Accelerated aging tests for MDD prediction

Predicting the shelf life of food products

It's important to distinguish between two key concepts in assessing the sustainability of food products: Shelf Life (SLS) and Minimum Durability Date (MDD). SLEDs, which are generally short, apply to perishable, refrigerated products; they extend over a few days or weeks, providing a direct measure of a product's (microbiological) shelf-life. On the other hand, DDM only applies to microbiologically stabilized products (dry products, canned foods, frozen foods, etc.), whose shelf life depends only on the slow evolution of their organoleptic and/or nutritional properties. For products with an extended DDM, often exceeding one year, durability tests under real conditions prove too long, necessitating the use of accelerated aging methods. 

 

Temperature-accelerated ageing

The most common method used to accelerate the ageing of a packaged product is to store it at a temperature higher than its actual storage temperature. The Arrhenius model, which shows the evolution of a chemical reaction or physical phenomenon as a function of temperature, can be used as a kinetic model:  

K = K0 exp (-Ea/RT)  

where K is, for example, the rate of change of a characteristic product parameter (composition parameter, sensory parameter), T the temperature (°K), Ea the activation energy, R the perfect gas constant, and K0 a constant 

K0 can be eliminated by calculating the ratio Q of degradation rates between two temperatures with a fixed deviation. A gap of 10°C is generally used, which gives the name to the "Q10" method: when the temperature is increased by 10°C, the aging rate is multiplied by the factor Q10. 

Q10 = exp (10 Ea / RT(T+10) )    

Since Q10 values often range from 1.2 to 3, it is sometimes suggested that an average value of 2 be taken as the default. For example it is sometimes accepted that the degradation rate of a product at 30°C is double that of a product at 20°C. But beware! This approximation often leads to very erroneous values for the shelf life of food products. It is always preferable to measure the Q10 value, based on a study of the product's behavior at different temperatures.  

 

Alternatives to thermal aging

Performing tests at variable temperatures involves time-consuming and costly testing. It may therefore be tempting to carry out highly accelerated ageing tests based on high temperatures. However, the greater the temperature difference between actual aging and accelerated aging, the greater the risk of observing different degradation mechanisms at high temperatures... in other words, accelerated aging is not representative of real product evolution conditions. This raises the question of not use temperature-accelerated ageing; this is possible (!), for certain degradation mechanisms controlled by a well-identified phenomenon : 

 

• Sterilized products stored in plastic packaging evolve mainly through oxidation, following the penetration of oxygen through the packaging.. While this transfer can be accelerated by temperature (with a Q10 acceleration factor yet to be determined!), it can be accelerated much more simply by carrying out a oxygen pressure test, simply at room temperature, for example. This test, carried out specifically by the CTCPA, must be carried out under suitable safety conditions, but it has two major advantages: (i) acceleration by oxygen enrichment is far more representative than thermal aging, and (ii) the acceleration factor is not not to determine (!); its value is proportional to the oxygen enrichment ratio.

• Dry products stored in plastic or paperboard packaging evolve mainly as a result of the permeation of water vapor which is then absorbed by the product. A test under a moisture-enriched atmosphere is then carried out. The principle is not exactly the same as oxidative aging. It is not possible to enrich an atmosphere with moisture at ratios such as those used for enriched oxygen. Mixed aging (temperature + humidity) is therefore required to achieve adequate acceleration coefficients. Acceleration coefficients are then determined in parallel by measuring water vapour permeability on the packaging.

• Products that evolve under the effect of light (photooxidation, discoloration, etc.) are subject to "light ageing". light aging "specific lighting systems representative of real-life storage conditions.

• It should be noted that it is perfectly possible to cumulate the constraints of wet ageing, oxygen and light, to simulate in accelerated mode the behavior of a product subjected to a combination of these factors.

The prediction of MDDs therefore calls on various acceleration tools which can be used to simulate the actual behavior of a product stored over long periods of time. The CTCPA assists food processors in defining and carrying out these tests. Product ageing can be monitored by means of various chemical (markers), physical, physicochemical or organoleptic tests, which can also be carried out by our Bourg en Bresse packaging laboratory, or by the company itself, using its in-house resources and expertise.