicts.

1. Vijayaraghavan R, Thompson B, MacFarlane D, et al. Biocompatibility of choline salts as crosslinking agents for collagen based biomaterials. Chemical Communications.2009;46(2):294‒296.
2. Tabarestani HS, Maghsoudlou Y, Motamedzadegan A, et al. Optimization of physico‒chemical properties of gelatin extracted from fish skin of rainbow trout (onchorhynchusmykiss). Bioresource technology.2010;101(15):6207‒6214.
3. Eysturskarð J, Haug IJ, Elharfaoui N, et al. Structural and mechanical properties of fish gelatin as a function of extraction conditions. Food Hydrocolloids. 2009;23:1702‒1711.
4. Rawdkuen S, Thitipramote N, Benjakul S. Preparation and functional characterisation of fish skin gelatin and comparison with commercial gelatin. International Journal of Food Science & Technology.2013;48(5):1093‒1102.
5. Karim A, Bhat R. Gelatin alternatives for the food industry: Recent developments, challenges and prospects. Trends in food science & technology. 2008;19(12):644‒656.
6. Schrieber R, Gareis H. Gelatine handbook: Theory and industrial practice. John Wiley & Sons; 2007.
7. Figueroa Lopez KJ, Andrade‒Mahecha MM, Torres‒Vargas OL. Development of antimicrobial biocomposite films to preserve the quality of bread. Molecules. 2018;23(1):212.
8. Jamaludin MA, Zaki NNM, Ramli MA, et al. Istihalah: Analysis on the utilization of gelatin in food products. 2nd International Conference on Humanities, Historical and Social Sciences. IPEDR. 2011:1‒5.
9. Easterbrook C, Maddern G. Porcine and bovine surgical products: Jewish, muslim, and hindu perspectives. Archives of Surgery. 2008;143:366‒370.
10. Boran G, Regenstein JM. Fish gelatin. Advances in food and nutrition research. 2010;60:119‒143.
11. Eriksson A, Burcharth J, Rosenberg J. Animal derived products may conflict with religious patients’ beliefs. BMC medical ethics. 2013;14:48.
12. Mahmoodani F, Ardekani VS, See S, et al. Optimization and physical properties of gelatin extracted from pangasius catfish (pangasius sutchi) bone. Journal of food science and technology. 2014;51(11):3104‒3113.
13. Andiç S, Boran G, Tunçtürk Y. Effects of carboxyl methyl cellulose and edible cow gelatin on physico‒ chemical, textural and sensory properties of yoghurt. International Journal of Agriculture & Biology. 2013;15.
14. Du L. Khiari Z, Pietrasik Z, et al. Physicochemical and functional properties of gelatins extracted from turkey and chicken heads. Poultry science. 2013;92(9):2463‒2474.
15. Babel W. Gelatine–ein vielseitiges biopolymer. Chemie in unserer Zeit. 1996;30:86‒95.
16. Mariod AA, Fadul H. Gelatin, source, extraction and industrial applications. Acta Scientiarum Polonorum Technologia Alimentaria.2013;12(2):135‒ 147.
17. Sai‒Ut S, Jongjareonrak A, Rawdkuen S. Re‒extraction, recovery, and characteristics of skin gelatin from farmed giant catfish. Food and Bioprocess Technology.2012;5(4):1197‒1205.
18. Gómez‒Guillén M, Giménez B, López‒Caballero MA, et al. Functional and bioactive properties of collagen and gelatin from alternative sources: A review. Food hydrocolloids. 2011;25(8):1813‒1827.
19. Morrison N, Clark R, Chen Y, et al. Gelatin alternatives for the food industry. Progr Colloid Polym Sci. 1999;114:127‒131.
20. Choi SS, Regenstein J. Physicochemical and sensory characteristics of fish gelatin. Journal of Food Science. 2000;65(2):194‒199.
21. Avena‐Bustillos R, Chiou BS, Olsen C, et al. Gelation, oxygen permeability, and mechanical properties of mammalian and fish gelatin films. Journal of food science. 2011;76(7):E519‒E524.
