Small Scale Solutions to Large Scale Problems

How much trust do you place in the “Best Before” date on your carton of milk or package of ground beef?

As consumers, we have to trust the date since it’s difficult – if not impossible – to tell if food has spoiled just by looking at the package. And while having to toss out a rancid piece of food can be simply annoying, it can potentially be a more serious matter. In fact, the Canadian Food Inspection Agency estimates that there are about four million cases of foodborne illness in Canada every year.

The use of nanotechnology has the potential to solve this issue and many other food production and agricultural issues. For food production specifically, nanotechnology could be used in food processing applications and food packaging. Through the use of improved packaging, active packaging and intelligent/ smart packaging, nanotechnology could result in safer food products, higher quality food, and providing more informative labelling.

What is Nanotechnology?

Nanotechnology is the engineering and production of structures at the nanoscale, which is about one to 100 nanometres (nms) – essentially, at the scale of individual atoms and molecules. For a better understanding of scale, take a sheet of newspaper (for those of you who still know what that is) which is about 100,000 nanometres thick. What is it about nanomaterials that has generated so much interest for the food packaging sector? Well, consider that polypropylene films are probably the most commonly used plastics for packaging because of their transparency, low specific weight (weight per unit volume of a material) and chemical inertness (lack of chemical reaction) to the contained food. However, one of polypropylene’s drawbacks is that it has a low barrier threshold to gases and other small molecules. That means relatively low protection for the food.

In order to enhance the properties of the polypropylene packaging, manufacturers often add a polymer blend filler made of high aspect ratio nanoparticles (HARNs), nanoparticles with a length many times that of their width. These HARNs create an obstacle for gas and moisture passing through packaging walls by increasing the path that penetrating gas/moisture molecules must travel.

Nano-clays and nano-silvers are the leading nanocomposites used to augment these barrier properties for food packaging. Montmorillonite clay – a member of the smectite family – is the most common type of clay used in plastic nanocomposites, as well as in many other industrial applications such as absorbents and drilling fluids. For nanotechnology applications, it’s the fact that smectite clay particles measure about one nm in thickness which makes them attractive for nanocomposite production. Nano-silvers have also been employed in nanocomposite packaging for quite some time, mainly because of their antimicrobial attributes. In fact, silver’s natural antibacterial and antifungal properties have been well known for thousands of years.

Silver as a metal was used in ancient times as a water container to keep water fresh and it’s been used for medical conditions for more than a century, even prior to the discovery of microbes as the cause of infection. So, it should come as no surprise that its nano-particles have also been adopted for their medical qualities. There are biological reasons why nano-silver is so beneficial. Specifically, it suppresses the respiration and basal metabolism of the microbial electron transfer system as well as transport of substrate in the microbial cell membrane, impeding propagation and growth of bacteria or fungi. In simple terms, the nano-silver particles suffocate the bacteria. Those properties are enhanced by the nano-silver particles’ size at about 25nm, which greatly exposes contact of the bacteria and fungi to the nano-silver particles’ antiseptic attributes.

Nanocomposite polypropylene films are generally manufactured using a melt mixing or hot melt extrusion (HME) technique. The HME process has been used consistently since the 1930s in the plastic, rubber and food sectors although extruders were already being developed in the mid-1800s.

Modern extruders retain the same principles as early extruders, but the screw geometry has undergone many changes. The basic premise behind melt extrusion is to heat the mix of polypropylene and nanoparticles, feed it into a screw extruder (or a twin-screw extruder), which pushes it through an opening in the form of pellets for further processing into various products such as clear packaging.

The mixture is melted by the frictional heating of the cylindrical barrel in which the screws are housed, as well as by heaters attached to the barrel. An important part of the manufacturing process is the shearing sustained by the mixture as it’s squeezed between the rotating screws and the wall of the barrel, resulting in a thorough mixture.

Solution blending can also be used to make sure the nanofillers are uniformly distributed. During this process, the monomer is combined with the fillers and a catalyst, such as a solvent. Another beneficial application of nanoparticles in food production is the use of molecule-sensing materials that can respond to food spoilage either by a hand-held device or a “smart” packaging system. The nanomaterials act as a sensor to indicate that food is starting to spoil in the early stages.

Kenneth Bosnick, research officer with National Research Council of Canada’s (NRC) Nanotechnology Research Centre in Edmonton, Alta., says the science behind the spoilage indicators is more or less mature, but the type of sensors on which his team is working is cutting edge.

“We’ve been working on the development of these types of sensors for several years and the progress looks quite promising,” Bosnick says. “The premise of the technology is that when meat starts to spoil, it stinks and that’s basically because it’s giving off amine* molecules. However, in the early stages, the smell is still very faint and not noticeable. The spoilage detectors are based on embedding conductive material into the plastic film packaging, which can respond to the amines. The basic response of the nanomaterial in the packaging is a result of a charge transfer from the amine molecule to the sensing surface. Because the nano-particles have nano-scale dimensions, the sensor provides a larger surface area to react to the amines given off by the spoiling food. The optical readout for the packaging is usually in the form a label that changes colour to indicate spoilage.”

Bosnick says his development team is also interested in hand-held devices to be used by a human inspector. In a hand-held device, the electrical readout measures changes in the resistance of the semi-conductor, usually zinc oxide or silicon, positioned near the food. The readout allows the inspector to detect spoilage and adjust conditions of storage.

Major corporations such as Nestle, British Airways and French supermarket chain MonoPrix are already using chemical sensors, which can easily detect chemical changes in food.