Shelf life curves

Figure 30.23 SAFT lithium-copper oxide cells voltage-time shelf-life curves (Courtesyof SAFT)...

An approach for analyzing data of a quantitative attribute that is expected to change with time is to determine the time at which the 95% one-sided confidence limit for the mean curve intersects the acceptance criterion. If analysis shows that the batch-to-batch variability is small, it is advantageous to combine the data into one overall estimate by applying appropriate statistical tests (e.g., p-values for level of significance of rejection of more than 0.25) to the slopes of the regression lines and zero-time intercepts for individual batches. If it is inappropriate to combine data from several batches, the overall shelf life should be based on the minimum time a batch can be expected to remain within the acceptance criteria. 

In most dry foods, starch is the main component responsible for a glassy state. This concerns several dry cereal products, like hard biscuits and various breakfast cereals. Figure 16.5 gives Tg for wheat starch, but if low-molar-mass components, generally sugars, are also present, the curve is shifted to lower temperatures, somewhere between those for starch and those for lactose. Another example is pasta, in the dry form in which it is commonly sold, which consists for the most part of wheat starch. Pasta has an impressive shelf life in the glassy state, and it readily takes up water upon cooking, to attain a soft rubbery texture. 

An appropriate approach to retest period or shelf-life estimation is to analyze a quantitative attribute by determining the earliest time at which the 95% confidence limit for the mean around the regression curve intersects the proposed acceptance criterion. 

As can be seen in the graph, the curvatures associated with first or second order processes do not have a significant impact on the functional form of the degradant formation with time curve compared to a zero-order function, up to at least 10% conversion. It is therefore reasonable to assume that all degradation processes follow a zero-order curve shape with respect to the shelf-life. This allows the rate constant for a degradation process to be determined using the initial slope of the degradant versus time plot. 

Freezing, drying, and increasing the content of solutes in a food system are practical methods of decreasing and increasing shelf life. The curve can be used to determine the critical value for water activity ( critical a ") equivalent to the bound or monolayer water activity value. Plots... 

For any type of product that gains or loses water, one can measure the moisture content as a function of relative humidity, or water activity, and determine a moisture isotherm. As shown in Fig. 14.14, moisture isotherms are usually sigmoid shaped curves. However, one can sometimes use only the linear portion of the curve for shelf life predictions. 

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