Ensuring Shelf-Life Through Oxidative Stability

Tuesday, September 12th, 2017

Manufacturers have to ensure that their products maintain the desired attributes and quality throughout the entire shelf-life. In this regard, monitoring the oxidative stability of the products is of paramount importance. By Dr Robert Griffiths, technical specialist, RSSL

One of the most important considerations when developing a new product of reformulating an existing one is to ensure that the desired attributes and quality are maintained throughout its entire shelf-life.

Ingredients, foods and supplements can undergo deteriorative changes during their shelf-life that can impact on their chemical, sensory and nutritional properties (texture, appearance, flavour, nutritional value, beneficial health effects).

The factors that can cause these changes are varied and include moisture loss/gain, fat degradation or migration, alterations in colour and reactions such as hydrolysis and oxidation that impact flavour compounds.

Often products are considered unacceptable and rejected by consumers due to changes in flavour, with one of the most pronounced effects being the generation of rancid off-flavours/notes caused by the oxidation of oils and fats (and other food components).

This chemical decomposition can result in the product being unpalatable. It must also be remembered however, that some of these flavours caused by degradation of fats can be desirable in products such as aged cheese.

Oxidation of fats or oils is a complex process initiated by free radical reactions at the double bonds of unsaturated fatty acids. Therefore, the greater the number of double bonds or degree of unsaturation of the fatty acids, the greater the susceptibility to oxidation.

The process of oxidation is affected by many factors including atmospheric oxygen, heat, heavy metals, exposure to light, and other chemical components that promote initiation of the oxidation process.

These factors can promote the formation of free radicals which lead to the formation of peroxide radicals and hydroperoxides, and subsequent chain reactions, leading to the formation of secondary oxidation products including aldehydes and ketones.

It is these secondary oxidation products that often produce the distinctive and generally undesirable rancid off flavours/notes and the accumulation of these components over time increases the likelihood of the product being rejected.

As mentioned earlier, some oils and fats are more prone to oxidation than others such as those high in unsaturated fatty acids, especially polyunsaturated ones which includes omega 3 and 6 fatty acids.

Some of these oils and fats will have natural levels of antioxidants that can counteract the process of oxidation to a certain degree; however the protective effective will eventually be exhausted. This often coincides with an increase in oxidation of the unsaturated fatty acids and generation of rancidity.

Reformulation & Processing Options

Reformulation Processing Options

There is of course the option for product developers to use more stable fats with less unsaturated and polyunsaturated fatty acids. However, with the trend in recent times to reduce the level of saturated fat, which is generally more stable, and increase the level of unsaturated/ polyunsaturated fatty acids and/or fortify with omega 3 fatty acids for health benefits, there is the challenge to produce formulations that can last the desired/required shelf-life.

Product developers might reformulate the fat used in a product to reduce the level of saturated fatty acid by blending with more unsaturated liquid oil. While this would reduce the level of saturated fatty acids, it would also affect the stability of the product. Therefore, it requires assessments to be made to optimise the blends to ensure that product integrity could be maintained throughout shelf-life.

Changes in formulation of product might also require changes in processing conditions and these can have an impact on the long term oxidative stability of the product.

Optimising process conditions can also lead to extension of shelf-life of the product which may result in less wastage, greater flexibility and improved profitability of a product.

Manufacturing processes can be very complex with multiple stages and many possible opportunities to improve oxidative stability of the finished product, for example, by reducing temperatures, residence time, aeration and type of aeration gas. It can often be beneficial to assess the impact of these different conditions and optimise to improve stability.

One tool that can be used to improve the stability of reformulated products that have more unsaturated fatty acids or have higher levels of (added) polyunsaturated fatty acids such as omega 3, is the addition of antioxidants.

As previously mentioned, these can delay the onset of oxidation and generation of rancidity. The selection of antioxidant and optimisation of levels requires assessment of the effectiveness in preventing/ delaying oxidation. This potentially involves assessing many different permutations of antioxidant and levels to be added.

