Effects of blanching conditions on the mechanical properties of french fry strips

Control of the various processing operations is crucial for achieving optimal texture in french fries. This, in turn, requires precise measurement of the effect of each processing operation on the mechanical properties of the fries. A puncture test was used to measure the effects of blanching and freezing on peak force of french fry strips. Potatoes (cv. Russet Burbank) were grown at two locations in Manitoba in 1994 and 1995. Two blanching treatments (a 1-stage process, and a 2-stage process with a low-temperature long-time step) were followed by blast freezing. Strip position within the tuber was found to be a major source of variation for mechanical properties. Because peak force varied widely within and between tubers, it was necessary to compare the efficacy of the two blanching regimes using a pair of strips taken from as close a position as possible in the tuber. For both locations and crop years, blanched french fry strips taken from the pith of the tuber (referred to as inner strips), exhibited higher peak force than outer strips taken from the cortex. The 2-stage blanching process decreased the variation seen in the mechanical properties of different potato strips. Overall, strips blanched by the 2-stage process had significantly higher peak force than strips blanched by a single blanching process. The effect of blanching was evident in the mechanical properties of strips even after freezing and thawing. Microstructural analysis revealed that cells in the outer strips had a “balloon-like” appearance due to the degree of starch swelling pressure generated by swollen granules. For inner strips, this “balloon-like” appearance was less evident. Such appearance is in support of the starch swelling pressure hypothesis. The research shows that measurement of the effect of processing can be achieved by comparing treatments on adjacent strips in order to minimize variation between strips, and that a 2-stage blanching treatment can ameliorate texture differences between strips taken from different parts of the tuber.