The effect of calcium concentration in delimed hides on the efficiency of bating

Introduction
The first part of the research showed that different deliming methods can result in different final concentrations of calcium in delimed hides. The incorporation of EDTA into the deliming process resulted in better removal of the calcium from the hides, and whereas most calcium is removed from the surface layers of the hide only with conventional deliming methods, EDTA has the ability to remove calcium from the middle layers of the hide as well. This is important as it could have significant effects on the efficiency of bating, and whether bating would be effective in the innermost regions of the hide.
In this second part of the research, the effect of the calcium content on enzymatic bating, elastin removal and collagen degradation during bating was investigated.

Experimental
The experimental procedure was discussed in the first part of this research paper. To summarize, the different deliming methods were carried out as follows.

Four different deliming processes in different drums were carried out in a 50% float @ 33°C.

Method 1
Sufficient sodium dihydrogen phosphate and 10% phosphoric acid solution were intermittently added to the drum containing the 50% float, and the drum was run for 240 minutes to obtain a final pH of 8.0 – 8.5. After deliming, the hides were washed with 200% water three times for 30 minutes.

Method 2
2.5% ammonium sulphate was added to the 50% float in the drum and the drum run for 40 minutes. The drum was then drained, the hides then washed with 200% water for 30 minutes, and then a second deliming was carried out using 1.5% ammonium sulphate. This deliming was run for 200 minutes, and after completion the drum was drained, and the hides washed with 200% water three times for 30 minutes.

Method 3
2.5% ammonium sulphate was added to the 50% float in the drum and the drum run for 40 minutes. The drum was then drained, the hides then washed with 200% water for 30 minutes, and then a second deliming was carried out using 1.0% ammonium sulphate and 1% EDTA for 200 minutes. After completion the drum was drained, and the hides washed with 200% water three times for 30 minutes.

Method 4
2.5% ammonium sulphate and 1% EDTA were added to the 50% float in the drum and the drum run for 40 minutes. The drum was then drained, the hides then washed with 200% water for 30 minutes, and then a second deliming was carried out using 0.5% ammonium sulphate and 2% EDTA for 200 minutes. After completion the drum was drained, and the hides washed with 200% water three times for 30 minutes.
The four different deliming processes outlined above produced delimed hides with different concentrations of residual calcium in the hides from the liming process, and the effect of these different calcium concentrations on bating effectiveness was investigated in this second part of the article.

Results
Effect of calcium content of hides on enzymatic bating
A high purity trypsin enzyme and a microbial protease enzyme with high elastin activity were used to bate the delimed hides. The overall effect of the enzymatic bating could be analysed by measuring the soluble protein concentration in the float. It can be seen from Figure 1 that the more effective the deliming method was in removing calcium from the delimed hide (Method 4), there was a slight increase in the concentration of soluble protein in the float.

Figure 1. Concentration of soluble protein in the float after bating with the different deliming methods

Effect of calcium content of hides on enzymatic bating with elastase
Elastin has a great impact on the softness of leather. It has good acid and alkali resistance and can generally only be removed using enzymes. It contains desmosine. Thus, the rate of hydrolysis of elastin can be determined by analysing the concentration of desmosine in the float after bating. As can be seen from Figure 2, the concentration of desmosine in the float increased the better the removal of calcium was from the delimed hide. This indicates that the elastase enzyme is more effective at removing elastin from the hide if the calcium concentration in the delimed hide is low.

Figure 2. Concentration of desmosine in the float after bating with different deliming methods

Effect of calcium content of hides on collagen degradation
During the bating process, it is possible for the collagen to be degraded by the bating enzymes. A breakdown product of collagen is hydroxyproline. The rate of collagen destruction during bating can be monitored by analysing the concentration of hydroxyproline in the bating float. As can be seen from Figure 3, the greater the removal of calcium from the delimed hide, the greater the collagen destruction by the bating enzymes.

Figure 3. Concentration of hydroxyproline in the float delimed with the four different deliming processes

Practical Application
In the limed hide, the calcium is found in three different forms, free calcium ions, dissolved calcium hydroxide, and calcium that is bound to collagen. The calcium that is bound to collagen changes the surface charge of collagen fibres, and weak crosslinks are formed between these fibres. This increases the resistance of the collagen to bating enzymes. In the conventional process of deliming, the ammonium sulphate easily removes the free calcium ions and the dissolved calcium hydroxide, but the calcium that is bound to the collagen is not easily removed. The collagen will thus be protected from degradation by the bating enzymes. If EDTA is used in deliming, the calcium that is bound to the collagen will be more easily removed. However, this reduces the resistance of the collagen to the bating enzymes, and some slight damage to the collagen will take place.
From a practical point of view, firmer leathers that require a tight grain should not be treated with EDTA in the deliming process, as otherwise the bating enzymes could cause loose grain in the final leather. For soft leathers such as nappa and upholstery, the removal of as much calcium during the deliming process will result in a much softer leather as the bating enzymes will cause some slight damage to the collagen, thereby enhancing the softness of the final leather.

Conclusion
The calcium content of delimed hides can affect the performance of bating enzymes. Adjusting the content and form of the calcium in delimed hides allows the tanner to regulate the degree of action of the bating enzymes on the collagen and elastin. This has the effect of improving the controllability of the bating process for different types of leather.

This article is a summary of the paper “Effects of Calcium Content on the Enzymatic Bating of Delimed Hides” Journal of the American Leather Chemists Association (JALCA) Vol 117, 507 - 514 (2022).