Shelf-life Analyses - Microchem
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shelf life determination

Shelf-life Analyses

Introduction

A shelf-life study provides the food manufacturer with storage quality vs. storage time data.

The decision of a shelf-life date is a business choice that needs to be made by the manufacturer of the food product.

Why is Shelf-Life Testing Necessary?

The increased availability of ready-to-eat food, with an extended refrigerated shelf-life, has resulted in the need for the food industry to set measures in place to minimise the potential for microorganisms to be present in foods in hazardous numbers.

The calculation of a realistic shelf-life and date mark for a food product helps to ensure the safety and quality of food sold.

What is Shelf-Life?

Shelf-life is the consumer’s guide of the period of time that food can be kept before it starts to deteriorate, provided any stated storage conditions have been followed.

The determination of shelf-life is therefore the length of time a product may be stored without becoming unsuitable for use or human consumption, and is the responsibility of the manufacturer. Shelf-life is indicated by labelling the product with a date mark.

The shelf-life of a product begins from the time the food is prepared or manufactured.

Its length is dependent on the following factors:

  • The types of ingredients
  • Manufacturing process
  • Type of packaging
  • Storage conditions

Reasons for the Determination of Shelf-Life

The objective of shelf-life testing is to determine how rapidly the following changes occur in a food product during distribution and storage:

  • Microbiological
  • Chemical
  • Physical

The shelf-life of products needs to be determined for the following reasons:

  • Safety
  • Health
    • Safety: Foods that become microbiologically unsafe before the food noticeably spoils, for example certain chilled, ready-to-eat foods like chilled meals, sandwiches and salads, need shelf-life determination. This is not always applicable to shelf stable, frozen or unprocessed foods.
    • Health: Where storage affects the critical nutrient profile of products, for example in infant formula, the acceptable shelf-life needs to be determined. For special dietary foods, manufactured to provide the sole source of nutrition for a person’s diet for a specified period, for example the ill or immune-compromised, shelf-life needs to be determined.

Who is Responsible for Calculating a Shelf-Life?

Everyone who packages and sells food that is required to be date marked is legally responsible for calculating how long their product can reasonably be expected to keep, without any appreciable change in quality.

The food label is required to detail the shelf-life as well as the storage instructions to meet that shelf-life.

In most cases this is the responsibility of the food manufacturer, but it can also be re-packers, secondary processors, food retailers, distributors and supermarkets.

Establishing Reliable Shelf-Life Limits

  • You need to understand the innate stability of your product – how normal variability in ingredient quality, processing and packaging will impact your storage quality.
  • You need to understand how distribution will impact your storage quality.
  • You need to know the quality level at which you no longer want your product to be available to the consumer.
  • You need to know if the storage quality of this product changes significantly in a matter of days, weeks, three months, nine months, etc.

Most shelf-life issues occur when a marginal quality product is stored under abusive, (but not atypical) storage conditions.

Deterioration of Food

Deterioration of food is an ongoing process in food production:

  • For most agricultural crops deterioration starts when harvested.
  • For most manufactured foods deterioration starts before they are packaged.

It is therefore important to keep the distribution time as short as possible and avoid unnecessary stockpiling.

Relevant Factors

Quality changes over time and the following factors play a role:

  • Storage temperature
  • Humidity
  • Package protection
  • Product composition
  • Water activity
  • Processing conditions
  • Ingredient quality

Shelf-life of the same food, stored in warehouses in different areas, may not be the same due to different storage temperatures. Since different storage conditions accelerate some changes but not others, it helps to know before testing what changes are likely to occur.

It is therefore almost impossible to define shelf-life as an unchanging or fixed amount of time. However, shelf-life determination is extremely important for the following reasons:

  • Selecting a shelf-life that is too short could mean paying more than required for special handling, packaging materials, controlled temperature storage and destroying an over-aged product.
  • Selecting a shelf that is too long could mean loss of repeat sales, or customer complaints due to decreased quality and safety of product.

