The initial stages of leather processing are critical for transforming raw pelts into high-quality finished leather. In Kenya's leather industry, ensuring proper pre-tanning operations such as fleshing, bating, degreasing, and pickling directly impacts the durability, texture, and appearance of leather products supplied to local manufacturers and export markets. These processes remove unwanted materials, prepare the pelt's collagen structure, and condition it for effective tanning. Mastery of these steps allows leather technologists to optimize material use and minimize defects, contributing to Kenya's competitive leather sector.

4.1 Definition of Terms

4.1.1 Fleshing and Bating

Fleshing and bating are preparatory processes that remove unwanted substances from pelts and condition the hides before tanning. These steps are vital in Kenya’s leather production, where raw materials often come from diverse sources with varying levels of contamination and fat content, such as hides from slaughterhouses in Nairobi or Kisumu.

Fleshing

Fleshing involves the mechanical removal of flesh, fat, and connective tissue from the inner side of the pelt after the hide is removed from the animal. This process ensures that the pelt surface is clean and free from residual organic matter that can cause bacterial growth and spoilage during subsequent processing.

• Purpose: To eliminate all adhering flesh and fat to prevent putrefaction and ensure uniform penetration of chemicals in later stages.
• Method: Typically carried out using fleshing machines or manually with fleshing knives, depending on the scale and resources of the tannery.
• Effect on Quality: Proper fleshing prevents uneven tanning and reduces the risk of bad odours developing during processing.
• Challenges: Inadequate fleshing leads to uneven tanning and increased waste due to rot.
• Kenyan Context: County slaughterhouses supplying raw hides often require effective fleshing before hides are transported to tanneries such as those in Eldoret or Thika.

Bating

Bating is a biochemical process that uses enzymes to remove non-fibrous proteins and loosen the pelt structure, making it supple and receptive to tanning agents. It follows liming and deliming and is critical for producing soft, pliable leather.

• Purpose: To remove residual proteins and mucopolysaccharides that cause stiffness and hinder tanning.
• Enzymes Used: Proteolytic enzymes such as pancreatic extracts or bacterial enzymes are common.
• Process Conditions: Controlled temperature and pH ensure optimal enzyme activity and prevent damage to the collagen.
• Impact on Leather: Results in improved softness, grain quality, and dye uptake.
• Local Practice: Tanneries in Nakuru often utilize bating in drums with enzyme solutions sourced locally or imported, adjusting conditions depending on the pelt type.

4.1.2 De-greasing

De-greasing is the removal of excess natural fats and oils from the pelts, which if left untreated, can cause uneven tanning and defects in the finished leather. In Kenya, where hides from animals such as cattle and goats may have variable fat content depending on diet and breed, de-greasing is essential.

Purpose of De-greasing

The main goal is to eliminate excess grease that can interfere with tanning chemicals and cause oily spots or weak leather fibers.

• Uniform Tanning: Grease can block chemical penetration, leading to uneven tanning.
• Prevention of Defects: Excess grease can cause spots and reduce the leather’s strength.
• Improved Finish: De-greased hides absorb dyes and finishes more uniformly.
• Waste Reduction: Removing grease early helps reduce downstream processing problems.
• Environmental Impact: Proper handling of grease waste is necessary to avoid pollution in nearby water bodies, a concern near tanneries such as those in Mombasa.

Methods of De-greasing

De-greasing can be performed using chemical, mechanical, or solvent-based methods, depending on the scale and resources available.

• Chemical De-greasing: Uses detergents and emulsifying agents to break down fats.
• Solvent Extraction: Organic solvents dissolve grease but require careful handling due to flammability.
• Mechanical Methods: Includes centrifuging or pressing to physically remove grease.
• Combination Approaches: Often a combination of enzymatic and chemical methods is used for thorough degreasing.
• Kenyan Tanneries: Medium-scale tanneries in Kisumu employ chemical de-greasing with biodegradable detergents to balance effectiveness and environmental safety.

4.1.3 Pickling

Pickling is a preservation and preparation step where pelts are treated with acid and salt solutions to stabilize them before tanning. This process is crucial in Kenya’s leather industry, especially when hides cannot be tanned immediately after curing due to logistical delays.

Purpose of Pickling

Pickling lowers the pH of the pelt, inhibiting microbial growth and preparing collagen for chrome tanning.

• Preservation: Prevents bacterial spoilage during storage and transport.
• Collagen Conditioning: Acid environment swells collagen fibers, improving tanning agent uptake.
• Control of pH: Typically achieved by adding formic or sulfuric acid with salt.
• Time Extension: Allows pelts to be stored for weeks without quality loss.
• Essential for Chrome Tanning: Ensures optimal chromium salt penetration for wet blue leather.

Pickling Process Conditions

Maintaining correct acid concentration, salt levels, and temperature is critical to avoid damage to pelts.

• Acid Concentration: Usually between 1.5% to 3% of acid, balanced with salt.
• Salt Concentration: Around 10% NaCl to prevent swelling and microbial growth.
• Temperature Control: Lower temperatures (10-15°C) reduce enzymatic activity and spoilage.
• Duration: Typically 12-24 hours, depending on pelt thickness.
• Kenyan Application: Tanneries in Nairobi adjust pickling parameters based on humidity and pelt source to optimize preservation.

4.1.4 De-pickling

De-pickling is the process of neutralizing the acid and salt from pickled pelts before tanning. It ensures pelts are conditioned for the next tanning stage, especially chrome tanning, by bringing pH to a suitable level.

Purpose of De-pickling

The objective is to remove excess acid and salt to prevent interference with tanning chemicals and maintain pelt integrity.

• pH Adjustment: Raises pH to between 2.5 and 3.5 for effective chrome uptake.
• Removal of Residual Chemicals: Prevents chemical imbalances during tanning.
• Softening of Pelts: Helps relax collagen fibers for better tanning penetration.
• Reduction of Salt Content: Minimizes corrosion risk to equipment and improves effluent quality.
• Preparation for Tanning: Ensures uniform and controlled tanning reactions.

De-pickling Methods

De-pickling involves washing and chemical neutralization steps.

• Water Washing: Multiple rinses remove soluble acids and salts.
• Alkaline Neutralization: Mild bases such as sodium bicarbonate adjust pH.
• Controlled Time: Over-washing can lead to swelling and damage, so timing is critical.
• Mechanical Assistance: Drum agitation ensures uniform treatment.
• Kenyan Practice: Leather processors in Eldoret implement de-pickling with automated drum washers to maintain consistent quality.

Chapter Summary

This chapter covered the essential pre-tanning and tanning processes of bate, de-greasing, and pickling pelts. It began with definitions of key terms including fleshing, bating, de-greasing, pickling, and de-pickling, followed by a discussion of fleshing methods and the importance of removing unwanted materials. Common faults encountered during fleshing and protective measures to avoid damage were explored. The significance of bating was explained alongside the chemicals used, with attention to defects such as over-bating and inadequate bating and their remedies. Various types of bates and factors influencing the bating process such as temperature, pH, time, recipes, and de-greasing chemicals were examined. The chapter then detailed the aims and methods of de-greasing, including squeezing, hand fleshing, machine fleshing, and saponifying. Pickling was introduced with its chemicals and importance, followed by defects like acid swelling, incorrect salt levels, and high pH, with solutions to address these issues. Different pickling methods were described along with factors affecting pickling such as mechanical action and acid type and quantity, concluding with process control and typical pickling recipes. The chapter ended by highlighting the importance of de-pickling as a final preparatory step before tanning pelts.