C.G.B. COLE.
 

Leiner Davis Gelatine SA (Pty) Ltd.
PO Box 5019
1742 West Krugersdorp.
Tel +27 (0)11 951 5800
Personal:
Tel./Fax. +27 (0)11 660 6444
E-mail. drcole@cybersmart.co.za
 

Presented at the DAIRY SYMPOSIUM 2001, Gordons Bay, South Africa. 7 March 2001.
 

INTRODUCTION.
 

It is difficult to know what aspects of gelatine should be stressed to Dairy Technologists but I have decided to limit the amount of gelatine chemistry to those aspects known to be responsible for its functionality and try to concentrate on gelatine functionality and applications in the dairy industry.

It is important to note that in this discussion we are dealing mainly with proteins which are natural products with very specific functions. Due to thermal treatment, or pH change, or chemical attack, these natural proteins can lose their functionality. Examples would be the irreversible gelling of egg albumin and the destruction of enzymes by heating and/or extremes of pH, the precipitation of casein due to acidification and the thermal conversion of insoluble collagen to water-soluble gelatine. These phenomena are known as protein "denaturation".
 

WHAT IS GELATINE ?
 

Gelatine is a protein derived by thermal denaturation of collagen which is the most common protein in the animal kingdom. Type I collagen is a fibrous protein which is the "ground
substance" of skin and bone and other collagens are also found in the connective tissues like veins, arteries and the alimentary canal. There are actually some 20 different collagens known at present, each with a very specific functionality. Type I collagen is an infinite linear polymer made up of units called alpha-chains with a molecular weight of about 100 000 and each chain contains about 1000 amino acids. The protein is unique in that it is made up of triplets of amino acids, gly-X-Y. The X and Y can be any amino acid but the most common are proline and 4-hydroxyproline, which have a five membered ring structure (Figure 1). This is most important because it lies at the root of gelatine's unique ability to form smooth, elastic, thermo-reversible gels. Mammalian gelatines contain about 22 molar % of proline and hydroxyproline whereas cold water fish skin gelatine with very low gelling capacity contains only 17 molar % proline and hydroxyproline.

The next important aspect of the primary structure of gelatine is that it contains significant amounts of basic ( lysine 2.5 %, arginine 5 %) and acidic (glutamic acid 7.2 % and aspartic acid 4.7 %) amino acids. This leaves collagen and gelatine containing some 80% of non-polar amino acids. Hence, along the alpha-chain there are non-polar regions which give gelatine emulsifying properties.

In collagen, about 30 % of the glutamic and aspartic acids are present as the acid amide, hence the iso-ionic point of natural collagen is 9.2:

                                R-CONH₂ + H₂O  ­­­OH^- / HEAT  ­­­   R-COOH + NH₃

During processing to gelatine, more and more of the acid amides are converted to the acids with the liberation of ammonia and as a result the iso-ionic point of gelatine varies between 9.2 and 4.8.

Type A gelatine is most commonly made from bone or pigskin (where the animal does not exceed 6 months in age) hence, the cross-linking of the collagen is minimal and the collagen can be denatured by acidification to about pH 4 and warming to 50°C. On denaturation, the collagen dissolves to give a gelatine with pI in the region of 7 to 9 depending on the severity (time and temperature) of treatment of the collagen.

Type B gelatine is usually derived from cow-hide or bone. Due to the greater animal age, it is necessary to pretreat the collagen with alkali, which hydrolyses some of the collagen cross-links as well as the acid amides to yield a gelatine with a pI nominally in the range of 4.8 to 5.2. However, cow-hide gelatines with pI up to 6.5 are quite common. The question of pI can be significant, especially if gelatine is used as a stabilizer with other hydrocolloids where molecular charge based interactions can cause "problems".
 

GELATINE PROPERTIES.
 

Gelatine, with its unique molecular structure has some very interesting properties which are virtually unaffected by "denaturants" like modification of the thermal or charge environments, because the protein is already denatured.

Gelation and / or water-binding.

Collagen, the parent protein, consists of alpha chains wound together to form a rope-like thread due to the twisted structure imparted by the pyrrolidine rings of proline and hydroxyproline (Figure 2).

