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3D micro X-ray images help answer questions about fried foods’ internal structure

3D solid network image of pores formed in a potato during frying. Image courtesy of Pawan Takhar.

URBANA, Ill. – What happens to food and its microstructure when it is fried is a complicated process, both scientifically and mathematically speaking. While consumers want a product that is crispy and tasty, food scientists seek to get a closer glimpse into what exactly is going on inside the food during frying in order to improve products.

Particularly, Pawan Takhar, a University of Illinois food scientist, is interested in the food’s uptake of oil during frying and how that oil gets distributed throughout the food. “Through conventional lab techniques we can already see how much oil content is in food material, but we didn’t know how it gets distributed throughout the material,” he says.

To understand the distribution of oil better, Takhar and his lab recently conducted a study using X-ray micro-computed tomography (micro-CT) to gain 3D images of the microstructure of fried potato disks after they had been fried for various lengths of time.

During deep frying, as food is immersed in hot oil, water in that food quickly evaporates and steam pressure builds. This pressure affects the microstructure, including the porosity—the number and size of pores in the food—as well as the twistiness of the pathways between those pores (tortuosity). This determines how and how much oil gets taken up into the food.

For the study, russet potatoes cut into disks that were 45-mm in diameter and 1.65 mm thick were fried at 190 degrees Celsius for 20, 40, 60, or 80 seconds, freeze dried, and scanned.

Takhar says about 986 2D images of the potato samples were collected and then combined to produce 3D images. Using the 3D images, they were able to gain more information about the pores and pore networks in the material.

The researchers observed that as frying time increased, pore size increased, allowing for greater uptake of oil. They also saw a correlation between oil content and how the network of pathways between the pores changed throughout the frying time. These pathways act like channels for water and vapor flow and oil penetration in the food.

“As you fry the material, you can see how those pore structures are forming,” Takhar says. “We found that in the beginning of frying, the pore network is very complicated. The waviness in the pathway, the tortuosity, is very complex in the beginning so the material resists oil penetration. But as the frying progresses, those pathways become simpler. Pores open up and are easily accessible from the outside and oil can be taken up.”

Takhar also explains that oil was observed distributed across the full thickness of the potato disks. In thicker materials with lots of moisture (like chicken nuggets and French fries), they have observed the oil to remain near the surface as continuous evaporation helps to resist oil penetration.

“It is not easy to make a product that has no oil and still provides taste, flavor and texture that consumers enjoy,” he says. “People like that fried flavor and the texture of crispiness outside and softness inside. At the same time you want to reduce the oil content to make the food healthier. With this network study we wanted to see how those networks are formed, because networks are also related to texture.” It’s a combination of the oil content and air pockets in the pore structure that provide the desired crispy texture.

The findings from the potato disks in the study can also be applied to other fried foods, Takhar says. His lab has done previous research on frying using chicken nuggets and French fries.

While Takhar and his lab have done mathematical modeling of what happens during frying—just one previous paper outlines over 100 mathematical equations involved in the process—he says this study provides some experimental validation as to what is happening inside the food material.

“I would say we still only understand about 10 percent of what is taking place during frying,” says Takhar. He and his lab have studied the effects of frying for 10 years. “For an engineer or a food scientist, it’s the ultimate problem because it’s so complicated.

“Our aim is to make these products healthier, so that they have the same taste and texture but, at the same time, have lower fat content. That is our long-term goal with our research,” Takhar says.

“Microstructural characterization of fried potato disks using X-Ray micro computed tomography,” is published in the Journal of Food Science and can be found online at Co-authors are Tanjila Alam, formerly a graduate student at the University of Illinois, and Pawan S. Takhar of the University of Illinois.

Funding was provided by USDA-NIFA.

The researchers acknowledge Beckman Institute at University of Illinois for providing assistance with the micro CT scanning experiments.

News Source:

Pawan Takhar, 217-300-0486

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Scientists tackle our addiction to salt and fat by altering foods’ pore size, number

URBANA, Ill. – Two University of Illinois food scientists have learned that understanding and manipulating porosity during food manufacturing can affect a food’s health benefits.

Youngsoo Lee reports that controlling the number and size of pores in processed foods allows manufacturers to use less salt while satisfying consumers’ taste buds. Pawan Takhar has found that meticulously managing pore pressure in foods during frying reduces oil uptake, which results in lower-fat snacks without sacrificing our predilection for fried foods’ texture and taste.

Both scientists are experts in food engineering and professors in the College of Agricultural, Consumer and Environmental Sciences’ Department of Food Science and Human Nutrition.

Regarding salt, Lee said, “Six in 10 American adults either have high blood pressure or are on the borderline of this diagnosis largely because they eat too much salt. Overconsuming salt is also associated with the development and severity of cardiovascular and bone diseases, kidney stones, gastric cancer, and asthma.”

