Internal factor influencing shelf life of bakery products

Shelf life of food -  This article will focus on the internal factors that influence microorganisms and thus shelf life of bakery products

In the Shelf life Introduction in the series about Shelf Life, all possible microbiological types of spoilage that can occur in bakery products were discussed. Moulds and bacteria were the biggest concern for the bakery industry and in a lesser extent yeasts. Rhizopus species, Penicillium species and Aspergillus species are moulds that are the most prevalent and can produce mycotoxins such as aflatoxins. Bacteria that create the most problems in bakery products are Bacillus species, which can cause ‘ropy’ bread and furthermore Staphylococcus aureus and Salmonella outbreaks are worth mentioning.

Part 2 and Part 3 are focused on the internal- and external factors that are influencing shelf life. In particular, a focus to prevent or delay the germination of microorganisms and other possible processes that cause quality losses in bakery products is applied. These factors be hurdles to take for microorganisms and compounds that affect shelf life. In combination, preservation factors (hurdles) reinforce each other, whereby individual techniques can be moderated due to synergetic effects.

The Hurdle Technology will be further extensive explained in part 4, the final part of this series about Shelf Life. This article will focus on the internal factors that influence microorganisms and thus shelf life of bakery products including pH, redox potential, raw materials, product formulation, product make-up and structure and water activity (a_w).

2.1 Water activity (a_w)

Water is often the major constituent in foods. Even relatively 'dry' foods like bread usually contain more than 35% water. The state of water in a food can be most usefully described in terms of water activity. Next to temperature (an external factor), a_w is considered as one of the important parameters in food preservation and processing. Figure 1 shows the relationship between Mould Free Shelf Life (MFSL) and water activity at different temperature¹⁴. Where Figure 2 shows deterioration reactions at different water activity levels.

Figure 1: Relation between MFSL in days, water activity and temperature

Figure 2: Food stability as a function of water activity

The concept of water activity is more than 60 years old when William James Scott showed that Staphylococcus aureus has a limiting a_w level for growth¹. Water activity (a_w) of a food is the ration between the vapour pressure of the food, when in a completely undisturbed balance with the surrounding air, and the vapour pressure of pure water under identical conditions².

Water activity, in practice, is measured as Equilibrium Relative Humidity (ERH), the humidity of the surrounding air. During Shelf life, the ERH will change, hence the equilibrium at the moment of measuring. For simplicity reasons, we will remain using water activity. The relation between ERH and water activity can by calculated by using equation 1:

a_w = ERH/100
ERH = a_w * 100%

If it’s not possible to determine the relative humidity of the surrounding air, the water activity can be predicted in theory by several formulas. The most conventional for bakery products is the Grover equation. This model has been successfully applied to bakery products such as bread³.  However, it works not properly for products with a low water activity, a large deviation is the outcome.

The Grover equation is based on the sucrose equivalent (SE). By setting the value of sucrose to 1, other ingredients can be compared towards ‘the effect that the ingredient has on water activity compared to the effect an equal quantity of sucrose would have’¹⁴.  An overview of the sucrose equivalents of common bakery compounds are displayed in Table 1. The other parameter that needs to be obtained is m_i, the moisture content in grams of water per grams of the ingredient. In other words; the moisture content of each ingredient. Grover’s equation is written below.

Table 1: Sucrose Equivalents of some common bakery ingredients

*depends on salt concentration
**DE = dextrose equivalent

the Grover equation:

a_w = 1.04 - 0.1 E_s   + 0.0045 (E_s)²
E_s = ∑ (SE/m_i)

There are many humectants that can decrease the water activity drastically and thus extend the shelf life of bakery products. Overall, the most popular are sugars or derived products of sugar such as glycerol, dextrose and glucose syrups. Probably the most important/common ingredient to decrease the water activity is salt (sodium chloride).

Reformulation with the help of these humectants will be discussed in paragraph 2.5. Furthermore, acids have an influence on Shelf Life extension by lowering the water activity. Although, the biggest impact of acids on shelf life extension is the pH decrease which will be discussed in the next paragraph.

