Determination of the Thermal Inactivation Point
The thermal inactivation point (TIP) is defined as the lowest temperature required for complete inactivation of a virus in crude sap heated for 10 min. In the case of an unidentified virus, virus-containing sap is first exposed to temperatures at 10 °C intervals. When repeating the test, the intervals are usually narrowed down to 5 °C in the range of inactivation. For example, if the inactivation occurred between 40 and 50 °C, repeat the test between 35 and 55 °C at 5 °C intervals. In view of the great variation occurring in this type of tests due to the aforementioned factors, it makes no sense to heat at intervals smaller than 5 °C. In the literature, TIP is usually reported as the two temperatures in between which the virus is inactivated completely. Viruses with elongate particles have a high temperature coefficient (Q10°; the ratio of velocity constants of inactivation at temperatures differing by 10 °C) and their inactivation is accompanied by denaturation of their coat protein. Viruses with isometric particles, on the other hand, have a low Q10°, and their inactivation takes place long before denaturation of their coat protein. With this type of viruses, unlike the former, temperature is not the most important factor determining inactivation and, therefore, there is a great variation in TIPs reported in the literature.
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