Using Time-Domain NMR to Determine Shelf Life of Products

With global consumers spending $374 billion* on snack foods annually, a number that is expected to continue rising, the industry is fiercely competitive as manufacturers look to capture market share.

Efficient and effective production and quality control procedures are critical and measuring oil and moisture content has become one of the most important concepts in the industry. There are a number of analytical techniques available, but TD-NMR has proven to be a fast, reliable and cost-effective method that can be easily implemented.

Why is Moisture Content Important?

There are a number of reasons why it is important to measure and know the moisture content of food materials.

• Quality of product – the texture, taste, appearance and stability of foods depend on the water content
• Spoilage – moisture directly affects the propensity of microorganisms to grow, therefore foods must be dried below specific moisture levels to minimize spoilage
• Production of product – understanding the moisture content is important during processing and packaging of food to maximize shelf life, quality and consistency of final products
• Labeling requirements – there are legal requirements to the maximum or minimum amount of water that must be present in certain types of food
• Fraud prevention – because the cost of many raw materials is based on weight, manufacturers may try to incorporate inexpensive water into their product. This increases the weight and customers are charged more but are getting less actual product

Because of the large number of molecules, it’s not possible to directly measure the number of water molecules present in a sample. A number of analytical techniques commonly used determine the moisture content of foods based on determination of the mass of water present in a known mass of a sample. However, water molecules are capable of forming strong hydrogen bonds with neighboring water molecules and the strength of these bonds cause many of the unique physicochemical properties of water. Determination of moisture content by analytical techniques depends on being able to distinguish water from the other components in the food. Also, the fact that water molecules can exist in different molecular environments, with different physicochemical properties, can be problematic in trying to accurate determine moisture content. NMR is capable of measuring the amounts of water in different molecular environments, thus making it a widely used technique.

Case Study – Food industry

TD-NMR methods have been widely employed in the food industry. Below, the application of TD-NMR for measuring oil and moisture content in snack foods is described. Components in the sample can be individually quantified using the different relaxation properties of oil, moisture, protein and carbohydrates (Figure 1).

TD-NMR relaxation profile for moisture and oil

The moisture and oil content of the sample are measured using the spin-echo pulse method (Figure 2). The difference in intensity of the FID and the echo correlates with the moisture content. The echo intensity correlates with the oil content.

Knowing the moisture and oil content enables the cooking time of snack food to be optimized (Figure 3). The food is cooked when the moisture is less than 1.5% and the oil content should not exceed 25-35%.

Moisture and oil to determine cooking time of snack food

The food is cooked when the moisture and oil content are below 1.5% and 25%, respectively. If the moisture is more than 1.5% and the oil less than 25%, cooking time is extended and if the moisture is more than 1.5% and the oil more than 25-35%, the temperature is increased and cooking time decreased.

TD-NMR enables moisture to be determined within 30s for 15 to 20g of sample, while Karl-Fischer titration completes the process in 30 to 60mins for 0.5g of sample and oven drying takes several hours for indefinite sample mass. The minispec can guide the process within a few seconds, therefore minimizing defective product and wastage.

Shelf-Life of Snack Food

Shelf-life studies are carried out to determine the amount of time a product can be stored for without there being any significant changes in its character, safety and quality. Typical parameters used for shelf testing of snack food include human sensory for appearance and taste, oxidation of fats, texture and moisture content (Figure 4). “End of Shelf Life” (EoSL) parameters for fried snack food are moisture and texture. Another critical factor for shelf-life determination is water activity (aw) which can be reduced by solutes, freezing, drying, and packaging.

Shelf-life determination by TD-NMR

EoSL parameters can be most accurately determined using direct methods. The minispec can directly determine the moisture content in snack food even through package (40 or 50 mm diameter probe setup). An increase in the moisture content of snack food leads to texture loss. For instance, when the moisture level of potato chips is over 2.5%, the chips develop a stale taste and become chewy.

The shelf life of snack food is highly influenced by permeability of the packaging material. TD-NMR can directly determine packaging-dependent shelf life by measuring the moisture content. Accelerated shelf life studies can also be carried out at temperatures ranging from -100˚C to +200˚C using a variable temperature probe.

*In 2013-2014, according to Nielsen retail sales data, adjusted for inflation