HUNGRY FOR NANO
The fruits of nanotechnology could transform the food industry
When it comes to nanotechnology, almost every industry wants a piece of the action. For more than a
decade, the electronics industry has been exploiting the tools of nanotechnology for the development
of high-performance computer chips. More recently, biotech companies have begun harnessing the
same tools for diagnosing and treating disease. Now, the food industry is turning to nanotechnology
as it searches for innovations that could bring safer, healthier, and tastier products to the public.
On the food industry's wish list are low-cost sensors that quickly signal the presence of foodborne
pathogens, filters that remove undesired compounds from food and beverages, and nanoparticles
that store flavors and nutrients inside food and release them at designated times and places in the
body.
"Food is the ultimate complex mixture," says materials scientist David Weitz of Harvard University.
"The texture of the food, the taste of the food, the functionality of the food are determined to a large
degree by how you mix the different components together."
Understanding how all the individual ingredients in foods interact at the molecular scale-the
nanometer scale, that is-should help researchers control these interactions more precisely, Weitz
says.
In 2000, Kraft Foods, headquartered in Northfield, Ill., began sponsoring one of the most concerted
efforts to apply nanotechnology to food R&D. Known as the NanoteK Consortium, its members
include researchers from 15 universities, 3 national labs, and 3 start-up companies. By funding
research in nanotechnology, the multibillion-dollar food giant expects to churn out a new generation of
"smarter" and more personalized food products, says Manuel Marquez, director of the consortium.
Kraft is not alone. Although officials at Nestlé's research center in Lausanne, Switzerland, are tightlipped about their plans, the company now has a dedicated staff of food scientists investigating the
potential benefits of nanotechnology. Meanwhile, many university labs have begun adapting their
nanoscale innovations in chemistry, materials science, and biotechnology to meet the needs of the
food business.
Marquez says that nanotechnology will enable companies to bring food research and development to
the next level. "If we don't understand the interactions at the molecular level, we will not be able to
develop new products or materials with new properties," he explains.
PLASTIC SENSE As they reach for that new level, some researchers are turning to lessons learned
in the electronics industry. Seeking to replace silicon with less-expensive and flexible materials for
applications including solar cells and computer displays (SN: 1/31/04, p. 67, SN: 8/11/01, p. 86),
many electronics firms have been investigating electrically conductive polymers. These same
materials could be molded into sensors with nanoscale features that would, within minutes of
exposure, detect extremely faint molecular signals of spoilage or foodborne pathogens.
Because the sensors are made from cheap, flexible materials, they could be incorporated directly into
packaging for continuous monitoring of food quality, says consortium member Gregory Sotzing at the
University of Connecticut in Storrs.
Sotzing and his colleagues are working on a food-spoilage sensor that changes color when it detects
a problem. Such a sensor could render obsolete those vague, and frequently inaccurate, "best
before" dates printed on many food packages, says Sotzing.
To make the sensors, the researchers coat a piece of glass with a layer of nonconducting, precursor
polymer and then pass the fine tip of an atomic force microscope (AFM) over the material's surface.
Wherever the electrode-like tip comes into contact with the polymer, it changes the molecular
structure of the material, rendering it conducting. With this technique, the researchers can rapidly
draw extremely thin electrical lines in the material. Each line would function as a single sensing
element, explains Sotzing.
In the Aug. 11 Journal of the American Chemical Society, Sotzing and his team describe how they
used their AFM technique to create conducting lines only 45 nanometers wide.
Certain chemicals associated with, say, the breakdown of mayonnaise, binding to the lines would
change their electrical resistance in characteristic ways. Sotzing says the materials can be designed
so that a change in the resistance of the polymer lines would cause the material to swell and emit
light of a particular color.
By varying the chemical composition of the polymer in each line, the researchers can tailor the sensor
to detect multiple chemicals or pathogens and to change color in response.
On a more futuristic front, polymer sensors could help a customer analyze his breath to discover his
own food preferences. A customer in a grocery store might breathe on a sensor and the device would
detect a set of breath chemicals that it would correlate with specific taste preferences or dietary
requirements.
Qingrong Huang, a food scientist at Rutgers University in New Brunswick, N.J., is also developing
nanosensors, but he's pursuing a different application: protecting the food supply from natural
pathogens and biowarfare agents.
While working at IBM's Almaden Research Center in San Jose, Calif., Huang and his coworkers
created an insulating material to prevent electronic signals from interfering with each other on
computer chips. Soon after moving to Rutgers in the fall of 2002, Huang realized that the same
material could serve as a substrate for detecting biological agents. His group is now making small
chips out of the material that would go into portable devices to detect pathogens in food products.
Huang's silicon-containing material is riddled with pores just 50 nm in diameter. As in many other
sensing schemes, the researchers attach DNA probes to the material's surface. When bits of DNA
from a specific pathogen attach to the probes, the complex fluoresces. Because the material is
porous, it has a large surface area, and therefore can hold many probes per unit area. This could
make the device more sensitive than other sensors, says Huang.
Huang's lab is adapting its design for detecting Escherichia coli and Listeria. The goal, Huang says, is
to produce inexpensive pathogen sensors that would enable food companies to monitor their
products from the production plant to the grocery store.