Oxidative Stability/ Shelf-Life Assessment

Oxidative Stability

It is important to ensure that a product is stable and resistant to oxidation under the conditions it will be exposed to over the period of its shelf-life. This can be a very time-consuming process particularly if products have long shelf-lives and can impact the time of product to market.

Accelerated studies can be conducted to speed up this process in which the product is exposed to harsher conditions such as higher temperature, aeration and exposure to UV light or trace metals. Typically, however, elevated temperature and aeration are used.

It is imperative to ensure that this accelerated testing can the n be extrapolated to ‘real time’ conditions. Both real time and accelerated studies involve storing samples for periods of time under carefully controlled conditions and then performing specific analyses at different time points to monitor the development of oxidation and rancidity.

This can be very time-consuming and costly, but is sometimes necessary to fully evaluate the shelf stability of the product. This can be a barrier particularly with new product development or reformulation, or changes in processing where a number of variants need evaluating and it is often impractical to carry out these long term evaluations at this stage of the product development.

Under these circumstances, it is useful to have rapid screening techniques that can give a quick indication of the relative performance of changes in formulation or processing and allows developers to narrow down the likely end formulation or process change before commencing a full shelf-life study.

Oxidation Measurement

Once product formulations or processing changes have been screened and narrowed down to a small number of candidates, these products can then be placed into carefully controlled storage conditions reflecting the conditions the product will be exposed to during its shelf-life or harsher conditions to accelerate the oxidation process .

At set time points, samples can be taken and analysed to determine the impact on the sensory quality and level of oxidation. This can be carried out over time to build up a picture of the evolution of oxidised components and link this to acceptability of product through sensory evaluation to guide the shelf-life that can be assigned to a product.

Sensory Evaluation

The use of sensory evaluation in assessing stability of products should not be underestimated. Sensory assessment can be completed by a consumer panel of untrained assessors who will assess based purely on ‘like’ or ‘dislike’.

This provides no information on sensory defects resulting from deterioration but reflects consumer acceptance of the products. On the other hand, trained assessors who are familiar with the product and have possible perceptions resulting from sound or defective products can be beneficial. Using standardised vocabulary gives reproducibility and precision.

The combination of scientific analysis and sensory assessment is a valuable tool particularly when looking at the development of new products or reformulation of existing ones. The combination of scientific analysis and sensory assessment is a valuable tool particularly when looking at the development of new products of reformulation of existing ones.

Peroxide Value (Pv) Determination

The peroxide value is probably one of the most commonly used methods to measure the initial stages of oxidation of oils and fats. The peroxide value is often conducted using a titration based method to determine the level of iodine liberated from potassium iodide by the oxidised species in the sample but there are also colorimetric methods.

Samples of oils and fats can be analysed directly using the peroxide value. However, foods and finished products need to be extracted to recover the fat for the peroxide value determination.

This extraction needs to be conducted very carefully to avoid further oxidation and also ensure that the fat is sufficiently recovered from the finished product. The peroxide value measures hydroperoxides that are produced in the early stages of the oxidation process.

Care needs to be taken in interpreting the peroxide value results since the hydroperoxides readily degrade, so samples with a low peroxide value can still have undergone significant oxidation.

The peroxide value increases as the oil/fat oxidises, but will decrease when the peroxides are degraded to secondary oxidation products such as aldehydes and ketones. Therefore, it is important to combine peroxide value analysis that measures the initial products of oxidation with methods that measure the secondary products of oxidation such as the anisidine value.

The combination of the peroxide value and anisidine value is referred to as the TOTOX value (2 x the peroxide value + anisidine value) and is a useful measure of the initial and secondary oxidation products.

Shelf-Life Extension

Shelf Life Extension

A common desire of food manufacturers is to extend the shelf-life of products. One product development challenge involved the use of antioxidants to improve the oxidative stability but also maintain a ‘natural’ ingredients label.

