Technical Note: Effect of animal age and conditioning method on the conversion of bovine hide into gelatine.

C. G. B. COLE AND A. E. J. McGILL

Published in:
International Journal of Food Science and Technology. (1988) 23, 525-529.
 

Introduction.

The manufacture of gelatine from bovine hide requires that the hide be 'conditioned' to increase solubilization and maximize yield and quality of the resulting gelatine. The traditional processof soaking in a lime suspension for an extended period involves large quantities of materials and any process which might reduce the conditioning time of hide is worthy of investigation. One such process is that of enzymic proteolysis, patented by Novo Industries (1977).

Alternatives to the liming of hide have been proposed. One of these, although not commercially viable, is known as the sodium sulphate/sodium hydroxide accelerated alkaline pretreatment of Ward (1953), and has been found to give excellent results for comparative studies, particularily in the laboratory.

Much is known about the age-related changes in collagen (McClain, 1977; Miller, 1981; Miller & Karmas, 1985), and it is accepted that these changes affect the conversion of collagen to gelatine.

The natural variance in hide makes it necessary to use 'twinned' experiments. Face pieces or masks are trinned for symetry around the central hair line, and then split into two equal parts about this line. The two halves are then used to compare treatments and any differences can be attributed to the treatment, rather than to between-hide variation. In the present work, hide from animals of various ages was conditioned by an alkaline soak method, and compared with a proteolytic enzymic method for yield and quality of the resulting gelatines.
 

Materials and methods.

Raw material

Masks from 20-25 calves, 4-5 'feedlot animals (18-24 months old) and 4-5 aged animals with long horns(>60 months age) were collected from the abattoir and salted with granular salt (50% w/w). The salted hides were trimmed for symmetry, and 'twinned' to give two equal portions to be conditioned by the two methods.

Alkali conditioning

After washing for 16 hr, hides (c. 7 kg) were soaked (2 kg Na2SO4, 460 g NaOH in 20 litres water) for 4 days at 21 to 22ºC (Ward 1953), before washing and acidification with 0.1 M H2SO3.

Enzyme conditioning.

The hide was treated in a tumbler with Novo Esperase in a sodium carbonate/sodium hydroxide (1.5M) buffer solution at pH 12.5 to 10.0 and temperature 24 to 28°C f6r 2 days, followed by washing and acid treatment as in the alkali process. The acidification step denatures the enzyme and prevents further proteolysis of extracted gelatine.

Gelatine preparation.

The individual twinned hide pairs were placed in separate beakers and covered with distilled water in a thermostatically controlled bath (see below) for 5 hr. Residual hide was separated from the gelatin solution with a stainless steel colander, and returned to the beakers for subsequent extractions at higher temperatures. The volume of gelatine solution was measured, and it was then filtered to high clarity through compressed paper pulp. Duplicate samples (10 ml) of filtered liquor were evaporated to dryness in stainless steel dishes at 105ºC, to determine the quantity of gelatine extracted at each stage.

Alkali conditioned hide was extracted for 5 hr at 45, 50. 55ºC and then at 93ºC (boiling point) for 7 hr to determine the final amount of available gelatine.

Enzyme conditioned hide was extracted at 50, 60, and 70ºC and then boiled for 7 hr to achieve similar amounts of extracted gelatine to the above for each extraction. This allowed valid comparison of properties of gelatine at each extraction stage. The filtered extracts (c. 3 %) were evaporated to about 10 % in a vacuum evaporator at below 42ºC. The concentrated liquor was refiltered, set at 5ºC, cut into slices and dried in a current of dry air. The sheets of dry gelatine were broken up using a Waring blender and the properties were determined.

Gelatine properties.

Bloom strength was measured (BS757, 1975) using the Boucher instrument. All results were corrected to 12.5 % moisture gelatine according to the relationship:

                        (Bloomm)½ x 87.5 / (100-Moisture %) = (Bloomc)½

where Bloomm is the measured Bloom strength at the prevailing moisture content and Bloomc is the corrected Bloom strength. Standards were analysed daily (c.v.± 2%).

Viscosity was measured (BS757, 1975) on a 6.7% solution at 60ºC, using a calibrated U-tube viscometer with a factor of c. 1 m Stoke/sec. Standards were run daily (s.e.±0.5 mSt). Again all results were corrected to 12.5% moisture content gelatine according to the relationship:

                        log Vc = log Vm x 87.5 / (100- Moisture %)

Isoionic point (pI) was determined by passing a 1% solution of gelatine through a column of mixed bed ion-exchange resin (Rohm & Haas MB3) at a flow rate of 8-10 bed volumes/hr and measuring the pH of the deionised solution (kenchington, 1951).

Results and discussion

Extractability

For both conditioning methods the ease of extractability of the hide was greatly decreased by animal age (Table 1). The parallel increase increase in insoluble residue with animal age is to be expected, but the same relationship of gelatine yield to animal age would not necessarily follow, because, yield depends upon the variable salt, hair and fat contents of the starting material, wet salted hide. If one assumes that calves average 6 months, young animals 20 months and aged animals 60 months, then for the conditioning given, the first extraction extractability decreases by some 0.5% for each month of animal age.