2.2 Preservation, pH value and type of acid

Increasing the acidity of bakery products has been used as a preservation method since ancient times. It is a well-established fact that microorganisms can only multiply within certain pH ranges. The pH of a system is related to the concentration of hydrogen ions which, in case of food, come from ‘acid’ ingredients that dissociate in water, releasing them in the process.

This can be achieved by adding acidulates like sorbic- and propionic acids (Table 2). Sorbates are more effective in inhibiting mould growth in bakery products, but have the disadvantage of a poor water solubility and that they are affecting yeasts as well. This last disadvantage can result in a reduction of loaf volume and making dough sticky, which makes it difficult to handle. To avoid this problem, sorbate could be sprayed onto the product’s surface after baking or mixing anhydrates of sorbic acid with fatty acids⁴.

Two properties of food preservatives are the most important for application: the dissociation constant (pK_a) and partition coefficient (P_{octanol/water}). Food preservatives exert antimicrobial effects by interfering with internal metabolism, which requires their passing through the membrane. Only the undisassociated form of organic acids can pass through the membrane. The degree of dissociation is presented by the negative logarithmic ionization constant; pK_a. The pH of a food directly affects the proportion of a preservative that can enter a cell.

In general:

pH = pK_a                               →           50% is undisassociated →            active as a preservative

pH = 1 unit below pKa    →           91% is undisassociated →            strongly active as a preservative

pH = 1 unit above pKa    →           9% is undisassociated   →             poorly active as a preservative

Table 2: Effectiveness, working pH ranges and applications of some widely-used preservatives

+ Decrease the growth of microorganisms, the more, the bigger the effect
-  No effect on the growth of this type of microorganism

The partition coefficient is a ratio for the affection of a compound in fat (octanol) or water.

Equation 3: Partition coefficient octanol/water:

log P_oct/wat = log ([solute] octanol / [solute] un-ionized water)

In general:

High P_{octanol/water}                 →           lipophilic              →                           low solubility in water

Low P_{octanol/water} →           hydrophilic          →                          high solubility in water

Chemical properties and the biological effectiveness of some frequently used preservatives are described in tables 2 and 3.

Table 3: Chemical properties, usage and antimicrobial effectiveness of some widely-used preservatives

Another, more natural way to preserve can be achieved with the help of bio-preservatives (e.g. lactic acid bacteria), which are commonly found in a sourdough. Now, sourdough is employed in the manufacture process of products such as bread, (some) cakes and crackers. The most prevalent used cultures are Lactobacillus species and can lower the pH of the dough to 3.8-4.5⁵. In this range, most pathogenic bacteria cannot grow. Although, moulds and yeast can grow at this relatively low pH, but it is not optimal. Lowering the pH is normally not the main goal of a fermentation process, improving quality and flavour are. However, there is a sourdough product developed especially for decreasing the pH. The manufacturer claims that it inhibits the growth of moulds by adding it as an ingredient or by spraying it on the surface without taste defects. 

Table 4: pH range for minimal and maximum growth plus optimum of growth

When choosing to work with preservatives it is not always easy to have your product within the working range of the preservative(s) of choice. Either an overdosing is then applied or a combination of two preservatives. One can also consider using an acidulant, some have also a preserving effect. Table 5 shows some applications:

It is not always achievable to induce adequate changes of pH in bakery products to have a significantly inhibiting effect on microbial activities. In some cases, the nature of the ingredients themselves makes it difficult to achieve pH changes because they may interact with acids. For example; sodium bicarbonate reacts with acids like citric acid that results in the formation carbon dioxide. Furthermore, if hydrocolloids are presented in the recipe, pH is also a critical parameter according to the product stability. Because many hydrocolloids will lose their solubility and thereby gelling properties if the pH is around the isoelectric point (where the net charge is neutral)⁷.