22. Aewsiri T, Benjakul S, Visessanguan W, et al. Antioxidative activity and emulsifying properties of cuttlefish skin gelatin modified by oxidised phenolic compounds. Food Chemistry. 2009;117(1):160‒168.
23. Bhat R, Karim A. Ultraviolet irradiation improves gel strength of fish gelatin. Food chemistry. 2009;113(4):1160‒1164.
24. Jongjareonrak A, Rawdkuen S, Chaijan M, et al. Chemical compositions and characterisation of skin gelatin from farmed giant catfish (pangasianodon gigas). LWT‒Food Science and Technology. 2010;43(1):161‒165.
25. Madhamuthanalli CV, Bangalore SA. Rheological and physico‐chemical properties of gelatin extracted from the skin of a few species of freshwater carp. International journal of food science & technology. 2014;49(7):1758‒1764.
26. Chandra M, Shamasundar B. Rheological properties of gelatin prepared from the swim bladders of freshwater fish catla catla. Food Hydrocolloids. 2015;48:47‒54.
27. Muyonga J, Cole C, Duodu K. Extraction and physico‒chemical characterisation of nile perch (lates niloticus) skin and bone gelatin. Food hydrocolloids. 2004;18(4):581‒592.
28. Zhou P, Mulvaney SJ, Regenstein JM. Properties of alaska pollock skin gelatin: A comparison with tilapia and pork skin gelatins. Journal of Food Science. 2006;71(6):C313‒C321.
29. Gómez‒Guillén M, Turnay J, Fernández‒Diaz M, et al. Structural and physical properties of gelatin extracted from different marine species: A comparative study. Food Hydrocolloids. 2002;16(1):25‒34.
30. Sinthusamran S, Benjakul S, Kishimura H. Molecular characteristics and properties of gelatin from skin of seabass with different sizes. International journal of biological macromolecules. 2015;73:146‒153.
31. Souissi N, Abdelhedi O, Mbarek A, et al. Gelatin based bio‒films prepared from grey triggerfish’skin influenced by enzymatic pretreatment. International journal of biological macromolecules. 2017;105(Pt 2):1384‒1390.
32. Koli JM, Basu S, Nayak BB, et al. Functional characteristics of gelatin extracted from skin and bone of tiger‒toothed croaker (otolithes ruber) and pink perch (nemipterus japonicus). Food and bioproducts processing. 2012;90(3):555‒562.
33. Samal SK, Goranov V, Dash M, et al. Multilayered magnetic gelatin membrane scaffolds. ACS applied materials & interfaces. 2015;7(41):23098‒23109.
34. Hafidz R, Yaakob C, Amin I, et al. Chemical and functional properties of bovine and porcine skin gelatin. International Food Research Journal. 2011;18:813‒817.
35. Mokrejs P, Langmaier F, Mládek M, et al. Extraction of collagen and gelatine from meat industry by‒products for food and non food uses. Waste Management & Research. 2009;27(1):31‒37.
36. Ran XG, Wang LY. Use of ultrasonic and pepsin treatment in tandem for collagen extraction from meat industry by‒products. Journal of the Science of Food and Agriculture. 2014;94(3):585‒590.
37. Mad‐Ali S, Benjakul S, Prodpran T, et al. Characteristics and gel properties of gelatin from goat skin as affected by pretreatments using sodium sulfate and hydrogen peroxide. Journal of the Science of Food and Agriculture. 2016;96(2):2193‒2203.
38. Xu M, Wei L, Xiao Y, et al. Physicochemical and functional properties of gelatin extracted from yak skin. International journal of biological macromolecules. 2017;95:1246‒1253.
39. Abdullah MSP, Noordin MI, Ismail SIM, et al. Physicochemical evaluation and spectroscopic characterisation of gelatine from shank and toes of gallus gallus domesticus. Sains Malaysiana. 2016;45(3):435‒449.
40. Almeida PF, Lannes SCdS. Extraction and physicochemical characterization of gelatin from chicken by‐product. Journal of Food Process Engineering. 2013;36(6):824‒833.
