Shelf Life (Part 1)
1674 marks the year man has first documented microorganisms, when Antoni van Leeuwenhoek first examined microorganisms in a sample of water. This discovery was probably a first step in explaing the deterioration of foods, as there is little documentation on when the first awareness on food spoilage in relation to microorganisms was. Perhaps, the first man to suggest the role of microorganisms in food spoilage was Anthanasius Kircher, who examined decaying bodies in meat and milk, and saw what he referred to as "worms", invisible for the naked eye27.
Lazzaro Spallanzani observed in 1765 that heated beef broth in a hermetically sealed flask remained unspoiled and free from microorganisms. Later on, Henry Pasteur showed that the acidification/ souring of milk was caused by microorganisms, and around 1860 he used heat to destroy undesirable organisms in wine and beer28. This process, known as pasteurization was the first method that was conscious adapted to improve shelf life of food products.
Nowadays, shelf life is such an important requirement, it should be of interest to everyone involved in the food chain. There is a growing realization that a high standard of food safety and quality can only be achieved by adopting a comprehensive and integrated approach, covering the whole of the food chain, from farm to table. Food manufactures are balancing between retaining food safety/quality and extending shelf life to reduce food waste, therefore shelf life forms such an important part of the Food Safety- and Quality department. Such a holistic integral approach is however not very common in the baking industry.
Shelf life is interpreted in different ways by a consumer, retailer and manufacturer. Dominic Man1 defines shelf life as "the period of time during under defined conditions of storage, after manufacture or packing, for which a food product will remain safe and be fit for use". In other words, during this period, it should retain its desired sensory, chemical, physical, functional and microbiological characteristics. Changes in these characteristics are considered as quality loss, Table 1 shows the major causes of quality loss attributed to food products2.
Table 1 Major Quality-Loss Mechanisms2
Manufacturers want to prevent or delay mechanisms of food deterioration and spoilage, to reduce costs, ensure food safety and extend shelf life, while maintaining food quality. Generically speaking three techniques are considered:
Before discussing these techniques we first explore the different spoilage types and its known favoured conditions in bakery products.
Molds are the major spoilage problem for bakeries and are responsible for big costs due to losses of raw materials and end products3. These microorganism are a type of fungi that consists multiple cells in contrast to yeasts, which are single celled. Molds favor the presence of water, thus appearing frequently in bakery products with a high water activity (aw) level such as bread, cakes and creams4 (Table 2).
Spoilage types for typical bakery products4Rhizopus stolonifera (Figure 1), better known as Rhizopus nigricians is probably the most well-known mold causing spoilage type in bread whereupon it owes its nickname: ‘bread mould’3. It grows at a water activity (aw) of 0.91 or higher3,5 and can be recognized by its black sporangium, where spores are formed (Table 3).
|Water activity (aw)||Products||Spoilage types|
|0.99||Moist cakes (e.g. carrot cake)||Moulds and yeast, bacterial spoilage (e.g. "rope")|
|0.97 - 0.90||Plain cakes||Moulds and yeast|
|0.95 - 0.90||Moist cakes (e.g. carrot cake)||Moulds and yeast, bacterial spoilage (e.g. "rope")|
|0.89 - 0.80||Plain cakes||Moulds and yeast|
|0.79 - 0.70||Fondants, fruited cakes||Osmophilic yeasts, xerophilic moulds and osmophilic yeasts|
|0.69 - 0.60||Some dried fruits, heavily fruited cakes||Specialized xerophilic moulds and osmophilic yeasts, sugar-tolerant yeasts|
|< 0.6||Biscuits, chocolate, some dried fruits||No microbial spoilage|
Penicillium and Aspergillus are other well-known species that can, under specifice circumstances, produce mycotoxins (e.g. Ochratoxin that damages kidney and liver). They are also frequently used in the development of medicine or food additives (e.g. enzymes). Penicillium cycolium is the most common type of Penicillium species found in bakery products and can produce Penicillic acid above a water activity of 0.87, while the mould can already grow at a water activity (aw) of 0.80 or higher. Penicillic acid is an antibiotic mycotoxin with mutagenic properties which can result in hepatocarcinogenesis (liver cancer).
Some molds are more hazardous to mankind when consumed, called mycotoxins.
Figure 1: Rhizopus Stolonifer ('bread mold')
Aflatoxins are the most widely studied mycotoxins and thank their name to the first isolated mold that produced the toxin: Aspergillus Flavus. They became well-known in 1960, when more than 100.000 turkey poults died in England after eating peanut meal imported from Africa and South America. Aspergillus Flavus produces Aflatoxin B1 and B2 and will only be formed between a temperature range of 7.5 – 40°C. Substantiated evidence suggests that aflatoxins are carcinogenic, especially in liver where the toxin will first pass after absorption in the intestinal tract.
Other relevant mycotoxins are Deoxynivalenol (DON) and Zearalenone (ZEN) produced by Fusarium species, one of the main contaminants in breads3,6. The exposure risk to human is directly through cereal grains or indirectly through food of animal origin (e.g. milk and eggs). Tolerable daily intake (TDI) levels are determined on 1.0 µg/kg bodyweight a day due to adverse effects like nausea, vomiting, headache, dizziness, diarrhea and fever8. A study of EFSA (European Food Safety Autority) reported that DON was detected in almost half of the 26.613 samples of feed and food derived from 21 European countries between 2007 and 2012. Highest levels were observed in wheat, maize and oat grains and derived products7.