LEAVING IMPRESSIONS The challenge for food companies goes beyond making sure that food is
safe to eat. With the aim of improving consumers' eating or drinking experiences, some NanoteK
members are using nanotechnology to develop products that give people more choice over which
ingredients they consume.
Consider Gustavo Larsen of the University of Nebraska in Lincoln. He's developing filter paper that
would remove caffeine from coffee during brewing. By varying the number of filters, users could brew
up a different degree of decaffeination with each pot, Larsen notes.
To make the filters, he and his colleagues modified cellulose fibers-the plant-derived material that
paper is made of-to recognize the molecular structure of caffeine. A process called molecular
imprinting creates billions of minuscule footprints of the caffeine molecule on the filter's surface. As
the coffee passes through the filter, the footprints catch the caffeine molecules, while letting the rest
of the hot beverage pass through.
The researchers created the footprints in a solution containing caffeine by mixing cellulose fibers with
silica, the mineral of sand and glass. The silica particles clustered around the caffeine molecules and
then stuck to the fibers. Selectively washing away the caffeine from the solution left behind billions of
silica clusters attached to the cellulose-each cluster harboring what was effectively a mold of the
caffeine molecule.
After drying the fibers in an oven, the Nebraska team tested the filtration capacity of its new material
and found that caffeine stuck to the modified fibers much more readily than to unmodified fibers.
When the researchers tested the cellulose on theophylline, a molecule with a chemical structure
almost identical to that of caffeine, the filter let most of the theophylline pass through.
"The fibers chiefly recognize one shape," Larsen concludes. Therefore, filter paper made from the
imprinted fibers should leave unaltered the essential mix of the coffee's flavor compounds. The
researchers' next project is to make actual filter paper out of the imprinted cellulose fibers.
Similarly imprinted filter materials could be used to remove other molecules that "we literally don't
want to have in the product," says Marquez. For instance, he notes, molecules such as cholesterol
and nicotine that are associated with diseases, or others that produce off tastes, might be removed
with properly imprinted filter materials.
IT'S GOOD FOR YOU As consumer demand for healthier products increases, so does the
commercial promise of nutraceuticals-foods and dietary supplements that contain health-promoting
ingredients. Incorporating these ingredients into foods is not always easy. For example, fish oil, which
contains omega-3 fatty acids and can help prevent heart disease, rapidly breaks down on the shelf.
Consumers have to take large doses every day to ensure that they ingest enough of the active form
of the fatty acids.
If the nutrients were protected inside nanosize capsules that unload their contents only when
ingested, then smaller doses would be sufficient. Fortifying foods with encapsulated material might be
an effective means of delivering health-promoting compounds to the body.
What's more, says Marquez, some nutraceuticals break down in the stomach before the body
absorbs them. Encapsulating materials that "know" when and where to release their contents-for
instance, in the intestine, which is less acidic than the stomach-could enhance absorption, Marquez
says.
Funded by Kraft, researchers at a company called Y-Flow in Seville, Spain, are developing a novel
way of making tiny droplets for encapsulating nutrients, as well as flavors and aromas. The
researchers squeeze two different liquids simultaneously through concentrically arranged nozzles. In
one type of droplet, the outer liquid is a slightly conducting polymer and the inner liquid contains the
nutrient.
This process produces fine coaxial jets that have one fluid positioned inside the other. An electric field
pulls the jets out of the nozzle and causes the stream to break up into nanosize droplets. When the
outer liquid is a photosensitive polymer, exposing the droplets to ultraviolet light hardens the shell,
encapsulating the second liquid, Y-Flow food scientists have shown.
Regardless of how they encapsulate the ingredients, food scientists can use different materials inside
and out to tune the particles to degrade at different rates or under different conditions. For instance,
Huang and his colleagues are making nanoparticles out of standard food ingredients such as gum
arabic and whey protein. By controlling the density of these natural polymers, they can design
capsules that degrade at specific acidities.
Huang's lab is already working on encapsulating an antioxidant compound found in green tea. "Lots
of clinical studies have demonstrated green tea's usefulness, but the body has a very low absorbency
of it," says Huang. "So, we need to create good delivery systems, and we can do that using natural
biopolymers."
Because nanoscale particles are more kinetically stable than larger particles, they are less prone to
break down or interact with other ingredients.
Huang says he's working with a beverage company to develop encapsulation materials that store
flavor compounds in a drink over a longer period of time than existing materials do. This will allow the
company to ship its beverages around the world without the drinks losing their signature taste.
With growing concern over the safety of nanomaterials, consumers might be wary that
nanotechnology is making its way into food. Indeed, recent studies have suggested that certain types
of nanoscale materials such as carbon nanotubes and buckyballs-structures being tested for use in
electronic devices-might be harmful because they could persist in the body and environment (SN:
4/3/04, p. 211).
As a result, "Kraft Foods has been extremely careful in selecting its projects," says Marquez. He is
quick to point out that the food industry is not working with those specific nanomaterials that have
raised concerns.
"We work with materials that are already in nature," he says. For instance, the materials that
researchers are using to develop flavor-encapsulating nanoparticles are derived from natural
ingredients that break down in the body. Using degradable and biocompatible polymers to fabricate
biosensors for food packaging could also address potential health and safety issues.
Like chefs, food nanoscientists are challenged to come up with the right recipe.
By Alexandra Goho
Source: Science News, 9/25/2004, Vol. 166 Issue 13, p200, 2p