Measuring the crispness of fried vegetable products

Cooking in oil is widely used in the production of snack foods of various kinds, and results in drastic changes to the structure of the food.

Penetration of oil is particularly important for structure development in products such as potato chips, tortilla chips and potato crisps. In products which become totally hard and crisp, the oil first coats the surface and begins to move into the chip. As the heat builds up inside, moisture turns to steam and exits the chip, leaving a sponge-like network of tunnels which fill with oil.

This process occurs within the first 20 seconds of frying. Starch granules gelatinise inside the chip, but not at the surface. The interior becomes smooth and plastic as protein, starch and lipids interact to form a continuous phase which hardens upon dehydration.

In products with a soft interior, such as potato chips, the dehydrated oil-infused layer is confined to the peripheral four or five layers of cells. The hard crisp surface pellicle (or crust) develops at the same time as the interior of the chip is changed to a structure comparable to that produced by moist heat.

A penetration test using a small cylinder probe can be used to puncture the sample and indicate the presence (or absence) of a distinct crust. Such a "crust" would be highlighted by much higher penetration forces than the force required to penetrate through the underlying potato tissue.

However, due to inherent variability in the product, the firmness of chips or French Fries can usually more repeatably be determined using a Multiple Chip Rig attached to a Texture Analyser. This rig measures the force obtained by the penetration of a multiple sample of chips within a single test, thereby creating an averaging effect.

Up to 10 chips can be tested at any one time and the rig ensures that each chip is completely penetrated by a 2mm diameter probe. The probe head has a fast release adapter to enable the probe to be rapidly cleaned and relocated in the same position. Two adjustable stainless steel rods are used to hold the samples in position on the sample holder. A typical curve produced from this test is shown highlighting the area calculation which is used for comparison purposes.

The assessment of potato crisp/chip crispiness is perhaps the most dominant characteristic and can greatly affect consumer acceptability. A sample possessing a crispy texture produces a curve featuring multiple force peaks when crushed in an Ottawa Cell. This type of curve (as shown below) can be analysed using special calculation features in Exponent Connect Software.

An Acoustic Envelope Detector is also able to collect the sound data associated with crispness during a compression test. This gives another dimension of analysis which helps texturally characterise the sample and create a sound profile or ‘signature sound’ for your crispy product.