Quality Concern – Coring

Coring occurs when pieces of the vial’s rubber stopper break off and fall into the extract due to repeated syringe insertion. These pieces appear as small grey or dark particles that float around the vial. These are not to be confused with protein precipitates, which typically appear as light-colored particles. Regarding this phenomenon, see the section Quality Concern – Precipitation.

The likelihood of coring is related to the composition of the rubber stopper as well as to the technique, frequency and force of needle insertion. Natural rubber provides good elasticity that allows the stopper to return to its original shape following needle insertion. However, with recent controversies over the allergenic properties of latex, a component of natural rubber, there has been a shift away from its use as an ingredient in rubber stoppers. Some of the synthetic materials now used show higher levels of rigidity and brittleness that may increase the likelihood of coring, especially in vials that undergo numerous syringe insertions.

To minimize the risk of coring, the following should be considered:

1. Do not use large bore needles, as these may cause excessive damage to the rubber stopper with repeated use. This can be counter-intuitive, but large bore needles are found on syringes with LOWER gauges. Try to use syringes with a HIGHER gauge (between 22- to 27-gauge).
2. Always insert the needle in the center of the rubber stopper (within the little circle). This area is thinner than the edges and is designed to handle repeated insertions.
3. When inserting the needle, use an angled entry with the tip on the bottom, closest to the stopper (see figure below). Insert carefully and try to minimize the force of entry.
4. If there is a large bulk vial (e.g., 50 mL) with many insertions anticipated, it may be helpful to take some empty 10 mL vials and transfer the contents into these vials. Use of separate vials will minimize the number of insertions into a single rubber stopper.

If you experience coring, please contact your allergen extract supplier for assistance.