The science of shelf life

The Institute of Food Science and Technology defines shelf life as: “the time during which the food product will remain safe, is certain to retain desired sensory, chemical, physical and microbiological characteristics, and will comply with any label declarations of nutritional data when stored under the recommended conditions.”

The primary goal is to make food safe to eat, but it is also important to make food desirable to eat. Shelf-life testing evaluates four areas: microbial, physical and chemical deterioration, and sensory attributes. The endpoint is product-specific and is defined by the food becoming unacceptable due to spoilage and/or sensory deterioration. Numerous factors—including pH, moisture content, water activity, packaging and storage, and preservative usage—influence the spoilage of bakery products.

Microbial spoilage is often the limiting factor in the shelf life for intermediate- and high-moisture bakery products. A means to managing this is to control water activity (Aw). Bacteria typically require a high Aw (0.94-0.99) and are limited to bakery products with high moisture content. “Rope” caused by Bacillus subtilis is a common problem in bread. The crumb becomes discoloured and sticky with a flavour of cantaloupe due to the growth of the bacteria.

Propionates, naturally found in raisin paste or puree or acetic acid, can remedy this problem while maintaining a clean label. Mould normally grows at Aw  levels greater than 0.8 and is a frequent problem in baked goods. Products generally become contaminated by mould spores in the environment or from additions such as glazes following the baking process. Packaging as soon as possible after baking can minimize contamination.

Yeast spoilage occurs in intermediate and high moisture baked goods. Visible growth on product surfaces is typical in products with high Aw  and a short shelf life. Fermentative spoilage is more common in low Aw  products like fruitcakes and is made apparent by alcoholic and other odours and visible gas production such as bubbles in jellies and bloated packaging.

Maintaining good manufacturing practices is essential, and the use of preservatives such as sorbates or benzoates can effectively inhibit yeast. The growth of contaminants may be controlled through product reformulation, to reduce pH and/or Aw, while still using ingredients seen as label-friendly. The pH can be decreased using cultured sugar, wheat and milk ingredients produced from lactic acid fermentation and Aw  can be reduced by adding sugars or salts or humectants such as honey or glycerol.

Staling is considered a serious physical spoilage concern in bakery products. Delaying this through the use of emulsifiers can help maintain a soft crumb and tenderness in bakery products. Mono- and di-glycerides are commonly used, while soy lecithin is a clean label alternative. Moisture transfer is another issue, where moisture loss can result in hardening, or drying seen often in breads. Moisture gain can cause undesirable softening or clumping, such as in a baking mix. Using packaging products with selective moisture and gas barriers can help extend the shelf life by reducing water vapour rates and oxygen transmission.

Chemical spoilage is common in high-fat bakery products due to rancidity. Lipid degradation, either oxidative or hydrolytic, produces off-odours and off-flavours often described as fishy or cardboard-like. Antioxidants such as BHA (butylated hydroxyanisole) and BHT (butylated hydroxy toluene) are used to prevent chemical spoilage. Rosemary extract, ascorbic acid (vitamin C), tocopherol (vitamin E) are alternative label friendly options. Modified atmosphere packaging (MAP) can also be effective, by changing the composition of the atmosphere around the product, typically with a mixture of carbon dioxide and nitrogen. Reducing the oxygen limits oxidation, and additionally, slows microbial growth.

Sensory evaluation of the bakery products assists in determining an end point. Flavours, textures, aromas, colours, and appearance are monitored at set intervals and may be compared to a control. Evaluators may discover rancid, or “off” flavours and odours, tough textures, dark or bleached colours, or changes to the appearance. These sensory changes coincide with the chemical, microbial, or physical spoilage in the food and should be used in conjunction with analysis whenever possible and safe.

Sensory testing can be descriptive, discriminative, or based on consumer acceptability. Descriptive testing can look at a product profile, monitoring a number of attributes (for example bitterness, sweetness, sourness, aroma, aftertaste) with trained panelists, or use a structured scale such as ranking bitterness from “Not Bitter” to “Extremely Bitter.” It is useful when changes in the product profile or appearance are expected but still acceptable.

Discrimination testing looks for differences from a control. This is useful when products are not expected to change considerably. A triangle test, where two samples are identical and a third sample is different, requires the panelist to identify the different sample is an example.

Affective testing for consumer acceptability is used when acceptability is the key driver. A large number of consumers (more than 100) should be used and a hedonic scale is an appropriate measure.

A shelf-life study should be completed in real time. When working with a product with a long shelf life, such as biscotti, people question how they can speed up a shelf-life study. An accelerated shelf-life study may be used for directional guidance in this instance. An example is storage of a product under high oxygen. This allows for a four-fold acceleration factor, where one-month storage under high oxygen is approximately equivalent to four months at ambient storage. Only qualitative changes due to exposure to oxygen are affected (such as rancidity), so it is important that a real time shelf-life study be completed to confirm the results of the accelerated shelf-life study.

Sarah Sorensen is a project manager with NSF-GFTC (http://www.gftc.ca). NSF-GFTC assists clients in reformulating products and redeveloping processes and packaging to meet shelf-life targets while maintaining food safety principals and sensory objectives.