The hot fill concept basically entails that it is the product itself, bottled at high temperatures (85-95°C) that sterilizes the bottle and the cap. This system is particularly efficient with glass bottles that are not subjected to deformations at high temperatures. In order to attain reliable hot fill line operations without sacrificing the product quality too much, it is necessary to take some critical points into account and to solve them.

In the first place, it is necessary to obtain a balanced adjustment between temperature and contact time in order to minimize possible variations in colour and taste and the wearing off of the aromas, vitamins, active principals inside the product; therefore it is imperative to cool the product gradually straight after the filling cycle.

Secondly, it is imperative to monitor the temperature of all the surfaces in contact with the product; if the temperature drops below the pre-set value, microbiological problems might arise in the production phase. This is why line stoppages represent a problem; many hot fill lines presently use a recirculation system that permits a defined product quantity to continue to circulate inside the filling valve even when the system is at a standstill; the product crosses a recovery manifold, it is collected in a second tank and relaunched to the pasteurizer to return afterwards back to the filler. This system permits the filling valve temperature to remain constant regardless of the filler speed and possible interruptions. On the other hand, if a significant quantity of product undergoes a further thermal treatment (following numerous interruptions), this will lead to degradation of the organoleptic qualities of the end product.

The diffusion of PET bottles has determined the rise of new concerns for the hot fill technology: in fact it is not possible to use standard PET bottles in a hot fill line: they would simply collapse due to the high temperature. Therefore “heat set” bottles have been developed to resist temperatures of 85-95°C. The neck area, since this is the most critical zone, is heavier as compared to the standard bottle and is subjected to a re-crystallization process during its production. Moreover, the shape of PET bottle for hot fill is subject to restrictions since it has to be studied with appropriate “panels” so as it may expand after filling and retract during cooling. Thereby these bottles are more complicated to produce, heavier and therefore more expensive.

The same remarks apply to the caps: heavier and more expensive. Even though the technology has provided solutions to these problems, some restrictions continue to apply for hot fill; in particular, the range of products that may be processed is rather limited and does not include low acid beverages. From an operational point of view, intermediate cleaning cycles for external surfaces are required during production in order to avoid the product from dirtying the filling valve, especially when dealing with products containing pulps and fibres.

A hot fill line requires an accurate product formulation to counteract the spoilage and/or degradation of some components, such as vitamins and aromas due to temperature. The high energy consumption in order to heat and cool the product must also be taken into account when determining running costs. On the other hand, the hot fill technology is generally simpler than an aseptic line and has a lower initial investment cost.