In this case, the product developers were interested in evaluating the effects of natural antioxidants based on plant extracts such as green tea and rosemary extract. They then wanted to compare the efficiency of these relative to the more common and effective synthetic antioxidants BHT and BHA.

Since the number of possible combinations of antioxidants and concentrations was quite large, a screening exercise was conducted by adding different levels of the antioxidants to the base oil used in the recipe and evaluating these compared to BHT using the rancimat instrument.

After several trials of differing antioxidants, concentrations and combinations, the more promising candidate oil/antioxidant blend was incorporated into finished product on a laboratory/pilot scale. This prototype finished product formulation was evaluated directly using a rapid accelerated evaluation of the oxidative stability and compared to the existing formulation.

After a slight adjustment of the concentrations used, a long-term study was initiated by storing the sample in controlled conditions that mimicked real storage conditions and conducting sensory analysis, peroxide value and thiobarbituric acid analysis determination at different time points during the shelf-life.

The development of any off flavours/notes or oxidised components was monitored over time. The products were also stored at elevated temperatures and evaluated at more frequent time points to gain a quicker insight into the products performance and provide guidance on whether the product was likely to meet the goal of increased shelf-life.

This approach enabled the developer to screen many formulation options in a much shorter time frame at relatively lower cost compared to assessing the product in the real-life conditions. It also gave some confi dence on performance before stability trial.

Fat Reformulation

There has been much publicity in the media about the negative impact of saturated fat on health, although this has been questioned more recently. Many manufacturers however are still targeting a reduction in the amount of saturated fat in their products to fulfil health and wellness commitments.

One manner of achieving this is to substitute a portion of the saturated fat, in this case from palm oil, in the recipe with rapeseed oil. There were, however, concerns about the stability of the final fat blend since rapeseed contains a significant amount of unsaturated and polyunsaturated fatty acids that are particularly prone to oxidation.

A similar approach to that previously described was used to assess the stability of different blends of the oils and fats using the rancimat instrument which resulted in the incorporation of antioxidants to meet the required shelf-life.

The resulting formulation in the finished product was compared to the original formulation with a known shelflife to give confidence that the new formulation would perform at least as well as the original formulation.

Monitor Susceptible Ingredients

Many food manufacturers have sought to incorporate food ingredients into their products with the aim to use a positive nutritional or health claim.

The omega 3 fatty acids have been used in the development of many new products which has been driven by the positive EFSA approved health claims associated with this ingredient. The use of omega 3 fatty acids does however generate a number of challenges, not least the oxidative stability of the ingredient.

To increase the stability of finished products, developers have incorporated these ingredients into their products as emulsions with added antioxidants and also as encapsulated oils.

The encapsulation reduces the exposure of the ingredient to air and reduces the level of oxidation. Development of products incorporating these ingredients has involved the assessment of the quality and stability of the initial raw material ingredients which could be from different sources such as fish oils, algal oils or from vegetable oils.

The use and effectiveness of antioxidants has also been monitored in addition to the impact of incorporating the ingredient as an emulsion or as encapsulated oil.

These developments have also been assessed using the rapid accelerated techniques described above to evaluate the oxidative stability before the finished products have been placed into real time shelf-life studies to monitor the level of oxidation.

Oxidation of oils and fats has a critical influence on the shelf-life of ingredients and finished products. However, there are several strategies available to improve the oxidative stability and therefore shelf-life of products.

To aid product development and reformulation, there are many tools that can be used to monitor oxidation and oxidative stability in oils, fats and finished products. These can be extremely useful to ensure that a product maintains the level of quality expected by the consumer over the shelf-life of the product.

There are many drivers for reformulation of products or changes to the production process and some of the tools can provide rapid guidance to the likely impact on the products stability.

It is often the case that one method is not sufficient to assess the stability alone, but it is the use of some of these complementary techniques at different stages of product development that can speed up the development process and offer a quicker route to market with the confidence that the new product will meet the consumer expectation throughout its shelf-life.