Ultrasonically Improved Frying of Food

Fried foods such as french fries, potato chips and other deep-fried foods can benefit from sonication before or during frying. Learn how ultrasonics leads to healthier fried food as they contain less oil and less acrylamide. Simultaneously, the frying time is shortened and crispness and color are improved.

Ultrasound-Assisted Frying for Healthier, Better Fries and Potato Chips

Ultrasonically assisted frying of French fries and potato chips offers several benefits, primarily centered around improved quality, optimized nutritional values and processing efficiency. One significant advantage is the reduction in oil uptake and frying time while maintaining or even enhancing product quality. Thereby, ultrasonic frying produces crispier exteriors and softer interiors in french fries, resulting in a more desirable texture.

Ultrasonic Cavitation improves Frying

Ultrasound treatment affects heat transfer, bubble formation and moisture reduction (water removal) during frying.
The mechanism behind ultrasonically-optimized frying outcomes in fries and chips lies in the phenomenon of acoustic cavitation generated by ultrasound waves. When the ultrasound waves pass through the frying oil, they create tiny vacuum bubbles known as cavitation. These bubbles collapse violently near the surface of the food, causing localized intense heat and pressure. This action aids in the rapid heat transfer to the food, promoting uniform cooking and reducing frying time.

Additionally, ultrasonic waves can enhance mass transfer processes, such as oil penetration and moisture removal, leading to improved product quality. In the production of French fries and potato chips, the rapid movement of the cavitation bubbles creates micro-convection currents within the frying medium. This helps in the rapid removal of moisture, and promotes an even distribution of heat and frying oil around the food.

Ultrasonic cavitation also causes a disruption of the surface structure of the potato, creating micro-channels and pores. As this means an increase in surface area, which results in better frying outcomes and thereby an enhanced crispiness of the final product. The disruption of cell matrices facilitates better oil penetration, resulting in reduced oil uptake by the food, which is beneficial for health-conscious consumers.

Due to an ultrasonically induced higher diffusion and water removal rate, the final moisture content can be quicker achieved.
Sonication promotes the formation of bubbles, meaning sonication during frying results in higher bubble volumes until the end of the frying. The number of bubbles, the size of the bubbles and the related water removal during frying are positively influences in an ultrasonically-assisted frying process. Water removal from potatoes is intensified by increased diffusion as well as increased heat and mass transfer.

Ultrasonic Frying results in Oil Reduction

Less oil adsorption during frying results in healthier French fries and potato chips.
 
How does ultrasound reduces oil absorption during potato frying?
As the potato slices are immersed in hot oil, tiny bubbles form around them due to the rapid expansion of water within the potato cells. These bubbles eventually burst, forming small pores in the potato structure. When ultrasonic waves are introduced into the frying oil, they create microscopic cavitation bubbles. These bubbles collapse with immense force near the surface of the potato, causing so-called microjetting.
These ultrasonically generated micro-jets remove excess starch so that less oil is absorped. Moreover, the intense agitation caused by the ultrasonic waves also promotes better contact between the potato surface and the hot oil, intensifying the heat transfer. This means that the potatoes fry more quickly, spending less time in the oil and thereby reducing the overall oil uptake.

Ultrasonic Frying results in Acrylamide Reduction

Acrylamide is a compound that is classified as a potential carcinogen. Therefore levels of acrylamide in foods are regulated and should be reduced. Acrylamide occurs when food is processed at temperatures above 120°C, and acrylamide molecules are formed through the so-called Maillard reaction, where the amino acid asparagine and reducing sugars are transformed. Fried potatoes such as French fries and chips are one of the main sources of acrylamide. In order to produce healthier food, low-frequency, high-intensity ultrasound (in the range of approx. 20-26kHz) is a beneficical frying pretreatment in water for a few minutes, in order to reduce the acrylamide content of fried potatoes.
Sonication results in an extraction and thereby removal of reducing sugars and asparagine from the potato cells. The ultrasonically extracted reducing sugars and asparagine are washed from the potato into the surrounding water. As reducing sugars and asparagine are the precursors for the Maillard reaction and for the formation of acrylamide, an ultrasonic pre-treatment before frying efficiently reduces acrylamide and 5-hydroxymethylfurfural formation.
Consequently, the ultrasonic pre-treatment decreases the content of reducing sugars, which resulted in less acrylamide and 5-hydroxymethylfurfural formation.

Bruno et al. (2024) achieved a reduced acrylamide formation up to 67.1% by a short ultrasound treatment of 2 minutes using the Hielscher UIP2000hdT sonicator followed by hot water wash.

Antunes-Rohling et al. (2018) showed an increase of sugar extraction and therefore 44% reduced acrylamide levels by sonicating potato sticks for 30 min in a water bath before frying (171◦C, 10 min).

Ultrasonic Starch Wash-Out and Blanching

Starch-rich potatoes turn into soft and crumbly French fries while frying or baking. That is why starches are removed from the surface of the potatoes by washing them. Ultrasonication improves the removal of starches not only from the surface, but even deeper layers of the potato. Sonication creates pores so that the surface erosion of starch granules is combined with starch extraction from the potato interior.
Thereby, an ultrasonically assisted starch wash-out results in an improved control over the browning process, resulting in evenly fried fries. This ultrasonically improved starch removal results in crispier and goldener fries.

Ultrasonic Effects on the Maillard Reaction

The Maillard reaction, which occurs between reducing sugars and protein sources, as well as oil uptake during frying affects the final color of the fried products.
Improved color in fired products: For ultrasound-assisted osmotic dehydration (UAOD)-treated fried potatoes, the values of a* were significantly lower (p ≤ 0.05) than those of untreated fried samples. Observing ΔE* and a* values, UAOD provided better color when applied prior to frying. Ultrasound pre-treatment of food before frying reduces enzyme activity and achieves positive color changes during frying.

Ultrasonic Osmotic Dehydration

An ultrasound-assisted osmotic dehydration (UAOD) has the advantage of improving the color of French fries. In addition, it shortened the pretreatment time of Osmotic Dehydration by about 67 %. Sonication increases the efficiency and speed of osmotic dehydration. Additionally, Ultrasonic Osmotic Dehydration improved the quality of fried potatoes by reducing oil and moisture content as compared to the untreated samples.

Ultrasonic Air Drying of Fries

Air drying is an alternative method to remove moisture from potatoes in order to reduce the initial water content and to minimize thereby the oil absorption and improve the properties of fried food. Ultrasonically-assisted convective air drying as pretreatment before potato chips frying improves properties and oil absorption of the final potato chips as ultrasound intensifies the water removal.

Sonicators for Improved Frying

The application of ultrasound in food processing results in better product yields, lower processing time, lower maintenance costs, and better, healthier quality characteristics. Sonication can be implemented either as pre-treatment before frying or directly during the frying process.
Hielscher Ultrasonics offers high-performance sonicators for the integration into fryers and potato washing bassins of all sizes. Sonotrodes and ultrasonic probes can be implemented into batches as well as over conveyor belts.
Contact us today to discuss your frying process and learn how the integration of a sonicator helps you to produce tastier, healthier French fries and potato chips!

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Hielscher ultrasonicators are well-known for their highest quality and design standards. Robustness and easy operation allow the smooth integration of our ultrasonicators into industrial facilities. Rough conditions and demanding environments are easily handled by Hielscher ultrasonicators.

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