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Biscuit, Cookie and Cracker Production
The Hurdle Technology: Shelf life part 4

This article is the fourth in a series about shelf life, where we predominantly focus on the microbiological part of spoilage of bakery products.

Moulds, bacteria and to a lesser extent yeasts were covered in part 1. Part 2 handled the internal factors influencing shelf life and in the third part, we have covered the external factors in relation to shelf life. In this part we try to bring all these items together in an approach that can facilitate a prolongation of the shelf life.

hurdle effect

Fig 1. Five examples of the hurdle effect used in food preservation. The individual hurdles may be encountered simultaneously or sequentially, depending on the type of hurdle and the overall processing. Symbols have the following meaning : F – heating, t – chilling, aw – low water activity, pH – acidification, Eh – low redox potential, pres – preservatives, V – vitamins,
N - nutrients

Mould-free Shelf life

Although the microbiological part is only one part of the what we call shelf life it is a quite important one from the aspect: safe and fit for use. Using Mould free shelf life and the techniques discussed in parts 2 and 3 will also address, although not fully, other aspects of the shelf life: staling and oxidation. In part we 1 we gained the knowledge that at almost any given pH bacteria, moulds and/ or yeast can occur, if we have a good environment that ‘feeds’ them. Only at elevated or truly reduced temperatures we seem to be able to stop them from growing. Apparently only an ERH or water activity below 0.60 seems to be then the solution to extending shelf life beyond limits.

rhizopus stonolifer
Fig 2. Rhizopus Stolonifer ( bread mould ) 

It was Leistner  who introduced a multi-layered approach to shelf life and how challenges could be met. The basic idea is that of an Athletic running the 60m hurdles: when the hurdles are far enough from each other he or she will pass them, but the closer the more difficult it becomes. 

Running the Hurdles

When projecting the spores, bacteria, moulds and yeast to be the runners in an estafette final, it will be our job as formulators, product and process engineers and manufacturing management as obstacle course designers, with the intention to have them all fall and be stopped by one of the hurdles. The moment of falling should be minimally on or after its expiry date. It is therefore vital we know and understand which deteriorating mechanisms are revealing themselves first. 
As our prime goals is to understand which mechanisms are working to deteriorate and spoil the food, so we understand what we can do to delay or prevent that food spoilage from happening, while maintaining food quality. From the microbiological point of view, we have a few options:
-    Inactivate microorganisms
-    Preventing growth
-    Restrict access to the product

As can be seen in Table 1, more mechanisms work and can have different outcomes:
-    Chemical
-    Physical
-    Mechanical 
-    Enzymatic.

Partly a relation can be made with internal aspects, such as quality of the raw materials, water activity or susceptibility to oxidation (called Redox-potential in article 2: internal factors influencing shelf life of bakery products ). An extra level of complexity is added when products are multi-layered or have multicomponent: cake/biscuit with a cream filling between another cake/ biscuit and a chocolate coating for instance (think of Frangipanes, Choco pies, etc). Potential interaction for migration of amongst others moisture and fat are prone to happen. In those multi-layered products it is vital to understand per layer or component what their intrinsic capacities are for the shelf life, as in general the weakest component determines the duration or the speed of deterioration. That also counts in relation to the more commonly looked at aspect of deterioration: staling.

As no country is alike and therefore the climate, consumer preferences, company goals differ a level of flexibility is required to adapt product and process characteristics. As temperature in relation to water activity and holding temperature has a clear relation to shorten or prolonged shelf life. The warmer the storage of product are, the more sense it makes to apply a multidisciplinary approach to extend shelf life to the maximum with the right quality from a sensory point of view as well. 

If we then add what we have shared during this article series and combine that with our product and process knowledge, we could create a model. With this model we can clarify interactions, in which we can influence our product quality and shelf life. To have a model in which we can work with, we are required to set clear, measurable goals and do need a good idea of what will happen during making, transportation and storage of our products. If one or more of these parameters are shifting, we require to adapt. In a somewhat simplified version we have merged shelf life parameter with formulation and production models to get to the following model of a bakery product:

Deciding on what parameters to consider

Every product has a base composition which will result in a certain shelf life. That implies that products with low water activity has different priorities to investigate compared to medium to high water activity. From the external factors the storage and consumption temperatures are the of second priority as the temperature facilitates many processes, but as can be seen in figure 8 it will influence a certain shelf life as well from the perspective of water activity and storage temperature. Perhaps needlessly to say that when there are high temperature variations in storage, a higher risk of reducing the shelf life is a result due to condensation. Condensation on products result in temporary higher water activities on the places of condensation, facilitating local growth of moulds, yeast and bacteria: and once the process has started it is challenging to stop.