Table 1. Gelatine extractability from bulked twinned hides (7 kg)
 

Animal age months: Calf 
(6)
Young
(18)
Aged
(60)
Conditioning* E A E A E A
1st extraction(%) 31.0 45.2 20.7 26.8 7.1 10.3
2nd extraction(%) 56.1 31.4 41.5 23.9 25.2 17.3
3rd extraction(%) - - 32.8 21.9 44.1 21.4
Boil‡(%) 12.9 23.4 5.0 27.4 23.6 51.0
Residue g 420 770 450 665 620 910
1st extraction pH 2.6 2.7 2.6 2.7 2.4 2.6
Yield on wet salted hide (%) 15.1 14.1 21.7 22.3 23.0 23.5

*E, enzyme;extraction temperatures of 50,60 and 70ºC for first second and third extractions respectively. A, alkali; extraction temperaturesof 45,50 and 55ºC for first, second and third extractions respectively.
‡ Boiling temperature is 93ºC
 

Gelatine Quality

Table 2 clearly demonstrates the well-known phenomenon that the more readily extractable (Table 1) the gelatine, the better are the properties, with viscosity a good indicator of this trend. Conditioning method had no effect on viscosity.

pI

Conventionally, high pI gelatines are derived from acid processed pigskin; it is too mild a process to have a significant effect upon ossein or bovine hide (veis, 1964).

Alkali treatment produces gelatines of lower pI (c. 5, Table 2), but enzymic conditioning produces gelatines of uniquely high pI from bovine hide (Table 2). The pI of gelatine is significant in determining the pH at which it reacts with acidic polymers. Hence, whereas only high pI (c. 9) pigskin gelatine is now used for microencapsulation, a possibly cheaper and more readily available bovine hide gelatine could now be used, opening up markets previously closed to bovine hide gelatine manufacturers.

Difficulties were encountered in gel chromatography of the high pI gelatines, due to their partial insolubility in the o.1% SDS phosphate buffer system of Takagi (1981), and in staining them with either amido black or coomassie blue after electrophoresis (Bartley, 1973).

Table 2. Gelatine quality data*
 
Animal age 
(months)
Calf
(6)
Young
(18)
Aged
(60)
Extraction No.
1
2
1
2
3
1
2
3
Alkali conditioned
Bloom (g)
300
311
275
282
259
238
279
259
Viscosity (ms)
43.1
41.2
35.9
35.5
31.4
26.3
26.9
28.9
Moisture (%)
11.2
10.8
11.7
11.5
11.1
11.6
10.5
11.3
Ash (%)
0.57
0.50
2.71
0.66
0.79
5.48
2.02
1.12
pI
5.17
 
5.17
   
5.03
   
Enzyme Conditioned
Bloom (g)
336
353
260
329
184
216
269
179
Viscosity (ms)
43.2
37.7
36.2
33.0
21.6
24.5
24.0
23.3
Moisture (%)
8.2
8.3
11.7
8.7
11.0
10.3
9.3
10.7
Ash (%)
0.69
0.34
4.09
0.46
1.00
7.11
1.90
1.38
pI
8.76
6.99
8.03
8.55
7.04
7.24
8.09
6.90

* Extracted from bulked twinned hides; single determinations with standards.
 

Bloom strength.

As seen from Table 2, the alkali process can produce gelatine with a maximum Bloom strength of some 300 g. However a feature of the enzyme process is the abnormally high Bloom strength of the gelatines extracted from calf skin and the second extraction from young animals. These exceptionally high gel strength gelatines would be of value to manufacturers whose product prices related to Bloom strength, and to consumers who can us less gelatine to obtain a desired gel strength.

Cost of conditioning

The alkaline conditioning process was not commercially viable due to the cost of sodium sulphate. However, the enzymic process was only marginally more expensive than the conventional liming process, and required only two days compared with 60 to 120 days, showing potentially significant savings in capital costs.
 

Conclusions

The response of bovine hide to constant lecels of conditioning showed marked variance with the age of the animal, for both alkaline and enzymic methods. The production of gelatines of unique and valuable Bloom strength and pI by enzymic conditioning of bovine hide was noteworthy, as was the potential saving in capital cost for this method.
 

Acknowledgement

The authors are grateful to Davis Gelatine Industries (Pty) Ltd. for permission to publish this work.
 

References

Bartley, J.P. (1973). The electrophoretic separation of gelatine, Part I. British Food Manufacturing Industries Research Association Technical Circular No. 531 (restricted circulation).

BS757 (1975). Methods for sampling and testing gelatine (Physical and Chemical Methods), 3rd edn. London: British Standards Institution.

Kenchington, A.W. (1951).The deionisation of gelatine by ion exchange resins. British Glue and Gelatin Research Association Research report A1.

McLain, P.E. (1997). Chemistry of collagen crosslinking: relationship to ageing and nutrition. Advances in Experimental Medical biology, 821, 603-618.

Miller, A.T (1981). Age related changes in the Collagen of Bovine Corium. Ph.D. Thesis, Rutgers University New Brunswick, N.J.

Miller, A.T. & Karmas, E. (1985). Age-related changes in collagen of bovine corium: relationship to histological and ultrastructure observations. Journal of the American Leather Chemistry Association,80, 106-117.

Novo Industries (1977). Gelatin Extraction. US Patent 4 064 008 (20.12.1977).

Takagi, T. (1981). High performance liquid chromatography of protein polypeptides on porus silica gel columns in the presence of SDS: comparison with SDS-PAGE. Journal of Chromatography, 219, 123-127.

Veis, A. (1964). The Macromolecular Chemistry of Gelatine. Pp. 186-202. New York: Academic press.

Ward, A.G. (1953). An accelerated alkaline pretreatment process for the preparation of gelatine from collagen. British Glue and Gelatin Research Association Research report C1 (limited circulation).