FIGURE 2.                                                   From Baily & Light 1989
 

When denatured, the helical regions of the individual alpha chains partially reform the collagen helix which results in a three-dimensional water holding network which we recognise as the typical gelatine gel. This gel is quite unique in that it exhibits excellent elasticity while it also exhibits smoothness due to the gel melting at below blood temperature. In addition, the gel is infinitely thermo-reversible. However, one aspect of gelatine gelation is that its formation is somewhat time dependent and full gel-strength development takes about 16 hours. This means that the gel forms slowly and melts slowly, hence, ice-cream stabilised with gelatine exhibits superior melt-down properties, although an ageing period of a few hours is required to fully develop the final texture.
 

Foaming.

Like many proteins, gelatine exhibits excellent foaming or air in water emulsification properties. This attribute is used in the manufacture of marshmallows where foam-generation is combined with the film-forming properties of gelatine. Hence gelatine stabilised ice-cream exhibits excellent over-run with greatly reduced tendency to shrink with age due to the strength of the air cell wall provided by gelatine.

Emulsifying.

Due to the non-polar, (oil-soluble) sections of the protein chain, gelatine has the ability to emulsify water in oils and this property is well demonstrated in the use of gelatine in very low-fat spreads. In dairy products it replaces the emulsifying properties of the natural milk proteins lost to thermal denaturation. Sometimes the emulsifying properties of gelatine may not be considered optimal in which case the addition of 0.2 to 0.4 % glyceryl mono-stearate (GMS) may be necessary.

Colloidal protection against crystallization.

Gelatine, being a colloid, has the property of preventing crystallization. In ice lollies and ice-cream this is demonstrated by the formation of a very fine crystal structure on freezing. This goes much further in that gelatine stabilised ice-cream does not become grainy or sandy due to the slow crystallisation of lactose with time.
 

USING GELATINE.
 

Gelatine is a denatured dry protein which is provided in moisture resistant multi-wall paper bags at a moisture content that ensures that the powder is wel below the "glass transition temperature". Gelatine can absorb foreign odours in storage and will react chemically with aldehydic vapours (like formalin vapour) but if kept in a cool, dry, well ventilated area, it has a virtually unlimited shelf life.
 

Gelatine is provided in many grades. Firstly there are the Types A and B which is only significant if molecular charge could be important. It has been claimed that Type A gelatine is preferred for whipped or aerated products. However, there does not seem to be any published evidence to substantiate this claim. From the point of view of gel-strength (Bloom strength), high gel-strength (> 200 g Bloom) gelatines do have superior water absorbing properties to low Bloom strength gelatines and high gel strength gelatines have a higher gel melting temperature and for these reasons alone they should be preferred in dairy products. Lower Bloom strength gelatines have to be used at a higher concentration and are recognised to give a more viscous product and a more chewy texture. Hence the quality of gelatine used may be dictated by customer preferences.
 

Specific functionality to be gained from the use of gelatine would be:
 

1. In top quality ice-cream and frozen products.

Use Rate: 0.3 to 0.5 % on mix weight for ice-cream and soft serve.

0.4 to 0.6% for milk-ice.
 

• Gelatine is easy to handle and disperse.

 
• Gelatine produces smooth body and velvety texture.

 
• Homogenisation brings about an increase in the surface area of the fat globules and gelatine can augment the plasma colloids supplying the added requirement for emulsifying agent.

 
• Gelatine assists in establishing the air cell structure formed during freezing and the strong cell wall helps prevent foam shrinkage and collapse.

 
• Gelatine prevents ice crystal growth during freezing and storage.

 
• Gelatine prevents the development of sandiness due to lactose crystal growth.

 
• Gelatine's gel structure provides heat shock resistance and better melt-down performance.

Cultured milk products are acidified by the fermentation of lactose to lactic acid. The three main products are Cultured Buttermilk, a low fat milk with 10-12 % milk solids, Sour Cream with about 40 % butterfat, and Yoghurt with 18-25 % milk solids. Stabilisers like gelatine are used to prevent syneresis and give a smooth uniformly textured product.

During fermentation some of the milk protein is coagulated thereby reducing the water binding capacity which can be countered by adding 0.2 to 0.4 % gelatine to prevent "whey off". The presence of gelatine also imparts smoothness to all cultured milk products.

The method by which gelatine prevents "whey off" has been the subject of a number of studies and Modler et al (1983) concluded that gelatine was the most effective stabiliser used but also from micrographs and other methods, concluded that gelatine is absorbed by the casein particles and this modified their properties and efficiently countered aggregation and precipitation. This protein-protein interaction is not unique to milk protein because in wine fining gelatine combines with and precipitates the unwanted haze-forming proteins in the wine.

