Shelf life: Part 2
In the previous part 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 (aw).
2.1 Water activity (aw)
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), aw 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 aw level for growth1. Water activity (aw) 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:
aw = ERH/100
ERH = aw * 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 mi, 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
|Water, fat, and whole egg||0.0|
|Glucose syrup 42 DE**||0.7|
|Starch (DE < 50)||0.8|
|Glucose syrup 64 DE||0.9|
|Glucose, fructose (invert sugar)||1.3|
|Salt (sodium chloride)||11.0|
*depends on salt concentration
**DE = dextrose equivalent
the Grover equation:
aw = 1.04 - 0.1 Es + 0.0045 (Es)²
Es = ∑ (SE/mi)
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 (pKa) and partition coefficient (Poctanol/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; pKa. The pH of a food directly affects the proportion of a preservative that can enter a cell.
pH = pKa → 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
|Preservatives||Bacteria||Moulds||Yeasts||Working pH range||Applications|
|Calcium sorbate||+||+++||+++||3.0-6.5||Cakes & pastries|
|Propionic acid||Breads, part-baked breads|
|Calcium propionate||+||++||-||2.5-5.5||Pre-packed rolls, buns and pitta|
|Sodium propionate||All types of bread|
|Acetic acid and acetates||++||+||+||3.0-5.0||All types of bread|
|Sodium diacetate||Cakes & some breads|
|Benzoic acid and its salts||++||++||+++||2.5-4.0||Fruit fillings, jams|
+ 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 Poct/wat = log ([solute] octanol / [solute] un-ionized water)
High Poctanol/water → lipophilic → low solubility in water
Low Poctanol/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
|Property||Sorbic acid||Propionic acid||Benzoic acid||Methyl paraben|
|Dissociation constant (pKa)||4.76||4.87||4.20||8.47|
|Principal usage|| Many foods,
|Yeast-leavened bakery products||Fruit drinks, soda||Beverages|
|Relative effectiveness against:|
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
|Type of microorganism||Minimum||Optimum||Maximum|
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:
|Property||Acetic acid||Lactic acid||Citric acid|
|Dissociation constant (pKa)||4.75||3.86||3.14|
|Relative effectivenes against:|
|Other interactions:|| Flavour (vinegar)
| Flavour (mild, sour)
| Leavening interaction
Chelating effect (binding)
|pH 3.5||-||- (3.7:+)||+|
|Required dosage to achieve||0.84%||0.67%||2.49%|
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 (Eh).
2.3 Redox potential (Eh)
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
|Type of ingredient||Presence of air||Eh (mV)||pH|
|Wheat (whole grain)||-||-320 to -360||6.0|
|Butter serum||-||+290 to +350||6.5|
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
|Categories of microorganism||Eh (mV)|
|Aerobes (e.g. moulds and bacteria like Aspergillus species)||+300 to +500|
|Facultative anaerobes (e.g. yeasts and bacteria like Bacillus species||+300 to -100|
|Anaerobes (e.g. bacteria like Clostridium species||+100 to -250|
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)
|Ingredient||(A) Weight (g)||(B) Weight (g)|
|Skimmed milk powder||8||8|
|Mould-free shelf life at 21°C||10 days||12 days|
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 (aw 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
|Ingredient||(A) Weight (g)||(B) Weight (g)|
|Skimmed milk powder||8||8|
|Mould-free shelf life at 21°C||12 days||10 days|
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
|Sugar||Relative sweetness||Sucrose equivalence|
|High-dextrose glycose syrup||0.65||0.9|
|Regular glucose syrup||0.50||0.8|
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)
|Ingredient||(A) Weight (g)||(B) Weight (g)|
|Skimmed milk powder||8||8|
|Mould-free shelf life at 21°C||21 days||10 days|
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
|Skimmed milk powder||8|
|Mould-free shelf life at 21°C||31 days|
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 (aw) throughout each domain. Diffusion and moisture kinetics play important roles in these types of dynamic systems. Water activity (aw) equilibrium and rate of diffusion are the two main factors influencing moisture migration. The rate of diffusion is defined as Deff (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
|Product||aw||Thickness (mm)||Deff (m²/sec) * 10-12|
|Non-fat dry milk||0.75||3.1||21.3|
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 aw 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.
In our next article, we will cover the external influences on a product to control shelf life.
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