Enzymatic reaction mechanism:
The speed of the chemical reactions depends on the energy barrier between the substrate and the product. This barrier is known as the ‘activation energy’ and for molecules to react, they must possess the energy to overcome the barrier.
However, enzymes do not increase the energy levels of substrate molecules, but provide an alternative low-energy route for the reaction to proceed. They achieve this by forming an intermediate enzyme-substrate complex, which alters the energy of the substrate such that it can be quite readily converted to the product. The enzyme itself is released unaltered at the end of the reaction, thus acting as a catalyst.
It can be schematically represented by the following equation:
E (Enzyme) + S (Substrate) ------→ ES -------→ E + P (Product)
(Initial state) (Intermediate state) (Final state)
Enzymes being highly specific catalysts, the substrate must fit precisely into the active site of the enzyme just like a key fitting into lock.
The formation of enzyme substrate complex also requires very little energy. Consequently, enzymes are very effective catalysts, enhancing reactions up to several thousand-fold more than the most effective chemical catalysts.
The substrate & reaction specificity is determined by the structure of enzyme. The primary structure is determined by the amino acid sequence, the secondary structure by the specific conformation of protein chain and the tertiary structure by the arrangement of chain segments.
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Most enzymes have a maximum activity at an optimum temperature and optimum pH. The reaction rate increases with increasing temperature until the optimum temperature is reached and activity decreases sharply on both sides of the optimum pH range.
Enzymatic Desizing:
- The enzymes alpha-amylases hydrolyse starch in an 'endo' fashion, cleaving at random internal alpha-1,4 glycosidic bonds in both amylose and amylopectin, showing little or no activity on the alpha-1,6 bonds.
- These enzymes degrade starch to shorter polymeric fragments, known as dextrins and maltose, a disaccharide, which contain two glucose residues.
Enzymatic desizing consists of three main steps:
To facilitate this, the components of enzymatic desizing bath are as follows:
- application of the enzyme,
- digestion of the starch and
- removal of the digestion products.
To facilitate this, the components of enzymatic desizing bath are as follows:
- Amylase enzyme.
- pH stabiliser.
- Chelating agent.
- Salt.
- Surfactant.
The general recipe of desizing process using enzyme is given below:
1. Take 0.5% to 2.0% desizing enzyme (on weight of fabric) in water
2. Add require quantity of Common salt
3. Maintain pH around neutral
Desizing in a jigger is a simple method where the fabric from one roll is processed in a bath and rewound on another roll. First, the sized fabric is washed in hot water (80 - 95°C) to gelatinize the starch. The desizing liquor is then adjusted to pH 5.5 - 7.5 and a temperature of 60 - 80°C depending on the enzyme. The amylase is added in an impregnation stage and degraded starch in the form of dextrins is then removed by washing at 90 - 95°C for two minutes. The jigger process is a batch process. In contrast, in modern continuous high-speed processes, the reaction time for the enzyme may be as little as 15 seconds.
Desizing on pad rolls is continuous in terms of the passage of the fabric. However, a holding time of 2 - 16 hours at 20 - 60°C is required using low-temperature alpha-amylases before the size is removed in washing chambers. With high-temperature amylases, desizing reactions can be performed in steam chambers at 95 - 100°C or even higher temperatures to allow a fully continuous process.
One of the important property of enzyme is that it targets only on the starch, it does not negatively affect on the cellulose as in the case with acidic or oxidative methods of desizing (in which degardation of cellulose is expected) . Thus, the specific advantages are:
- Effective solubilisation and removal of starches.
- Excellent biodegradability.
- No aggressive chemicals needed, thus maintaining the tensile strength of the substrate.
- Safe handling and operation.
- Improved wettability.
- Improved fabric quality.
- Reproducible performance and ease of use.
Factors affecting enzyme activity:
- Substrate concentration - the rate of enzyme activity increases with substrate concentration at lower level up to a certain point and then slows down.
- pH value - the amino acids and other ionisable groups in enzyme may get ionised at lower or higher pH effecting its activity.
- Temperature - with increase in temperature, the reaction rate increases due to "thermal energy", but with further increase in temperature, the rate decreases due to thermal denaturation.
- Activators - presence of specific bivalent metal cation can activate enzyme reaction. Such metal ions stabilise the structure of enzyme-substrate complex or sensitize the substrate to the attack of enzyme or take part in the ion exchange process.
- Inhibitors - certain alkalis, acids and antiseptic tend to inhibit enzyme activity.
Effect of inadequate desizing:
Following problem may arise, if there is a inadequate:
- Reduced absorbency
- Inadequate whiteness
- Paler dyeing
- Spotty dyeing
- Reserved marks in dyeing and printings
- Increased tendency to creasing due to hard handle
- Reduced tear strength
Checking effectiveness of desizing:
The effectiveness of desizing can be checked using TEGEWA solution. This solution is prepare using 10 g potassium iodide (KI) and 0.635 g iodine in 1000 ml distilled water.
A drop of TEGEWA solution is placed on the fabric and the color change observed and assessed by TEGEWA scale. The fabric is not spotted randomly but from side-centre-side at different points along the length of the fabric. A dark bluish violet color indicates excessive starch and if so, the fabric or garments must be reprocessed.
