A.A. Karim and Rajeev Bhat, Contributing Editors
Editor's note: This article has been extensively condensed and adapted from "Gelatin Alternatives for the Food Industry: Recent Developments, Challenges and Prospects," first published in Elsevier Ltd.ís Trends in Food Science & Technology. For more information on the authors and the publication, see this articleís end.
Gelatin, a unique hydrocolloid, serves multiple functions in a wide range of applications. The main sources of gelatin include pigskin, cattle bones and cattle hide. Gelatin replacement is a major interest, due to emerging and lucrative halal, kosher and vegetarian (including Hindu) markets. In the 1980s, gelatin replacement gained increased attention, especially in Europe, with the emergence of bovine spongiform encephalopathy (BSE). It has been less of a concern to consumers in the U.S., and, in 2003, the FDA stated that [in gelatin processing] "the reduction in BSE infectivity is sufficient to protect human health." This article discusses gelatinís unique properties, the rationale for developing gelatin alternatives, the various studies and approaches that have been undertaken to develop gelatin alternatives, as well as challenges and prospects. Finally, directions for future studies are suggested.
Rationale for Developing Gelatin Alternatives
Gelatin is used as a beverage clarifier, including for beer, fruit and vegetable juices. It is used in desserts at 8-10% of dry weight, in yogurt at 0.3-0.5% as a thickener, in ham coatings at 2-3%, and in confectionery and dietary supplement capsules at 1.5-2.5%. Other uses include pastry fruit toppings; instant gravies, sauces and soups; edible films for confectionery products; as a stabilizer in ice cream, cream cheese and cottage cheese; and, also, in food foams and fruit salads. Religious and vegetarian lifestyle choices may prohibit certain consumer groups from eating foods like yogurt, whipped desserts, low-fat margarine spreads, marshmallows, ice cream and other products containing gelatin, an animal-based ingredient.
The development of gelatin alternatives provides market opportunities for food processors. For example, one estimate places halal food products at some 12% of total global trade in agri-food products, and the market is growing. Muslims are projected to account for 30% of the worldís population by 2025.
Unique Properties and Uses of Gelatin
The functional properties of gelatin can be divided into two groups. The first group is associated with gelling and includes gel strength, gelling time, setting and melting temperatures, viscosity, thickening, texturizing and water binding. The second group of properties relates to gelatinís surface behavior--for example, emulsion formation and stabilization, protective colloid function, foam formation and stabilization (such as in marshmallow), film formation and adhesion/cohesion. Comparison of gelatinís rheological functions with other common food hydrocolloids can be seen in the chart "Comparison of Frequently Used Hydrocolloids--Rheological Properties."
The most commonly used gelatin property is its ability to form thermoreversible gels. At a few percent in water, gelatinís gel-melting temperature (<35∞C) is below body temperature, which can provide gelatin products with a unique 'melt-in-mouth' quality. Gelatinís most important attribute is its gel strength and, when determined by the standard method, is called the 'Bloom strength' or 'Bloom value.' Commercial products normally have Bloom values that fall between 50-280.
Gelatin offers special properties not easily imitated by other hydrocolloids. They include the following.
* "Melt-in-mouth" perception that leads to intensive flavor and aroma release. Scientists have not yet been able to find a gelling protein or polysaccharide that universally replicates this property.
* Thermally reversible gel. Some plant hydrocolloids, such as carrageenan and agar, form thermally reversible gels, but melting points are significantly higher.
* Surface activity. Although gelatin does not perform as well as gum Arabic, in regards to emulsifying/stabilizing properties, it still is an important characteristic.
* Customization ability. Gelatin is available in different gel strengths and particle sizes.
* Easy to use. Gelatin gels within the pH range typical of foods and does not require salts, sugars or food acid additions to set.
The best approach to developing gelatin alternatives is application-/process-specific. Approaches to mammalian gelatin replacement include fish gelatin, thermoreversible gels from polysaccharides and mixed polysaccharide-based systems.
Gelatin from Fish
Researchers have studied obtaining gelatin from warm- and cold-water fish skins, bones and fins. Fish gelatin is acceptable for Islam and, with minimum restriction, for Judaism. Fish skin is a major by-product of the fish processing industry.
The main difference between fish and mammalian gelatin is the content of the amino acids proline and hydroxyproline. The lower content of proline and hydroxyproline probably gives fish gelatin its low gel modulus, gelling and melting temperature. Gelatin extracted from coldwater fish, such as pollock, cod and salmon, has very low gelling (typically below 8∞C) and melting temperatures. Factors impacting gelling temperature include gelatin concentration, average molecular weight, ionic strength, pH, cooling rate and method of determination. For example, 10% mammalian gelatin forms a gel at about room temperature, whereas 10% cod gelatin will gel at ~2∞C. Some researchers have rated fish gelatin superior in a blind sensory test. The lower melting point of coldwater fish gelatin enhanced flavor release, fruit aroma and melt rate in water gel desserts.
