Food Research at the Microscale

Recent advances in polarized thermal stage microscopy have served to enhance the research performed in laboratories within the food industry, and allow for the study of a wide range of substances. These advances include the addition of pressurized chambers, rheological control, temperature ranges from -196 to 600 C, humidity control and the ability to couple the stage to a wide variety of instruments, such as infrared and Raman microscopes. These features allow the food science microscopist to carry out their micro-experiments under a range of conditions, thereby replicating conditions that may exist in development, production and post-production.

Differential scanning calorimetry

Starch gelatinization is the process in which the intermolecular bonds within the starch molecule are broken down in the presence of water and heat. The starch grains are irreversibly dissolved. Differential scanning calorimetry (DSC) is one method commonly used to examine the properties of gelatinized starch, and is typically correlated with optical changes within the sample.

As the starch grains are heated in the presence of water, DSC measures the endothermic reaction that occurs. Optically, under polarized light, starch grains normally display a Maltese cross (Figure 1). This is because of their crystalline nature. Heat causes the grains to break down into an amorphous form, and the tell-tale cross is lost.

Recently, the gelatinization process of starch grains has also been studied using high-pressure techniques, such as high pressure DSCs—microscope thermal stages possessing the ability to heat, cool and withstand high pressure. Increasing pressure during the gelatinization process causes an increase in the peak reaction temperature, or the temperature at which the endothermic reaction occurs at a maximum. These experiments can be carried out on a micro-scale, using a polarized light microscope and thermal stage capable of adding pressure. By pressurizing the sample chamber, such instruments can be used to investigate the effects of pressure on the sample during heating and cooling experiments.

Rheological cells

Using a rheological cell on a microscope stage is useful to study substances, including foods and additives, that have a complex microstructure. Rheology is the study of the flow of matter, primarily in a liquid state, but also as soft solids or solids under conditions in which they respond with plastic flow rather than deforming elastically in response to an applied force. The Linkam Optical Shearing System (CSS450), for example, allows structural dynamics of complex fluids to be directly observed via standard optical microscope while they are under precisely controlled temperature and various shear modes. Using an optical shearing cell, it is possible to study the micro-structural evolution of complex fluids in great detail for many physical processes, such as coarsening of binary fluids during their phase separations, flow-induced mixing and demixing of polymer blends, defects dynamics of liquid crystals and aggregation of red blood cells and their deformation in flows. Researchers can then correlate micro-structural dynamics with rheological data to gain insight into rheology of complex fluids.

Humidity controls

New technology can control humidity within a sealed microscope thermal stage. These new humidity controllers can regulate humidity inside a thermal stage or any other sealed chamber up to a volume of 2000 cc from 5 to 95 percent. No dry air supply is required, and ambient air is dried through a specially designed automatic recycling desiccant system, which can control humidity for months at a time. Rather than simply monitoring the humidity of the air supplied to the chamber, a sensor is placed inside the chamber to create a feedback loop to the controller.

Humidity control is imperative in the food and agricultural sciences, as humidity can have a profound effect on how food cooks. Water molecules conduct heat much faster than air, and humid air transfers heat more efficiently to food, cooking it more quickly. It also helps prevent the water in the food from evaporating, keeping the moisture locked inside.

Food packaging remains a critical component in the food analysis industry and may include investigation of corrosion and/or leakage in a can or the analysis of a multi-layered film. Polarized light microscopy combined with FTIR microscopy can aid a scientist in analyzing a variety of layers (Figure 2). Plastic packaging often relies on a heat seal to seal its product, and the ability to image these multi-layered plastics while exposing them to a variety of environmental conditions (heating/cooling) has made the thermal stage a critical component in the food analysis laboratory.

General improvements

Recent advancements in polarized light microscopes have also served to aid the food microscopist. In particular, Nikon’s Eclipse polarizing microscopes are known for their ability to produce brighter, clearer and higher-contrast images. The LV100 POL, available in diascopic and episcopic microscope illumination types, offers a reengineered base for even easier operation. Its exclusive, high-intensity halogen light source delivers brighter images, lower power consumption and less heat generation–thereby reducing the chance of heat-induced focus drift. The high-intensity 50 W halogen light source incorporates a fly-eye lens design that outputs more light than a 100 W lamp. The greater brightness is achieved by optimizing the lamp filament size. Features such as these allow food microscopists to better resolve their specimens.

The reversed-type quadruple nosepiece of Nikon’s Eclipse provides more space at the front of the stage, making it easier for a food science microscopist to handle specimen slides. The increased objective lengths and longer working distances provide more space around the nosepiece to exchange specimens or oil the slide. The stage is large-sized, pre-adjusted and click-stops in 45° increments.

Studying the microscopic structure of food provides information needed to understand and control its characteristics. On any microscope, the critical factor is the objective. Microscope objectives are perhaps the most important components of an optical microscope because they are responsible for primary image formation and play a central role in determining the quality of images the microscope can produce. Objectives are also instrumental in determining the magnification of a food specimen, and the resolution under which fine specimen detail can be observed in the microscope.

Polarized thermal stage microscopy serves to enhance the research of laboratories within the food industry, allowing the food science microscopist to carry out micro-experiments under a range of conditions. These micro-scale experiments allow companies to save time and money in the longrun, troubleshooting issues well before mass production takes place.