The effect of irregular cooling phenomena on the lethality of thermal processes

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Abstract
Summary: Irregular temperature/time curves in the cooling phase of thermal processes for cans containing semi‐fluid foods have been previously attributed to internal boiling and mixing of the can contents. A physical model is proposed for this behaviour. This model can in part be simulated by numerical (finite difference) heat transfer calculations, but the actual physical behaviour is too complex for complete simulation. Experimental investigation for a baby food showed that substantial variations in total process lethality (±18% around the mean value) occurred as a result of changing process variables that affected the extent of internal boiling. Of these retort pressure during cooling was most important, but there were also complex interactions between this variable and three others: headspace, steam temperature and can pressure during seaming. Because the extent of internal boiling has a substantial effect on the total process lethality, determination of heating phase target F values for foods in which irregular cooling phenomena occur must consider these effects. The canner should consider the sensitivity of Fvalues to variations that could occur in practice of parameters such as can seaming pressure, headspace and retort pressure during cooling, as well as steam temperature.