41. Abedinia A, Ariffin F, Huda N, et al. Extraction and characterization of gelatin from the feet of pekin duck (anas platyrhynchos domestica) as affected by acid, alkaline, and enzyme pretreatment. International journal of biological macromolecules. 2017;98:586‒594.
42. Djagny KB, Wang Z, Xu S. Gelatin: A valuable protein for food and pharmaceutical industries. Critical reviews in food science and nutrition.2001;41(6):481‒492.
43. Howe AM. Some aspects of colloids in photography. Current opinion in colloid & interface science. 2000;5(5‒6):288‒300.
44. Van Eerd JE, Vegt E, Wetzels JF, et al. Gelatin‒based plasma expander effectively reduces renal uptake of 111 in‒ octreotide in mice and rats. Journal of Nuclear Medicine. 2006;47(3):528‒533.
45. Ulubayram K, Cakar AN, Korkusuz P, et al. Egf containing gelatin‒based wound dressings. Biomaterials. 2001;22(11):1345‒1356.
46. Wang T, Zhu XK, Xue XT, et al. Hydrogel sheets of chitosan, honey and gelatin as burn wound dressings. Carbohydrate polymers. 2012;88(1):75‒83.
47. Johnston Banks F. Gelatine. In: Food gels. Springer: 1990; p. 233‒289.
48. Jamilah B, Harvinder K. Properties of gelatins from skins of fish‒black tilapia (oreochromis mossambicus) and red tilapia (oreochromis nilotica). Food chemistry.2002;77(1):81‒84.
49. Hidaka S, Liu S. Effects of gelatins on calcium phosphate precipitation: A possible application for distinguishing bovine bone gelatin from porcine skin gelatin. Journal of Food Composition and Analysis. 2003;16(4):477‒483.
50. Phillips GO, Williams PA. Handbook of food proteins. 1st ed. Elsevier: 2011. 464 p.
51. Kavoosi G, Dadfar SMM, Mohammadi Purfard A, et al. Antioxidant and antibacterial properties of gelatin films incorporated with carvacrol. Journal of Food Safety. 2013;33(4):423‒432.
52. Arvanitoyannis IS. Formation and properties of collagen and gelatin films and coatings. Protein‒based films and coatings. 2002;467:484.
53. Haug IJ, Draget KI, Smidsrød O. Physical behaviour of fish gelatin‒κ‒carrageenan mixtures. Carbohydrate Polymers. 2004;56(1):11‒19.
54. Kim SK, Byun HG, Park PJ, et al. Angiotensin i converting enzyme inhibitory peptides purified from bovine skin gelatin hydrolysate. Journal of Agricultural and Food Chemistry. 2001;49(6):2992‒2997.
55. Mendis E, Rajapakse N, Kim SK. Antioxidant properties of a radical‒scavenging peptide purified from enzymatically prepared fish skin gelatin hydrolysate. Journal of agricultural and food chemistry. 2005;53(3):581‒587.
56. You L, Regenstein JM, Liu RH. Optimization of hydrolysis conditions for the production of antioxidant peptides from fish gelatin using response surface methodology. Journal of food science. 2010;75(6):C582‒C587.
57. Kato S, Matsui H, Saitoh Y, et al. Fish collagen‒containing drink is subcutaneously absorbed and attenuates the uva‒ induced tissue‒ integrity destruction and DNA damages in 3d‒human skin tissue model. Journal of Functional Foods .2011;3(1):50‒55.
58. Ngo DH, Vo TS, Ryu B, et al. Angiotensin‒i‒converting enzyme (ace) inhibitory peptides from pacific cod skin gelatin using ultrafiltration membranes. Process Biochemistry. 2016;51(10):1622‒1628.
59. Jridi M, Souissi N, Mbarek A, et al. Comparative study of physico‒mechanical and antioxidant properties of edible gelatin films from the skin of cuttlefish. International journal of biological macromolecules. 2013;61:17‒25.