Another important characteristic when using an acidulant is the buffering capacity. The buffering capacity is its ability to resist changes in pH. Foods with a low buffering capacity will decrease the pH immediately when an acidic compound is added. Baked products that lend themselves to manipulation of pH for shelf life extension include those where an acid flavour is an advantage, is not that pronounced (as with lactic acid) or can be masked, like citric acid in fruit fillings. Besides regulating the acidity, citric acid enhances the antioxidation properties of compounds which bring us to the next internal factor: The Redox Potential (E_h).

2.3 Redox potential (E_h)

The redox potential is literally the ease by which a substance loses (oxidation) or gains (reduction) electrons. When electrons are transferred from one compound to another, a potential difference is created between these compounds. The difference can be measured with the help of electronic meters and is expressed in mV. A few redox potentials of frequently used bakery ingredients are shown in Table 6.

Oxidation is also achieved when a compound reacts with oxygen. The availability of oxygen therefore affects the oxidation reduction (redox) potential. Because of that, the redox potential is crucial to the biochemical reactions in food that require oxygen. If nutrients are exposed to oxidation, it will result in quality losses and thus affecting shelf life of bakery products.

Table 6: Redox potential and pH of some bakery ingredients

Some microorganism requires a relatively low redox potential like anaerobic bacteria (e.g. Clostridium Botulinum), while others require a relatively high redox potential like the ‘bread mould’ Aspergillus niger⁶ (Table 7).

Table 7: Redox potential ranges of categories of microorganisms

Especially lipids in e.g. oil and butter are sensitive for oxidation reactions. The oxidation process of lipids is called rancidity and can be either slightly or intense. Paradoxically, slightly rancidity is a bigger concern for bakeries than intense rancidity. Because small degrees of taste and flavour deviations will not be observed as rancidity, therefore blame is usually given to other possible causes. Meanwhile, intense rancidity is obviously noticeable where after measures can be taken, such as applying antioxidants. 

Antioxidants is an umbrella name for several compounds which all have the function of preventing or delaying deterioration by oxidation according to the EFSA (European Food Safety Authority). The most widely known antioxidants are vitamin C (ascorbic acid), vitamin E (alpha tocopherol), and flavonoids like lycopene (e.g. present in tomatoes) and Bèta-carotene (e.g. present in carrots)⁸. Antioxidants significantly delay or inhibit oxidation and thus enhance product stability⁹.

2.4 Raw materials

Almost as a rule, the quality of a finished product is a reflection of its raw materials, according to Dominic Man⁹.  Not all the quality characteristics and parameters of a raw material will have an influence on shelf life. Those that do will need to be recognized and their effect on shelf life established. An important characteristic of raw materials is the nutrient content.

Like us, microorganisms need beside water, a source of carbon, an energy source, a source of nitrogen, minerals, vitamins and other growth factors such as pH, that we already discussed. Since bakery products are a rich source of these compounds, microorganisms can use them for growth and maintenance of metabolic functions. The inability to utilize a major component of the food material will limit its growth and put it at a competitive disadvantage compared to those that can. In general, moulds have the lowest requirement, followed by yeast and bacteria. Although, because the abundance of nutrients in each bakery products is sufficient, whereby it complicated to estimate the growth of microorganisms and thus shelf life.

Many food microorganisms can utilize sugars, alcohols, and amino acids as sources of energy. Few others are able to utilize complex carbohydrates such as starches and cellulose. This ability will favour the growth of an organism on cereals and other farinaceous products. The addition of fruits containing sucrose and other sugars increase the range of available carbohydrates and allows the development of a more diverse spoilage microflora of yeasts³. 

Another concern in the selection of raw materials is replacing substitutes in for example, E-number free products where eggs can replace hydrocolloids. Eggs will be a new source to grow for microorganisms like Salmonella, which must be considered. This critical point leads to the next paragraph about the influence of the product composition and formulation.