Because 70 percent of the salt Americans consume comes from processed foods, Lee began to study the relationship between the microstructural properties of these foods and the way salt is released when it is chewed.

“Much of the salt that is added to these foods is not released in our mouths where we can taste it, and that means the rest of the salt is wasted,” he said. “We wanted to alter porosity in processed food, targeting a certain fat–protein emulsion structure, to see if we could get more of the salt released during chewing. Then food manufacturers won’t have to add as much salt as before, but the consumer will taste almost the same amount of saltiness.”

Increasing porosity also changed the way the foods broke apart when they were chewed, exposing more surface area and increasing saltiness, he said.

“When foods crumble easily, we further reduce the amount of salt that is needed. Changing the number or size of pores in the food’s surface can help us to accomplish this,” he said.

Takhar said that his porous media approach to understanding the behavior of water, oil, and gas during frying will help create strategies that optimize the frying process, reduce oil uptake, and produce lower-fat foods.

The articles Takhar publishes in academic journals feature page after page of complex mathematical equations that describe the physics involved in the transport of fluids and in textural changes in foods. These equations then guide the simulations that he performs in his laboratory.

“Frying is such a complicated process involving more than 100 equations. In a matter of seconds, when you put the food in the fryer, water starts evaporating, vapors form and escape the surface, oil penetration starts, and heat begins to rise while at the same time there’s evaporative cooling off at different points in the food. Some polymers in the food matrix may also change their state, and chemical reactions can occur. It’s not an easy set of changes to describe,” he said.

Within 40 seconds of frying, the texture of gently fried processed foods like crackers is fully developed, the scientist said. “That’s the cracker’s peak texture. Any longer and you’re just allowing more oil to penetrate the food.

“A lot of frying research has focused on capillary pressure in the oil phase of the process, but we have found that capillary pressure in the water phase also critically affects oil uptake,” Takhar said.

Capillary pressure makes overall pore pressure negative, and that negative pressure tends to suck oil from inside. His simulations show when that pressure is becoming more negative.

“The trick is to stop when pore pressure is still positive (or less negative)—that is, when oil has had less penetration. Of course, other variables such as moisture level, texture, taste, and structure formation, must be monitored as well. It’s an optimization problem,” he noted.

When this exquisite balance is achieved, lower-fat, healthier fried foods are the result, he added.

“Temporal Sodium Release Related to Gel Microstructural Properties—Implications for Sodium Reduction” was published in a recent issue of Journal of Food Science. Lee and Wan-Yuan Kuo are co-authors of the study, which will continue to be funded by USDA. “Modeling Multiscale Transport Mechanisms, Phase Changes, and Thermomechanics during Frying” was published in a recent issue of Food Research International. Co-authors are Takhar and Harkirat S. Bansal of the U of I and Jirawan Maneerote of Kasetsart University in Bangkok, Thailand. The Takhar study was funded by USDA and the Royal Thai Government.

News Sources:
Youngsoo Lee, 217-333-9335
Pawan Takhar
, 217-300-0486

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Two University of Illinois food scientists receive USDA food safety grants

URBANA, Ill. – Two University of Illinois professors have received $861,714 in grant money from USDA’s National Institute of Food and Agriculture (NIFA) to fund research that will improve the nation’s food quality.

A four-year grant for nearly $500,000 was awarded to Pawan Takhar, a U of I associate professor of food engineering, to study damage to foods caused by ice recrystallization during freeze-thaw cycles. Shyam S. Sablani, associate professor of biological systems engineering at Washington State University, is a co–principal investigator on the project.

“Millions of dollars’ worth of food products are damaged during shipping and storing due to moisture migration and ice crystal growth caused by freeze-thaw cycles. Data generated from our physics-based mathematical modeling and experimentation will help the food industry improve the operation and design of its freezing units,” Takhar said.

Youngsoo Lee, a U of I assistant professor of food science, was awarded a USDA NIFA grant for over $360,000 for research that will enable food manufacturers to design solid food systems that will enhance saltiness and achieve sodium reduction in a broad range of products.

“Six in 10 American adults either have high blood pressure or are on the borderline of this diagnosis largely because they eat too much salt,” he explained.

Because 70 percent of the salt Americans consume comes from processed foods, Lee studies the relationship between the microstructural properties of these foods and the way salt is released when it is chewed.

“Much of the salt that is added to processed foods is not released in our mouths where we can taste it, and that means the rest of the salt is wasted,” he said. “We want to alter porosity in these foods to see if we can get more of the salt to be released during chewing. Then food manufacturers won’t have to add as much salt as before, but the consumer will taste almost the same amount of saltiness.”

Soo-Yeun Lee, a U of I associate professor of food science, and Jan Ilavsky, a physicist at Argonne National Laboratory, are co–principal investigators on Lee’s grant-funded research.

