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The time required to develop a product – from conception to launch on the market – is steadily shrinking. Since recipes are also becoming ever more complex and a wide range of different types of packaging are now employed, forced stability tests have become absolutely essential, in order to establish a realistic indication of a product’s shelf-life.

Inferences about the shelf-life of a product can only be made if the entire beverage concept is taken into consideration, such as the recipe, filling technology, packaging and distribution.

The most important stress factors in the aging process are heat, light and oxygen.

PET bottles have become a popular form of packaging for non-alcoholic beverages, and their permeability to gas, most especially oxygen, is therefore a critical parameter in the aging process.

The testing process described below operates, of course, on the assumption that the chemical reactions in the aging process are subject to the same mechanisms, whether they occur at temperatures typical for beverage storage or at somewhat elevated temperatures, and that they follow a linear relationship dependent on temperature. The same applies to forced photochemical reactions and to reactions brought about by an increase in the partial pressure of oxygen on the beverage. To verify results from forced testing, they can be compared and correlated to results from real-time tests on the same product. 

Dissolved nitrogen in a liquid medium is measured using the same procedure as the CO₂ determination, i.e., using heat conductivity.

CO₂ is employed as a purge gas in the beverage industry. Therefore, in order to measure nitrogen, the change in thermal activity and CO₂ and N₂ is used. The thermal conductivity is determined in a small measurement chamber, which in turn is separated from the material being measured by a semipermeable membrane. Diffusion through the membrane changes the thermal conductivity in the measurement chamber.

The gas volume in the measurement chamber is fully replaced in cycles of 10–20 s. The changes in thermal conductivity over time are a measure of the diffusion of N₂ through the membrane, which allows the concentration in the medium to be calculated, taking temperature into account.

The calculation for the concentration of N₂ is achieved using the change in thermal conductivity in the measurement chamber, also taking the temperature into account.

Since the thermal conductivity of oxygen is similar to that of nitrogen, a second channel may need to be used to compensate for any oxygen in the medium [1].

The thermal conductivity is measured in a small chamber, which is in turn separated using a semi-permeable membrane from the medium being measured. The diffusion through the membrane alters the thermal conductivity in the measurement chamber. The gas volume in the measurement chamber is completely replaced in 10–20 s cycles. The changes in the thermal conductivity over time are a function of the quantity of CO₂ diffusing across the membrane. Using this value and taking into account the temperature, the concentration in the medium being measured can be calculated. Other dissolved gases, such as nitrogen and oxygen, do not affect the result of the measurement, since either nitrogen or air is used to replace the gas in the measurement chamber [1].