3. Whipping or thickened cream.

The usage rate is 0.3 to 0.5 % w/w.

Gelatine increases the viscosity and shortens the time required to whip the cream. Gelatine also effectively increases the stability of the foam giving excellent over-run characteristics and a very light smooth texture. There is an exponential increase in cream viscosity with the

amount of gelatine used and the quality of the gelatine used can have a marked effect:

                     Figure 3. .      Cream Viscosity.
 

4. Processed cheese spreads.

Due to high pasturisation temperatures of these blended cheese products, much of the milk protein is denatured with loss of natural water-binding property which needs to be replaced. The addition of 0.5 to 0.8 % gelatine prevents syneresis, and the gel formed increases the spread-ability while imparting a smooth body.
 

5. Cottage Cheese.

Gelatine, when added to cottage cheese dressing, increases the viscosity giving a creamed curd with no free cream or whey and an even distribution of the dressing throughout the product. In fact the manufacturer is able to reduce the fat content and still produce an excellent creamed cottage cheese.
 

6. Cheese Cake.

In this popular product, gelatine is used in the filling at about the 1 % level because its gelling functionality is paramount.

PROCESSING GELATINE IN DAIRY PRODUCTS.

Powdered food grade gelatine is a pure protein. The South African product is made from cow-hide and conforms to the requirements of the Foods Drugs and Disinfectants Act of 1972 and in most instances this standard of purity is greatly exceeded. Gelatine manufactured in South Africa also carries the SABS (49 of 1985) and Halal marks, thus, this gelatine is safe to use as a food ingredient at any level. Today, the only gelatine that is accepted as kosher is fish skin gelatine. Some fish skin gelatines have a very low gel strength and are not suitable for use in dairy products. Also, all kosher fish skin gelatines are very expensive.

There is a low level of aerobic bacterial contamination of gelatine generally in the range of 1 to 100 cfu/g at 37°C but this has never been known to affect the flavour of cultured products. Gelatine also has a feint soupy odour and taste which, at the recommended low levels of usage, will not affect the products in which it is used.
 

Dissolving gelatine in water can be a problem and one must keep in mind that:

• Gelatine swells in cold (<30°C) water requiring some 30 minutes for complete hydration.
• Gelatine does NOT dissolve in water at under 40°C.
• Gelatine in concentrated (>10 %) solution is very viscous.
• Gelatine solutions when cooled from above 40°C will gel and the strength of the gel will increase over some 16 hours. The strength of the gel is proportional to the concentration of the gelatine in the water phase. (In ice-cream and cream the water phase is only some 60 % of the product).
• Gelatine, like milk proteins, is subject to hydrolysis or degradation, the extent of which is both pH and temperature dependent. Due to the short time processes used in the dairy industry, this will seldom be a problem.

Hence there are two processes that can be used to dissolve gelatine:

1. The gelatine can be added to the cold milk or cream with stirring and the gelatine should be allowed to swell for about 30 min. with continued agitation to hold the swollen gel in suspension. The product should then be heated to pasteurisation temperature, during which the gelatine will dissolve to form a homogeneous solution.

• The gelatine can be added to from 2 to9 times its weight of water at 95°C with stirring. The stirring should be continued until the gelatine is completely dissolved. This solution can then be added with stirring, to the product at >40°C.

 

CONCLUSIONS.
 

Gelatine's use as a stabiliser in dairy products has many advantages due to its multiple functionality:

gelling & water binding,
emulsification,
foaming,
strong foam cell wall,
crystal growth control,
casein precipitation control.

Goodman Fielder Ingredients specialises in providing hydrocolloids, so whether you prefer the gelatine or alternative approaches you can be assisted by them in meeting your product needs.

REFERENCES.

Bailey, A.J. and Light, N.D. (1989). Molecular and Fibre Structure of Collagen in Connective tissue in meat and meat products. Elsevier Applied Science. London and New York.
 

Modler, H.W., Larmond, M.E., Froehlich, D., Emmons, D.B. & Lin, C.S. 1983. Physical and sensory properties of yoghurt stabilized with milk proteins. Journal of Dairy Science, 66, 422-429.
 

SABS 49-1985. South African Bureau of Standards Standard Specification for Edible gelatine. The Council of the South African Bureau of Standards, Private Bag X191, Pretoria, R.S.A.