60. Omar WHW, Sarbon N. Effect of drying method on functional properties and antioxidant activities of chicken skin gelatin hydrolysate. Journal of food science and technology. 2016;53(11):3928‒3938.
61. Saiga A, Iwai K, Hayakawa T, et al. Angiotensin i‒converting enzyme‒inhibitory peptides obtained from chicken collagen hydrolysate. Journal of Agricultural and Food Chemistry. 2008;56(20):9586‒9591.
62. Nakade K, Kamishima R, Inoue Y, et al. Identification of an antihypertensive peptide derived from chicken bone extract. Animal Science Journal. 2008;79(6):710‒715.
63. Jayathilakan K, Sultana K, Radhakrishna K, et al. Utilization of byproducts and waste materials from meat, poultry and fish processing industries: A review. Journal of food science and technology. 2012;49(3):278‒293.
64. Dg health and consumers, european commission. Horse meatissue, dg health and consumers, european commission. Brussels; 2013.
65. Von Bargen C, Dojahn Jr, Waidelich D, et al. New sensitive high‒ performance liquid chromatography–tandem mass spectrometry method for the detection of horse and pork in halal beef. Journal of agricultural and food chemistry. 2013;61(49):11986‒11994.
66. Shabani H, Mehdizadeh M, Mousavi SM, et al. Halal authenticity of gelatin using species‒specific pcr. Food chemistr. 2015;184:203‒206.
67. Batu A, Regenstein JM, Dogan IS. Gelatin issues in halal food processing for muslim societies. Electronic Turkish Studies. 2015:10.
68. Riaz MN, Chaudry MM. Halal food production. CRC press; 2003. 400 p.
69. Sudjadi, Wardani HS, Sepminarti T, et al. Analysis of porcine gelatin DNA in a commercial capsule shell using real‒time polymerase chain reaction for halal authentication. International Journal of Food Properties. 2016;19(9):2127‒2134.
70. Demirhan Y, Ulca P, Senyuva HZ. Detection of porcine DNA in gelatine and gelatine‒containing processed food products‒halal/kosher authentication. Meat science. 2012;90(3):686‒689.
71. Nikzad J, Shahhosseini S, Tabarzad M, et al. Simultaneous detection of bovine and porcine DNA in pharmaceutical gelatin capsules by duplex pcr assay for halal authentication. DARU Journal of Pharmaceutical Sciences. 2017;25(1):3.
72. Wardani DP, Arifin M, Suharyadi E, et al. Quantitative detection of bovine and porcine gelatin difference using surface plasmon resonance based biosensor. SPIE. 2015;9506:1‒8.
73. Lee JH, Kim MR, Jo CH, et al. Specific pcr assays to determine bovine, porcine, fish and plant origin of gelatin capsules of dietary supplements. Food chemistry. 2016;211:253‒259.
74. Tasara T, Schumacher S, Stephan R. Conventional and real‒ time pcr–based approaches for molecular detection and quantitation of bovine species material in edible gelatin. Journal of food protection. 2005;68(11):2420‒2426.
75. Alao BO, Falowo AB, Chulayo A, et al. The potential of animal by‒products in food systems: Production, prospects and challenges. Sustainability. 2017;9(7):1089.
76. Kim SK, Kim YT, Byun HG, et al. Isolation and characterization of antioxidative peptides from gelatin hydrolysate of alaska pollack skin. Journal of agricultural and food chemistry. 2001;49(4):1984‒1989.
77. Nur Hanani ZA, Beatty E, Roos YH, et al. Development and characterization of biodegradable composite films based on gelatin derived from beef, pork and fish sources. Foods. 2013;2(1):1‒17.
78. Rose JB, Pacelli S, Haj AJE, et al. Gelatin‒based materials in ocular tissue engineering. Materials. 2014;7(4):3106‒3135.
79. Salamon A, Van Vlierberghe S, Van Nieuwenhove I, et al.. Gelatin‒based hydrogels promote chondrogenic differentiation of human adipose tissue‒derived mesenchymal stem cells in vitro. Materials. 2014;7(2):1342‒1359.