The Process Of Sugar Coating And Problems That One Can Encounter

Sugar coating of tablets is not as common in the pharmaceutical industry as it once was, having been superceded by polymer film coating. Nevertheless, a knowledge of the issues that may occur when sugar coating is applied may help to avoid loss of production when the process is used.

1. Benefits of Sugar Coating

There are some clear benefits of sugar-coated pharmaceuticals -

• The process requires low capital cost, unsophisticated equipment and is relatively easy to perform
• It is affordable in terms of running costs as the most common ingredients used are sugar and water.
• It provides an attractive and glossy finish.
• Sugar coated tablets are easy to swallow as the sugar masks the taste.

2. Disadvantages of Sugar Coating

• It requires specialized and trained personnel
• The process is Labour intensive
• It requires long processing time
• Sugar coated tablets are not amenable to embossing, use of print for identification or labelling
• Sugar coating increases the weight and size of the tablet

The sugar coating is generally water-soluble making it easier to dissolve quickly when it comes into contact with any liquid medium such as gastrointestinal fluids. One purpose of the sugar coating is to protect the drug inside the tablet and act as barrier to external contaminants. It may also hide or mask the unsavoury taste of the drug that some patients may find unpalatable. Sugar coating insulates and masks the colour and texture of the tablet’s core. It may also influence the release of the encased drug in the body.

3. The coating process

The coating process involves the repetitive application of a sucrose-based solution in the coating pan or drum to cover the tablet. Four process steps are commonly employed, in the following order:

• Sealing (Water proofing) – sealing involves hardening the external surface of the tablet by providing a moisture barrier.
• Grossing (Smoothing) – this is often carried out in stages. The first step is to build up the shape and size of the tablet, and the second involves smoothing out the coated area while increasing the size of the tablet to the required dimensions.
• Colouring – provides the tablet the appropriate colour for the end product.
• Polishing – finishes the tablet by polishing to give the tablet a glossy and professional appearance.

The processes are summarised below.

3.1 Sealing (Water Proofing)

• Before any sugar or syrup is added to the tablet, the core must be sealed and thoroughly dried so that it is free from any moisture or residual solvents. Once the core is dry, a sealing coat is applied to protect the core from absorbing any moisture. The sealing layer or coating hardens the surface to provide protection for the core in subsequent process steps. The core of the tablet is usually weak and has high propensity for disintegration. The core would therefore likely be the first to disintegrate during the coating process. Common water-proofing sealants are usually water-soluble and are formulated from organic solutions.
• The choice of sealant formulation is influenced by the porosity of the tablet since highly porous materials would soak up the sealant solution, preventing the creation of a uniform coating over the whole tablet. A second coating of course can be applied once the first is dried, to ensure that the core is effectively sealed.
• Sealants that are commonly used include: shellac, zinc oxide, cellulose acetate phthalate, polyvinylacetate phthalate, hyroxylpropylcellulose, hyroxypropylmethylcellulose

3.2 Subcoating

• The sugar coating process actually starts with a subcoating stage in which the sides of the tablet are rounded or shaped up to the desired appearance. This stage is where the tablet is smoothed and colour may be added.
• There are two methods of Subcoating:
  • A gum based solution or syrup is coated on the tablet. The tablet is then dried. The procedure is repeated until the desired tablet shape is achieved.
  • Dry powder is applied followed by gum/sucrose solution. Once applied, the gum/ solution excess water is removed by drying so as to harden the surface again.
• Subcoating usually involves alternating gum solution with dusting powder (talc or calcium carbonate) until the desired shape and consistency is achieved. Excess water is removed after every syrup application.

3.3 Grossing (Smoothing)

• The Grossing or smoothing step involves filling out and smoothing irregular features or areas of the tablet. After each filling step, the tablet is smoothed by polishing, refinishing the tablet so that a smooth and finished look is achieved. The process gradually also increases the size of the tablet to the required dimensions.
• If there are a large number of irregularities in the tablet, the worker may use a grossing syrup which contains suspended solid particles which can better fill the voids in the tablet than a sugar solution. An application of approximately 60-70% of sugar solids is generally enough to smooth the tablet. The solution usually contains a mixture of starch, pigments, acacia, gelatine, and opacifier if needed.
• In certain instances, colour may be added in during the grossing stage give the appearance of a homogenous coating layer.

3.4 Colouring

• Colouring coating is the most crucial penultimate stage in the coating process as multiple sugar solution is added to ensure that the predetermined colour is achieved. Historically soluble dyes have been used to give the desired colour as these stick to the surface of the tablet during the drying process. Nowadays, however, pharmaceutical companies prefer the use of insoluble certified aluminium lake pigments. These are materials that tint the tablet by dispersion. Lakes are produced from dyes but are oil dispersible (but generally not oil soluble) and thus can be mixed with oils and fats. They can also be dispersed or suspended in other carriers such as propylene glycol, glycerin and sucrose (water and sugar).

3.5 Polishing and/or printing

• The final step in the sugar coating process is a polishing step. This is carried out to impart a shiny distinctive appearance to the tablet. An application of wax such as carnuba wax, candelila wax, beeswax and hard paraffin is used to polish the tablets inside a polishing pan.
• Finally, tablet markings or brands may be printed on the surface of the tablet using edible ink.

4. Common Problems that can occur during the sugar coating process and their solutions

• 1. Chipping of the tablet coating
     • This is often caused by a low concentration or absence of polymer
     • add more polymer to the coating solution.
     • Another cause is the excessive use of pigments and insoluble fibres
     • reduce the number of fillers
  2. Cracking
     • Expansion of the tablet during or after coating caused by stress relaxation or moisture absorption –
     • consider the use of seal coat or extend the time between the sugar coating and compaction
  3. Non-drying of the coating
     • Excessive presence of invert sugar –
     • avoid the excess heating of the sucrose syrup under acidic conditions
  4. Twinning
     • High edge walls/flat surface
     • modify or alter the punch design
  5. Uneven colouring
     • Poor distribution of coating solution
     • add sufficient liquid and ensure that it is fully mixed
     • Colour migration due to under/rapid drying
     • optimize the drying condition and use pigments
     • Uneven sub coat layer
     • achieve the required smoothness during the sub coat application
     • Wash back of pigment colour coating
     • avoid excess colour coating liquid
     • Excessive drying between colour application
     • optimize the drying conditions
  6. Blooming and Sweating
     • Extra moisture in the finished tablet coat
     • dry properly to ensure appropriate moisture level
  7. Marbling
     • Uneven coating surface
     • make sure to achieve smooth coating before the polishing step