CHE-421: Tanning Technology Intro

Introduction to Tanning Technology

• Tanning technology is covered in course CHE-421.
• The instructor is Salaheddin Abu Yahya.
• This course is being delivered in Spring 24-25

Industry Overview

• The leather industry is a by-product valorization industry with a direct relationship to diet and nutrition.
• It bridges the gap between hide production from the food industry and manufacturing apparel.
• The tanning process involves mechanical and chemical processes to turn raw material into a final product.
• The leather produced is used in footwear, upholstery, garments and automotive applications.

Raw Materials and Origin

• Bovine sources contribute to approximately 70% of raw materials.
• Ovine sources contribute to approximately 20% of raw materials.
• Caprine and Swine sources contribute to approximately 10% of raw materials.

Skin Structure and Influences

• Skin functions to protect against mechanical risks and bacterial entry.
• It regulates body temperature, eliminates residues, accommodates sensorial organs, and stores fat.
• External factors influencing skin characteristics include age, nutrition, habits of life, environment, sex, health condition, and animal type.
• Representation of skin includes parts with no commercial value, such as nostril hole, snout and lip, eyehole, dewclaw holes, pizzle pockets, navels, cow bags, teats, cod bags, tail, and hind shank.
• A flying pattern involves specific flaying patterns and ripping cuts to obtain a standard shape of flayed hide/skin.

Skin Sections

• Parts of hides and skins have varying properties based on location.
• The green hide/skin has three parts: shoulder, butt and belly (2 Belly).

Shoulder

• Characterized by medium-density fibers, less thick

Butt (2 Bends)

• Thickest and strongest part with tightly packed fibers
• It is the most valuable area, for high-quality leather products

Bellies (2 Belly)

• Fibers are less dense, more elastic
• It is less durable and are for products not requiring much strength.

Skin Composition

• Skin structure varies by the sampled part, even within the same species.
• Key influent factors are breed, stock farming conditions and origin.
• Mammal skin structure is generally similar across species.

Skin Layers

• Hides and skins have three main layers which are Epidermis, Dermis, Subcutaneous tissue.
• The epidermis is approximately 1% of the total thickness of the raw hide.
• The dermis is approximately 84% of the total thickness of the raw hide.
• The subcutaneous tissue is approximately 15% of the total thickness of the raw hide.

Skin Structure - Dermis

• The dermis has two layers: Grain layer and Corium.
• The dermis has Colagen Fibres containing Collagen Molecules and amino acids chains.
• The dermis has layers of Fiber Bundle, Elemental Colagen Fiber, Fibril, and Protofibrils

Chemical Composition

• Freshly flayed hide composition: 64% water, 33% protein, 2% fats, and 0.5% each of mineral salts and other substances.
• Structural proteins include elastin, collagen, and keratin, with collagen at 29-31%.

Proteins

• Proteins combine 2 or several amino acids.
• 50% carbon, 25% oxygen, 17.8% nitrogen, and 7% hydrogen with traces of mineral salts and sulfur.
• Main proteins are 94-95% collagen, 1% elastin, and 1-2% keratin.
• There are also non-fibrous proteins (globulars).

Collagen

• Collagen corresponds to 94-95% of the proteins.
• It has white colour and swells strongly in acidic or basic baths.
• It is non-elastic and contracts to about 1/3 when subjected to high temperatures.
• At temperatures above the contraction temperature the hide converts into soluble gelatine by prolonged treatment.
• Contraction temperature, the temperature at which collagen loses its natural elasticity, ranges from 60 to 70°C (140 to 158°F).

Keratin

• Keratin fibres are proteins forming hair and epidermis and is water insoluble
• They have great stability due to sulphur bridges.
• Easily hydrolyzed by reducing substances in alkaline mediums
• Sulphide and sodium sulfhydrate promote chemical hair removal.

Fats

• The approximate percentage of fats ≈ 2% on bovine, 2 - 10% on caprine, 5 - 30% on ovine, 4 - 40% on swine.