News Sources:
Youngsoo Lee, 217-333-9335
Pawan Takhar
, 217-300-0486

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Meeting Food Processing Challenges through the Physics of Food

When considering a slice of pizza, most would see cheese, sauce, crust, maybe a topping or two. What Dr. Pawan Takhar sees is movement of fluids through a porous matrix, a structure undergoing constant change and requiring consideration of concepts such as mass exchange, viscoelastic properties, fluid transport and other physico-chemical processes. In sum, what Dr. Takhar sees in food is physics. And seeing food through the lens of physics has allowed Dr. Takhar to address significant challenges in food processing, with significant results for food quality and industrial efficiency.

The uncommon approach taken to addressing food processing challenges reflect Takhar’s own unique background in food science. On the path to obtaining degrees in Agricultural Engineering from India’s Punjab Agricultural University, Post Harvest and Food Processing Engineering from Thailand’s Asian Institute of Technology, and ultimately a Ph.D. in Food Engineering from Purdue University, Takhar also held positions as a software programmer for an information technology consulting firm, and as a design engineer for a refrigeration and food machinery company. Now an Associate Professor in the University of Illinois at Urbana-Champaign Department of Food Science and Human Nutrition, Dr. Takhar brings a unique blend of physics and computational modeling to the analysis of food and food processing methods such as drying and frying.

Takhar’s approach stems from continuum mechanics, a body of physics for studying laws of mechanics and material behavior by treating matter as continuous. Foods are not simple materials where simple statements of physics laws easily apply; foods are continually changing (due to temperature, pressure, etc.), and need examination at micro-, meso-, and macro- scales. However, studying all the changes starting at the smallest micro-scale level and applying it for the whole food material would require an impractical level of computing resources and microscale material properties. To remedy this need, Takhar and his group use (and improve upon) hybrid mixture theory (HMT), which scales up from the micro level to higher scales to generate mathematical laws specifically for food and biomaterials, and which are more general than laws produced for simpler materials. In practice, the researchers combine HMT and macroscale or microstructural experiments (using data from X-ray tomography, scanning electron microscopy, etc.) to investigate a particular science challenge.

Demonstrations of the strength of Takhar’s approach in improving food quality, nutrition, and industrial energy are found across multiple industrial food processing settings. In the soybean oil industry, the amount of oil extracted from soybean flakes results from the equipment used and the physical, thermal, and viscoelastic qualities of the soybeans in the feed stream. Takhar’s group was able to model the factors involved to produce soybean drying profiles and flaking roll performance adjustments that ultimately saved $2 million for one industrial producer. Stress cracks in corn are a problem for the corn processing industry, as cracked grains are a source of inferior quality products, produce more dust, and are more prone to insect and microbial damage. Typically, stress cracks arise from moisture fluctuations and drying times, but experimentally identifying the best combination of moisture and drying time would take a great deal of experimentation. By simulating the moisture/drying process, Takhar’s group was able to generate a graph of intermittent drying times, resulting in 50% less stress cracks in a food material. For industrial frying of rice crackers and potatoes, Takhar has been able to find the frying conditions at which both products are at the most desirable level; any further frying only results in excess oil absorption, which represents waste, poorer food quality, and even a greater risk for obesity in consumers.

Currently, Takhar and his group are applying their approach to biopolymer expansion in the form of starch extrusion, with the goal of finding the optimal moisture and temperature distributions of a variety of starch-using products, ranging from cereals and snacks to lubricants for gloves. Looking to the future, Takhar sees a wealth of research possibilities on applying porous media theories to solve transport problems in foods such as frying, drying, freeze-thaw cycles etc.. In the next step Takhar will also be using molecular-scale simulations with food and bio materials via molecular modeling, molecular visualization tools and supercomputing resources. Please see Dr. Takhar’s website ( for more information.

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Pawan Takhar

Dr. Pawan Takhar

Dr. Takhar uses porous media physics to study food and bioprocessing applications via mathematical modeling and experimental validation. He focuses on making further improvements in the multiscale hybrid mixture theory (HMT) with applications in swelling biopolymers. His research group has applied the theory for predicting fluid transport and stress-crack initiation in foods; performing macroscale and microstructural experiments (using NMR imaging, X-ray tomography and scanning electron microscopy) for obtaining further insight into transport mechanisms; solving unsaturated transport problems such as frying and starch expansion during extrusion; designing controlled release applications; and adapting the concepts from transport theories to work with food safety problems (fractional differential equations based new application in this field). Numerous bioprocesses such as drying, sorption, solvent transport, controlled release, conditioning, cooking, storage etc. can be addressed using his general approach on transport mechanisms. His team has also performed validation experiments on continuous and intermittent drying, and sorption to develop strategies for obtaining foods with reduced fractures and fried foods with lower fat content.

Associate Professor;; more detail here.


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