However, challenges exist beyond low gelling temperatures. The amino acid compositions of mammalian gelatins are remarkably constant, when compared to those from different species of fish, resulting in large quality variations. Also, collagenous material (the protein source for gelatin) from fish skin is more susceptible to degradation during chemical treatment (acid or alkali), in contrast to the more stable mammalian collagen. Thus, in addition to the source or species, gelatin properties also greatly depend on raw material preservation.
To overcome or minimize problems associated with the low gelling and melting point properties of fish gelatin, three approaches have been tried: enzymatic cross-linking of gelatin; mixed gelling systems of fish gelatin and suitable plant hydrocolloids; and manipulating gelatin characteristics by the addition of solutes, such as salts.
Certain enzymes, such as transglutaminase (TGase) and tyrosinase, have been shown to cross-link gelatin. The covalent cross-linking action of TGase modifies gelatinís rheological properties, but also alters its thermoreversibility. The challenge is to maintain the physical character of the gelatin, even after substantial covalent cross-linking. Future studies could involve cross-linking gelatin with another protein. Work already has been done with casein.
Use of mixed gelling systems is another approach. With mixed gelling systems, gelation could be inhibited or enhanced, and gel textures could be very different from that formed by individual components. Patents have described improved gelatin gel strength using modified starch with gelatin. Researchers also report that a gelatin-k-carrageenan gel system might produce systems with improved gel strength, gelling and melting temperature.
Blending gelatin with other hydrocolloids can have both positive and negative results. Gellan gum, for example, accelerates the gelling speed of gelatin and substantially increases gel firmness, but reduces color and clarity. On the other hand, blending gelatin with citrus pectin reduces the firmness of the gelatin gel. Other researchers have reported carrageenan has a negative effect on the firmness, color and clarity of gelatin gels. Still others report combining gelatin and agar-agar considerably increases the degree of gel firmness compared with gelatin alone, with the gel melting point increased up to 80∞C. In the case of fruit gummies, the melting temperature can be increased to about 50∞C, hence making the product suitable for marketing in tropical climates.
Another way to manipulate a given gelatinís properties is by triggering interactions through the addition of solutes, such as salts. In one study, the functional properties of fish gelatins, such as megrim skin gelatin, were made more similar to those of mammalian gelatins, chiefly with regard to melting point, by the addition of neutral salts in appropriate conditions of pH and ionic strength.
Interesting Developments in the Search for Gelatin Alternatives
Interesting and significant developments in the search for gelatin alternatives are based on the application of carbohydrate-active enzymes. For example, researchers report thermostable amylomaltase modifies potato starch to form a thermoreversible gel with gelatin-like properties. The enzyme produces a starch product that forms a strong, white gel with a thermoreversible character, when incubated at 4∞C. The amylomaltase-modified gels retrograded reversibly, comparable to gelatin gels. Both amylomaltase-modified starch and gelatin form firm gels, but the gelatin Bloom 250 gel is stronger and sets faster. The two gels also differ in mouthfeel, and the gelatin Bloom 250 gels melt completely at 37∞C, while the amylomaltase-modified starch gels are only 50% melted at 37∞C and 100% melted at 60∞C.
Many proposed gelatin alternatives are polysaccharides, which form gels, but which do not have the defined melt set characteristics of gelatin, such as gellan-, alginate- or carrageenan-based gels. For example, pectin, carrageenan or combinations of pectin/carrageenan give similar textures as gelatin, but not quite the exact melt-in-mouth temperature profiles. These polysaccharide-based gelatin alternatives also generally have higher viscosities than gelatin.
Exploiting synergistic relationships between certain hydrocolloids has shown promise. Xanthan gum cannot form a gel on its own, but forms strong, cohesive gels with certain plant polysaccharides, notably locust bean gum and konjac glucomannan (KGM). It has been suggested that mixtures of pyruvate-free xanthan and KGM could provide gelatin replacement, where ëëmelt-in-mouthíí characteristics are important, and where moderate acidity is acceptable or necessary (e.g., fruit jellies). Other hydrocolloids considered for gelatin replacement include the following:
* Mixed high-methoxyl/low-methoxyl pectin gels. High-methoxyl (HM) pectin is not considered a good candidate as a gelatin alternative, since it forms thermally irreversible gel and requires a low pH and high-soluble solids. However, low-methoxyl (LM) pectin appears to be more flexible in terms of manipulation of gelling conditions, although at high sucrose concentrations, LM pectin also tends to pre-gel. Research has found gel properties can be controlled using HM and LM pectin mixtures, along with judicious control of Ca2++, sugar, pH and types (degree of esterification) of HM pectin.
* Modified starch/wheat fiber gel. Another study used a combination of a dual modified starch and wheat fiber gel to replace gelatin in yogurt. The starch-to-wheat-fiber-gel ratio was critical, with the optimum ratio at 60% starch to 40% wheat fiber gel. Yogurts with gelatin replacer showed higher stability against storage temperatures over 20∞C. No significant sensorial differences between the yogurts made with gelatin and gelatin replacer appeared.
