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Some six months later a senior scientist from a large multinational visited us, apologising for having missed the symposium – his colleagues said it had been the best ever! Our main topic of research was fracture, and he suggested we include food materials in our studies. Food science was good on chemistry, but very poor on texture. At least half our appreciation of food is textural, which is governed by its failure properties. Texture is routinely assessed by a trained taste panel; mechanical testing devices are distinctly non-parametric, reminiscent of medieval instruments of torture. I felt sorry for the food.
Soft and hard
Some years later I spent a few months at the food science institute at Wageningen in the Netherlands to work on the texture of apples. Apples (there are thousands of varieties) are pollinated at about the same time of year but ripen at different rates. “Early” apples mature in a few weeks, have a specific gravity of about 0.5 and tend to be soft. “Late” apples can take the next six months and are very hard with an S.G. of about 1.0. The cells are all about the same size. The differences in texture are likely to be due to the air spaces between the cells, suggesting that cell-cell contact area is variable, resulting in differences in shear stiffness of the flesh and of the transmission of strain energy to a fracture surface.
The arrangement of the cells is related to rate of maturation. “Early” apples tend to have a rather random arrangement of the cells, so their texture is uniform. “Late” apples have the peripheral cells similarly random, but the deeper, slower-growing cells tend to be tightly packed, radiating in columns. This is easily sensed. Prepare two cube-shaped samples from a freshly picked apple, removing skin and core. The cube will be 15 to 20mm along its edge. Slowly bite into one cube along a radius relative to the whole apple, the other parallel to the core. The first cube will give a crisp fracture, not very juicy, making a sharp noise. The second will need a deeper bite, will be juicy, and sound squelchy. If you don’t experience these differences your apple is either an “early” one, or is an over-ripe “late” one with the cells separating. Eventually the cells swell so much that the apple splits its skin.
Search for crispness
Biting with the front teeth is equivalent to Mode I fracture with a wedge of 10 degrees that can be replicated on a test machine. Many people now use this simple test (colloquially the Wedge Fracture Test) to measure crispness of fruit and vegetables. It’s cheaper and quicker than a conventional taste panel and gives the same ranking with reliable, parametric, results.
In a later project we looked at the fracture properties of potato crisps (=chips). From tensile tests we calculated a Weibull modulus similar to a ceramic and a strength of 1.5MPa. This brittleness and weakness give crispness. Both these tests could be used in quality control and be cheaper, quicker, more objective and consistent than a taste panel.
Food is commonly derived from strong and tough material processed to make it weaker and more brittle, the opposite requirement to engineering materials. Our results found no comparisons in the properties tables. Where are the standards?
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