Based on article 1 where we investigate the different microorganisms we have seen that under virtually all pH’s microorganism will grow and in normal ambient conditions many aerobic and anaerobic can grow. Nearly all moulds are stopped at 0,70 or lower, yeast at 0,60 or lower and the toxin producing bacteria are stopped at 0,86 or lower.
That would imply that based on a water activity of 0,70 a shelf life of nearly 200 days could be reached when stored well near 21˚C. A storage of 27˚C would bring this back to approximately 75 days. Experience shows that a reduction of 0,05 in water activity relates to almost double the mould free shelf life.
However, it can be imagined that lowering Aw further that it could affect eating quality and is therefore not desirable. Most companies then apply (or have even done before so) the use of preservatives. Unfortunately, the pH is in many cases not favourable for an optimal action of the preservative(s). Simply by lowering the pH within the right functionality range of the preservative, an increase of shelf life can be reached . Considered that every acid requires different dosages and depending on its salt has different flavour profile. When using cocoa powder, it can also be a challenge to keep the pH as constant as possible during the shelf life; the search for the right acidity regulator can therefore be quite the challenge. As the saliva of humans vary between a pH of 5.6 to 7.9 and averaging around 7: Children around 7.5 and adults around 6.5. A too low pH will therefore also affect the taste appearance, albeit that the type of acidifier and its sourness perception are different. In certain cases, leavening acids, such as for example gluco-delta-lactone (GDL), can assist as well in lowering the pH by a slight overdosage (up to 30%) of this, compared to what is required to neutralise the carbon dioxide carrier (such as sodiumbicarbonate).

The usage of certain fats or fatty acid rich ingredients can also induce oxidation reactions, in low Aw and high interaction with air can speed up the process, for example in Digestive Biscuits. When tackling this issue at the source the use of antioxidants can postpone and, in many cases, prevent detectable oxidation within the shelf life. Choosing the right type of packaging is most certainly of influence.
In products with higher Aw, packaging in combination with either active packaging or modified atmospheric packaging showed improvements of shelf life extension up to 700%. This is however very product dependent. The use of antimicrobial coatings or ethanol spraying have also a lengthening effect, albeit less than MAP or active packaging.

All in all, the first hurdle to introduce however is of course proper hygiene measurements. Considering basic HACCP-rules to be applied a higher level will predominantly only bring a small incremental improvement, reports suggests about only 15%. Not negligible, but neither the highest gain. If, however a contaminated product goes into a package, neither Aw, pH or MAP will make it survive the shelf life period. Therefore, hygiene is the first, basic, hurdle to be applied: simply washing your hands well (most do not do well) before handling any open product. Then sanitation with cleaning down the lines, flours, walls and ceilings in a regular way in combination with an environment you control are already prone to give you enough opportunity to extend shelf life. 

To Summarize

When designing a product or redesigning towards new required conditions, many choices must be made by formulators. On top of that, formulators should understand the mechanisms in deterioration, so we can decide based on that which hurdle we should put first to have all the runner fall and therefore delay the product decay.

Depending on the length of the required shelf life and how much money one would like to invest, most likely the routes to prolonging shelf life are:
-    Lowering your Aw
Although having a logarithmic scale, as a rule of thumb reducing your water activity with 0.05 will double your mould-free shelf life. However, many analyses already require this tolerance.
-    Reducing pH.
Some research indicates that this can give an increase up to 1100% of current shelf life if combined with other measurements, such as preservatives and Aw.
-    Introducing preservatives
-    Modified Atmospheric packaging or active packaging
Reports show that shelf life can be extended up to 700% by using this technology
-    Hygiene & Sanitation
Hygienic measurements are reported to give up to 15% extension of shelf life if you work according to ‘normal’ standards as emphasised in Europe. It is not reported how much it will improve when hygiene is poor, one can only estimate it will be over 100%.
-    Spraying ethanol
Although the use of ethanol-based flavouring is not widely used, due to religious concerns, it will give an improvement of nearly 150% if dosed around 1%/w%. There are also other antimicrobial coatings available of which we unfortunately do not know yet its efficacy.

As spraying ethanol is currently on its way back due to more and more people intend to produce either HALAL or do simply not want to expose children to alcohol; alternatives can be sought into different antibacterial coatings based on:
-    Nisin
-    Polycyclic antibacterial peptides


[1] Leistner, L. (1992). Food Preservation by combining methods, Magazine for Food Research International, 25, 1992

[1] Duisterwinkel, W. and Vast, U. J. (2017). Article Shelf life part 2: Internal factors influencing Shelf life, published at

[1] Jay et al. (2005). Modern Food Microbiology, seventh Edition, Springer Media.

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