2.5 Product composition and formulation

The composition and formulation of bakery products is another important shelf life determining factor. In contrast to the use of eggs in E-number free products, many other ingredients have positive effect on shelf life extension. Margarine for example, contains at least 80% fat that limit the growth of most microorganisms. The size of the aqueous phase droplets and the inability of microorganisms to move been droplet reduce the growth of microorganisms. On a side note: margarine can be prone to oxidation that must be taken into account as discussed above.

Table 8: Reference cake recipe (A) and Extended shelf life by use of salt (B)

The addition of salt in bakery products can also extend shelf life through its powerful water-binding properties which decreases the availability of ‘free’ water (a_w level). Reducing the water activity makes it harder for most microorganisms to grow¹⁰. A relatively small amount of salt can be added to achieve a large effect on the water activity of the product because of its high sucrose equivalent (11, see Table 1). Although, there is limit to the quantity that can be added to baked products because of its effects on processing. Salt induces changes to the viscoelastic properties of gluten and inhibition of yeast in (yeasted) doughs. In general, salt has a strong effect on flavour and nowadays the addition of salt should be considered because of its relationship with chronic disorders like heart and vascular diseases¹⁴.

Table 9: Extended shelf life by dextrose (A), by sugar (B), by sugar

Furthermore, high refractometric solids in traditional fruit jams results in long ambient shelf life. However, increasing sucrose levels in a cake recipes is not always practical because of the excessive sweetness or possible formulation imbalance within the recipe.

Table 10: Relative sweetness and sucrose equivalence of some frequently used sugars

 
Humectants are another group of food additives that are used to reformulate products. A humectant can be defined as an ingredient that is hygroscopic, which means that it has the ability to absorb a greater quantity of moisture form the surrounding environment than suggested by its molecular weight. By adding humectants, it is possible to maintain ERH and increase the moisture content, or to reduce the ERH without reducing the moisture content (shown in table 11). Sorbitol and glycerine are examples of the most commonly used humectants in the bakery industry.

Table 11: Shelf life extension by glycerol (A); reference recipe (B)

As discussed previously, salt, sugars and humectants can be used individually to reformulate products and thus extend shelf life. However, they can also be implemented together in bakery recipes where through less of each individual component needs to be added and thus negative properties can be tempered. An example of a reformulated cake recipe including sugars, salt and humectants whereby water is reduced is shown in table 12.

Table 12: Combined strategy for shelf life extension

 2.6 Product - make-up and structure

Product- make-up and structure are often underestimated intrinsic parameters that can influence shelf life. Bakery products, which are mainly solid or semi-solid, do not have a truly homogenous and uniform structure. Therefore chemical and physical conditions relevant to microbial growth, chemical- or biochemical reactions can strongly depend on the position in the food. The smaller mobility of micro-organisms in solid foods allows spatial segregation which causes pattern formation. Evidence is given for the fact that taking space into account has an influence on the behaviour of micro-organisms¹⁵. Significant differences in lipid oxidation between bulk fat and emulsified fat, which can be presented in cakes and crèmes are observed. This is an example of a biochemical reaction that depends on product structure, in particular the micro structure¹⁶.

On the other side, macrostructure, better known as product make-up must be considered when determining shelf life. There are several bakery products that consist of different components such as croissants, cakes, pies and muffins that can be filled and/or coated with fruit jams, chocolate, nuts and/or cream. These components all have different chemical and biochemical compositions. Through contact between components migration of moisture, colours, flavourings or oil from one component to another can occur.

In fruit pies, migration of moisture from the filling to the pastry leads to a gradual loss of texture. Moisture is exchanged because of the chemical potential difference between the components until the system finally reaches equilibrium water activity (a_w) throughout each domain. Diffusion and moisture kinetics play important roles in these types of dynamic systems. Water activity (a_w) equilibrium and rate of diffusion are the two main factors influencing moisture migration. The rate of diffusion is defined as D_eff (effective moisture diffusivity), which is an overall transport property incorporating all transport mechanisms. Table 13 shows the effective moisture diffusion of some frequently used ingredients in bakeries.

