FRENCH FRY RESEARCH
As part of the class 2.671 (Measurement and Instrumentation), I conducted research on the mechanical properties of French fries.
An abbreviated version of the final research paper is below.
The full version is available for download here.
ABSTRACT
In this investigation, fries were prepared using 5 different frying times in order to characterize the relationship between the crust hardness (crunchiness) and flexural modulus (crispiness) of the fries and the duration of frying. Two tests, a puncture test and a three-point bend test, were used to analyze the mechanical properties of the fries. Both crust hardness and flexural modulus were found to increase with frying time, and the relationships can be characterized by power functions.
INTRODUCTION
French fries, like other fried foods, are usually prepared by deep-frying in a pan or fryer. This study investigated the effects of frying time on fry hardness and flexural modulus for the simpler, one-step frying method. Five batches of fries, each cooked for a time ranging from 6 to 14 minutes, were prepared. Crust hardness was used to indicate the crunchiness of the fries and was measured using a puncture test, during which the peak force was recorded for each of 12 samples per batch. A three-point bend test was used to indicate the crispiness of the fries, and was used calculate the flexural modulus using equations describing the deformation behavior of rectangular beams as an approximation of the fries.
BACKGROUND
Deep-fat frying is a complex process that results in structural changes in the fried food due to mass and heat transfer between the food and the oil. When food is submerged in hot oil (350-400° F), the water molecules in the food are rapidly replaced with the oil while the outside is cooked by the hot oil, resulting in a crunchy exterior texture and a strengthened crust. This crunchy texture is a highly desirable trait of fried food for many consumers. The crust then blocks more oil from entering the interior of the fry, and conduction within the potato cooks and softens the interior.
The intention of deep-fat frying French fries is to create a fry that has a crunchy, hard outer crust. For this experiment, two parameters were chosen: hardness of the outer crust and flexural modulus. The hardness of the crust is a measure of the peak force required to puncture the outer crust of the fry, and is reliant on both the amount of oil allowed to seep into the fry and how quickly the outer layer of the potato cooks. To measure the fries’ crispiness, the flexural modulus was found by using a three-point bend test, as shown in Figure 1. The fry is supported on either end and a downward force is applied to the middle.
Figure 1: Diagram of a three-point bending test of a rectangular beam. The fry is supported at either end and a force is applied by the Texture Analyzer machine, which records the force and deflection of the beam.
EXPERIMENTAL DESIGN
To obtain the flexural modulus and hardness of fries in relation to frying time and oil temperature, fries were cooked for several different time intervals and then subjected to a three-point bend test and a puncture test. The hardness was found by identifying the peak force during the puncture test, and the flexural modulus was determined from the dimensions of the fry and the three-point bend test Force versus Deformation plot.
Each batch of fries was cooked for 6, 8, 10, 12, or 14 minutes before testing. For each frying time, six fries were tested using a puncture test, as seen in Figure 2. Each fry was tested twice, once at each end, resulting in 12 data points for each batch.
For the three-point bend test, the fry was laid across a pair of beams. The Texture Analyzer was used to apply a downward force to bend the fries, as seen in Figure 3.
Figure 3: Diagram of the experimental setup of a three-point bending test of a rectangular beam. The fry is supported at either end and a downward force is applied by the Texture Analyzer, which records the force and deflection of the beam.
Figure 2: Diagram of the experimental setup for a puncture test. During the test, the probe applies a force onto the fry until it punctures through the outer crust.
RESULTS AND DISCUSSION
The hardness of the outer crust was found to increase over time. This behavior can be attributed to the decrease in moisture content in the crust with increased frying time, which has been shown to contribute to an increased crust strength of fried foods.
The mean Hardness value for each frying time was calculated. Figure 3 displays the mean Hardness value from each frying time, and the data has been fitted with a power curve.
Figure 4: Mean value of Hardness (g) plotted against frying time. The vertical error bars represent the uncertainty in the mean. The data has been fitted with a power curve with equation 𝑓(𝑥)=𝐴∗𝑥^3, where A = 0.00371 ± 0.0004.
The Flexural Modulus was also found to correlate positively with frying time, and the relationship can be modeled well with a power function. As with the hardness, this increase can be explained by the basic process of frying in which oil replaces water in the fry’s crust.
The average slope of the initial linear portion of each Force vs Distance plot was found and used with the fry’s dimensions to calculate the Flexural Modulus of each fry using Equation 2. The moduli from each frying time were averaged and the means were plotted and fit with a power curve.
As may be expected, the crispiness of French fries increases with increased frying time. Both the hardness of the outer crust and the elasticity modulus of the fries increased exponentially with frying time. The power functions found to characterize the properties in this study can be used to predict the mechanical properties of fries fried for any given time within the range of 6 to 14 minutes.
CONCLUSIONS
The relationships between frying time and both hardness and flexural modulus can each be modeled with a power function with equation 𝑓(𝑥)=𝐴∗𝑥^3. The relationship between the coefficients A from each relationship is not statistically significant, as is to be expected due to drastically differing scales of the flexural modulus and hardness. However, the shared factor 𝑥^3 shows that the two mechanical properties increase at the same rate. This relationship implies the surface hardness governs the flexural modulus of the fries. This makes sense, as both measures are dependent on the increased strength and thickness of the outer crust of the fry with increased frying time. Now that the relationship between frying time and the mechanical properties of the fries has been found, there is motivation to correlate these values with the perceived taste of the fries. Future work on this project would involve taste-tests of fry samples prepared at different fry times to determine the mechanical properties of an “ideal” fry.
FINAL POSTER