* High acyl gellan gum. Gellan gum provides a range of textures, from soft, elastic gels to firm, brittle gels with one label declaration. Recent studies show levels of glycerate and acetate substituents in gellan gum can be controlled independently. Blends of high (HA) and low (LA) acyl gellan gum can produce intermediate gel textures. High acyl gellan produces soft, elastic, thermoreversible gels for applications such as cultured dairy, dressings, jams and jellies, dessert gels, dairy and fruit beverages, milk puddings and confectionery. One study showed that in the water-based dessert gel formation (15% solids), a partially deacylated form of HA gellan closely matched the gelatin texture, but had a higher melt-set temperature, which is advantageous for rapid-set formulations and for stability in hot climates.
* Carrageenan. One paper described the development of new iota carrageenan extract by using a new, proprietary extraction process. A line of ingredients has been developed for use in confectioneries, particularly for gummi-type, or molded candies. The new iota carrageenan-based products allow for shorter conditioning times, easier demolding and alternate molding processes. The paperís author also claimed the use of carrageenan instead of gelatin produced finished products that are more tolerant of excessively high temperatures in shipping or storage.
Direction for Future Studies
Currently, fish gelatin is used mainly in niche markets. Efforts to improve its quality for broader use have involved modification via cross-linking of the protein molecules. The ability of other enzymes, beyond the extensively-studied TGase, to modify fish gelatinís functional properties could be further explored, as well as the role of chemical cross-linking.
A search for new food-applicable, chemical cross-linking agents has led to polyphenols. Researchers have found that gelatin gels cross-linked with plant-derived phenolic acids (caffeic, chlorogenic, ferulic) and flavonoids had greater mechanical strength, reduced swelling and fewer free amino groups.
The role of ferulic acid and tannin warrant further investigation, particularly for their ability to improve gelatin film-making properties. Genipin is another food-compatible, chemical cross-linking agent that has recently received attention.
Protein cross-linking via Maillard reaction (so-called "Maillard cross-link") could be exploited to enhance the strength of fish gelatin gel. It is known the incubation of proteins with sugars often results in a loss of solubility. It has been suggested this is due to the polymerization of protein molecules by covalent cross-links formed as a result of the Maillard reaction.
Depending on the degree of modification required, different cross-linking treatments can be used, including enzymatic, chemical and physical methods. The primary advantage of physical methods is they do not cause potential harm; their limitation is that obtaining the desired amount of cross-linking is difficult.
Although fish gelatin does not form strong gels, it is well-suited for applications such as micro-encapsulations, light-sensitive coatings and low-set time glues. Unmodified coldwater fish gelatins also could possibly be used in refrigerated products and in situations where low gelling temperature is needed.
Extraction of gelatin from non-fish marine sources, such as sponges, tunicates and mollusks, has not yet been reported. Collagen from fish has been identified as a potential allergen. More research is required to elucidate the nature of the allergenic reactions and possible ways to circumvent the problem.
In the area of thermally reversible gels, altering starch molecular properties by microbial amylomaltase enzymes appears promising. The molecular weight of starch chains could be reduced to decrease paste viscosity, yet remain great enough to form rigid (but thermally reversible) gel through re-association. Textural profiles of starch gels are very different from gelatin; gelatin gel elasticity is much higher, making it firmer than starch gels. In addition, amylomaltase-modified starches formed opaque gels, meaning they are appropriate for only some applications.
Future research can be directed towards use of combinations of enzymatic and/or chemical modifications; elucidation of the mechanism of the enzyme action; and identification of reaction condition factors to enable proper control of the process.
Genetic engineering has made great progress in the areas of recombinant collagen and gelatin expression; this approach could be extended to produce bovine collagen/gelatin.
Recombinant gelatin has been expressed in the yeasts, Pichia pastoris and Hansenula polymorpha. Pichia was highly productive for gelatin production, but the functional, rheological and sensory properties of the recombinant gelatin have not been reported. The prospect of producing microbial gelatin through genetic engineering akin to microbial polysaccharide via fermentation systems (e.g., xanthan, gellan) is very exciting. Scientists at Iowa State University have established transgenic corn as a viable way to produce gelatin. Someday, this technique may produce high-grade designer gelatin in a safe and inexpensive manner.
The development of effective, inexpensive alternatives for mammalian-based gelatin will definitely be a boon not only for consumers, but also for the food industry. pf
This article has been extensively condensed and adapted by Claudia D. OíDonnell, chief editor, Prepared Foods, with permission from Elsevier Ltd. The original article, by A.A. Karim and Rajeev Bhat, Food Biopolymer Research Group, Food Technology Division, School of Industrial Technology, Universiti Sains Malaysia, first appeared in Trends in Food Science & Technology, Vol. 19: 644-656, under the title, "Gelatin alternatives for the food industry: Recent developments, challenges and prospects." ©Elsevier Ltd. 2008. A.A. Karim, the corresponding author, can be reached at firstname.lastname@example.org. The original article, which includes additional sections on gelatin and collagen chemistry and nearly 90 references, can be obtained from Elsevier by visiting http://tinyurl.com/26vnjtj.