Table 13: Water activity, thickness and effective moisture diffusivity of some bakery products and ingredients

The addition of stabilizers can inhibit migration or diffusion of moisture and other compounds between product components. Widely used stabilizers in bakery products are starches and hydrocolloids such as guar gum, xanthan or carboxymethylcellulose (CMC)¹⁴.

Based on this, control of initial a_w and moisture migration is critical to the quality of many bakery products. It is important to note that moisture migration happens from a higher water activity to a lower water activity. This can be recognised by looking at filled croissants or cakes with a standard chocolate/hazelnut (fat based) filling: the croissant or cake becomes dry and hard, while the filling remains more or less soft.

Related:

Shelf Life Introduction

External factors influencing shelf life of bakery products

The Hurdle Technology: Shelf life

Visit TechTalks Discussion 

Shelf-Life of Lactation Cookies
I am a home-based manufacturer of Lactation Cookies for breastfeeding mothers. I am now slowly scaling up my business and am in need of some help increasing shelf-life of the product as I have now started supplying to retailers. Currently the shelf-life is around two weeks. The ingredients that I use are Wheat Flour, Cake Flour, Eggs, Butter, Margarine, White Sugar, Brown Sugar, Oats, Vanilla Essence, Baking Powder, Baking Soda, Semi-sweet dark chocolate chunks, nutritional yeast and flax meal. The product is packaged in a resealable laminated kraft pouch and heat-sealed.

Another point is that I live in Sri Lanka, a tropical country, with an ambient temperature of around 30C and humidity of around 80%.

Since my product is sold to lactating mothers I am not really willing to add any chemical preservatives, would anyone be able to give me some tips on where to begin?

A few ideas I have been given are to reduce water activity by substituting butter and margarine with shortening, but the shortening available here is quite horrible.

Shelf-life test
What is the best way to test products for shelf life in short time? Our product is plain flat wafer. We are currently shocking our packaged wafers in different environments several times. What is your expert opinion?

Improvement of shelf life and stability of recipe for hardough biscuits

I manufacture an artisan recipe for hard dough biscuits/crackers. I use natural ingredients only, no preservatives. The main ingredients are wheat flour, vegetable shortening, instant dry yeast for fermentation, sodium bicarbonate, cream of tartar, salt, and sugar. Very simple but nicely accepted. I am introducing the product in mass markets, but I cannot make it last long on shelves. The product is only stable for 3 months; after that, flavor changes and not nicely. I pack products in pouches of BOPP CPP bags, 30 microns, sealed by heat. I would like to keep the formula stable for at least 4 months, preserve flavor without chemical additions to the recipe. Any suggestions?

How to improve my product filling, shelf life & texture?

I am opening this topic to ask you for advice on different issues that I have in the production of my Chocolate Cookie. As you can see the picture attached, my product is a chocolate filled biscuit. 

My challenges are:

1- I can not have a Shelf life of more than 3 months and I wish to have 1 year to be able to compete with biscuits.

2- The major issue is either a very dry product or it loses the crunchiness after 1 month

3 - The filling we are using loses the paste form (becomes very dry, not liquid but hard form)

Here are the ingredients we put in the products :

- Wheat flour, Sugar, Vegetable Oil, Fresh eggs, Milk Powder, Peanut Paste, Cocoa powder, Iodized salt, Baking powder, Sodium bicarbonate (E-500ii), Sorbitol, Soya lecithin (E-322), Antioxidant (E-319), Vanillin.

Water Quantity in Biscuits Using Oat Flour for Dogs and Shelf Life
I am an artisan creating biscuits for dogs (treats) and I am experimenting some recipes.The plan is to scale up and improve the shelf-life of the products (target set to 12 months) by using adequate Doypack with sealing.Today, I use a classic oven (convection with fan) and a standard kenwood mixer, when scaling up, I am planning to use a rotary moulder and a rack oven.

I only use organic products and because the biscuits are for dogs, I need limited the sugar. And I am struggling with the following problems that your expert eyes could probably clarify.

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18. Leading image: By NANTa SamRan/Shutterstock.com