Skin Swelling

• Isoelectric point: The pH at which proteins have equal positive and negative charges.
• Physical properties like swelling, reactivity with acids and bases, osmotic pressure, and viscosity are at their lowest levels here.
• Minimum swelling power, combination power with acids and bases, osmotic pressure value, and viscosity at the isoelectric point.
• Skin swelling refers to the increase in size due to chemical treatments
• This affects fiber structure, porosity, and overall leather quality.
• There are 4 main conditions which bring about skin swelling: Alkaline, Acidic, Enzymatic, and Osmotic.

Alkaline Swelling (Lime Swelling)

• Occurs in the Liming process
• A high pH causes collagen fibers to absorb water, leading to fiber expansion.

Acidic Swelling (Pickling Swelling)

• Occurs in the Pickling process
• Sudden exposure to strong acids witout buffering causes the swelling

Enzymatic Swelling

• Occurs in Soaking and bating stages
• Moisture is absorbed as unwanted proteins break down

Osmotic Swelling

• Occurs in Water absorption stages
• Absorption can occur if hides are immersed in water without correct osmotic balance

Skin Types

• Bovine skins are classified as Calfs, Steers, Cows, and Oxen and Bulls.
• Ovine skins are classified according to wool length, wool type/quality, and skin size/weight
• Caprine skins are classified as Kid (under 2 months), Billy-Kid (2-6 months), Goat (females + 6 months), Buck (males + 6 months).
• Swine skins include Bacquira wild pig or Pecari and European/English/American Domestic Pig.

Hide and Skin Preservation

• Hides/skins should be processed within hours of slaughter or refrigerated/preserved quickly.
• Skin conservation is generally carried out by dehydration.
• Basic preservation methods include salting, drying, cold and chemical preservation.
• Mixed processes combine drying and salting.
• Main types: Air drying, Brining, Wet-salting, Dry-salting, Pickling, and Tanning

Air Drying

• Involves hanging hide/skin in a well-ventilated area.
• It is one of the oldest methods but may not be as effective as others.
• Frame and tent drying techniques are used.

Brining

• Uses a saltwater solution to soak the hide or skin.
• This draws out moisture, prevents bacteria, and preserves the skin.
• It's used when immediate drying is not possible.
• A relation of 1:5, for a minimum time of 15-16 hours, between 15 – 21 °C should be followed.
• An initial fleshing and an additional final salting is required.

Wet-Salting

• Hide or skin is coated with a layer of salt while still wet.
• Retains moisture and slows decomposition
• Mine and Sea salt, and Sodium chloride (NaCl) are used

Dry-Salting

• Salt is rubbed directly onto the surface of the hide or skin.
• Absorbs moisture and acts as a preservative.
• Used in dry, cool places for extended storage.

Pickling

• Hide/skin is soaked in a solution of salt, water, and acid (sulfuric or acetic).
• Lowers the pH to prevent bacterial growth.
• This prepares the skin for tanning.

Tanning (Preservation)

• Tanning is the final and most advanced preservation method.
• It involves treating skin with tanning agents (chromium salts or vegetable tannins).
• It prevents decomposition as it changes the chemical structure which makes the skin durable

Skin Defects

• Main skin defects originate from the living animal, mechanical sources, sanitary conditions, parasitic infestations, slaughterhouse operations, and preservation operations.
• Defects from living animals include scars, injuries, and diseases.
• Mechanical defects are caused by physical damage during handling or transport.
• Sanitary defects are from bacterial or fungal infections during slaughter or processing.
• Parasitic defects are due to infestations that leave marks, scars, or lesions.

Skin Defects - Slaughterhouse Operations

• Slaughterhouse defects originate during the slaughtering process from improper handling, equipment or techniques.
• Examples of such defects include flaying cuts and holes, grain breaks (mechanical flaying), and vein marks.
• Insects such as Moth (Dermestes maculatus and lardarius) can cause defects in slaughterhouse operations.

Skin Defects - Preservation Operations

• Heating can be a defect that occurs during preservation operations.
• This is caused by several conditions.
• There is Insufficient skin cooling and too much time between flaying and conservation
• Ambient temperature of 15°C or or high relative humidity, insufficient salt dosage, too high stacks of skin and high concentration of dung also bring about defects.