Possible changes that may occur and can easily be detected:

  • Organoleptic (sensory changes like taste, appearance and odour)
  • Moisture
  • Colour changes
  • Lipid deterioration

Testing Factors and Requirements

Test Package Concerns
What type of packages should be used for shelf-life testing?
The packages in which the product will be sold may be the best choice.
One might want to test the standard package against packages with intentional defects.

Microbiological Testing
In order to determine the effect of shelf-Life from a microbiological standpoint, several aspects have to be considered:

  • You need to determine if the product will support microbiological activity and if so,
  • You need to determine if this growth would result in product spoilage or potential consumer safety issues.

Analytical Determinations
Some analytical determinations are required to obtain a better understanding of a food before shelf-life testing.

These include:

  • pH
  • Moisture (high moisture foods – microbiological tests are needed to track spoilage)
  • Water activity (Aw)
  • The amount and type of fat
  • Protein content
  • Type of sugars and starches
  • Ash content
  • Preservatives used
  • Type of packaging
  • Manner of distribution and retail

Refrigerated Shelf-Life Testing

  • Refrigerated storage trials should be run at 5°C and under conditions of mild temperature abuse equal to what might be occur in the commercial cold chain.
  • Samples should be inspected and tested for stability of the critical physical and chemical characteristics, as well microbiological testing for both spoilage indicator organisms and the cold-tolerant pathogens, at suitable intervals.
  • Trials should continue beyond the targeted shelf-life, unless the product fails earlier.

The following questions should be asked:

  • Does the pH change due to microbial growth or failing buffering systems?
  • Is water redistributed in the product due to condensation or broken emulsion?
  • Is the preservative system stable?
  • Are there any signs of surface growth of microorganisms?
  • Does any modified atmosphere persist for required times?
  • Do chemicals migrate from the food-contact materials into the food?
  • Are there any unforeseen changes to the product?

A number of microbial growth modelling programs are available but interpretation of results requires experience and technical expertise.

Accelerated Shelf-Life

The trial period is shortened by deliberately increasing the rate of deterioration. Accelerated shelf-life trials are only suitable for shelf-stable products.

  • Accelerated shelf-life tests are performed under a set of abusive conditions:
    • High temperature
    • Humidity
    • Light intensity or similar
  • Products are tested following storage for different times and conditions.
  • Calculations to convert the results of testing into an estimated shelf-life are complex (one week at these abusive conditions might equal approximately two weeks of shelf-life). Consulting laboratories usually supervise the studies.
  • These results are extrapolated to results expected under normal conditions.

Shelf-Life Testing for Frozen Foods

Just like any other food, frozen foods deteriorate during storage by different modes.

Microbes are usually not a problem, since they cannot grow at freezing temperatures unless subjected to extensive temperature abuse above the freezing point.

Enzymes are a big concern for frozen foods.

Vitamin C loss is often a major concern for frozen vegetables.
Physical changes, such as package ice formation, moisture loss, emulsion destabilization or re-crystallization of sugars and ice of frozen desserts are often accelerated by fluctuating temperatures. Accelerated shelf-life testing is not possible on frozen products.

End of Shelf-Life Testing

  • We need to ask the question: “To what sensory quality level do we no longer want this product available to consumers?”
    Do we want the storage quality endpoint to be when the product is “not fresh” or when it is “clearly objectionable” to the consumer.

The end of shelf life can therefore be determined by taking the following into account:

  • Product quality: This often declines more gradually with changes impacting sensory attributes (appearance and taste) rather than food safety.
  • Product safety: When product quality reaches a rapid degradation reaction point of both microbial spoilage and lipid oxidation, and may be a risk to the consumer.

Expiry of Shelf-Life Testing

  • Retained samples from commercial batches of product should be tested for microbiological and chemical quality upon expiry of shelf-life.
  • Newly launched products should be tested more frequently, while proven products should be verified occasionally.
  • Samples for testing should be stored under realistic rather than ideal temperature conditions.
  • Expiry testing provides further assurance that manufacturing systems are under control.