Besides the molds that grow at a relatively high water activity, there are molds that can grow in bakery products with a low water activity, such as fruited cakes and fondants, called Xerophilic molds. Eurotium Amstelodami and Eurotium Chevalieri are examples of extremophilic species that are usually the first fungi to colonize these products, allowing later on other species in the product.
Overall losses of bakery products due to mold spoilage vary between 1-5% depending on seasons, type of products and methods of processing. The mold spore counts are higher in the summer months than in the winter due to airborne contamination in the warmer weather and more humid storage conditions. Mold spores are generally killed by the baking process. However, this does not apply for the eventual formed mycotoxins during cultivation and storage of raw materials. Mycotoxins, are very stable compounds that can resist temperatures between 200-300°C.
Bacteria also have a potential to contaminate baked products. The spores of Bacillus subtilis for example are heat resistant and will not be killed by the baking process. Several Bacillus species can contribute in the process that makes bread "ropy" like Bacillus cereus14. Warm humid conditions are perfect for the spores to germinate and grow, forming the characteristic stringy brown mass with the odor of fruit. As the spoilage continues, the crumb is degraded and becomes soft and sticky due to the production of extracellular slimy polysaccharides typical of "rope"12.
Figure 2 :Ropy bread due to Bacillus species
Staphylococcus aureus is one type of bacteria known to contaminate pie fillings. This microorganism has also been implicated in food poisoning outbreaks from cream filled bakery products. Food poisoning is caused by the fact that Staphylococcus aureus produces enterotoxins under appropriate conditions. Other bakery ingredients, such as egg (products), chocolate, desiccated coconut and cocoa powder were found to be contaminated with Salmonella.
Salmonella is part of the enterobacteria family, just as Escherichia Coli, which means that these bacteria can live with- or without the present of oxygen. In The European Union, around 100.000 human cases related to Salmonella (e.g. Salmonella Enteritidis) are reported each year by the EFSA, 4% are dedicated to bakery products according to a report of the EFSA published in 2014. Salmonella and in particular E.coli, can cause big complications in small quantities, due to the Shiga toxin production, mainly related to E. coli. This toxin can lead to hemolytic-uremic syndrome (HUS), which results in thrombosis, destruction of red blood cells and kidney failure24.
Next to molds and bacteria, a less obvious microorganism involved in the spoilage of bakery products namely: yeasts. Problems caused by yeasts can be divided into two types. The first type are visible yeasts which grows on the surface of the bread in white or pinkish patches. Surface spoilage on bread is mainly caused by Pichia burtonii, whereby white spots are visible leading to the term "chalk" bread. Furthermore, fermentative spoilage can occur, associated with alcoholic and essence odors and hence osmophilic yeasts such as Zygosaccharomyces rouxii, that are used to live in high sugar concentrations such as highly sugared toppings and fillings3.
In order to manage microbial growth and (post-baking) contamination to retain quality, it is crucial to have a substantiated approach. Knowledge about the possible spoilage types, and their growth range, is the first step (see Table 3). The next step is to minimize the spoilage by adopting strategies to prevent contamination, destruct contaminants and control the growth of contaminants. These strategies will be discussed in the next part of this series about shelf life.
Table 3: Spoilage microorganism that can occur in bakery products
|Type of microorganism||Water activity awa||Temperatureb||pHc||Effect on product||Characteristics|
|Aspergillus flavus2,19||0.78||7-45°C||2.1 - 11.2||Produces Aflatoxin B1, B2 > aw 0.84 (13-37°C) pH 3.5 - 8||Green/blue|
1.4 - 9.8
|Produces Aflatoxin > aw 0.84
Ochratoxing A > aw 0
|Aspergillus ochraceus2,19||0.77||8-37°C||2.2 - 10||Produces Ochratoxin A > aw 0.83 (12-37°C)|
|Chrysonilia (neurospora) sitophila12,23||0.88||5-?||-||Red/orange/pink|
|Eurotium chevalieri9||0.70||-||-||Produces Xanthocilin||Yellow|
|Fusarium culmorum10,23||0.89||-||-||Produces DON > aw 0.96
|Fusarium graminearum2,10,11||0.96||4-32°C||2.4 - 9.5||
Produces DON > aw 0.98 (4-32°C) pH 2.4–9.5
|Fusarium moniliforme11||0.91||4-36°C||-||Produces Fumonisin B1||White/orange/brown|
|Mucor species (e.g. M. spinosus)11,21||0.92||-3 - 36°C||2 - 8.4||Grey/black|
|Penicilium expansum2||0.83||-||-||Produces Patulin > aw 0.99||Blue|
|Penicillium cyclopium2,21||0.82||-2 - 30°C||2 - 10||Produces Penicillic acid > aw 0.97
Produces Ochratoxin > aw 0.87
|Penicillium ochraceus2||0.76||-||-||Produces Penicillic acid > aw 0.80
Produces Ochratoxin A acid > aw 0.85
|Rhizopus microsporus18||0.90||6 - 46°C||-||Produces Rhizonin A||White/Grey|
|Rhizopus stolonifera9,17 (Rhizopus nigricans)||0.91||1 - 34°C||2.5 - 10||Black|
|Bacillus cereus2,9,13||0.93||4 - 55°C||4.5 - 9.5||Causing “ropy” bread > aw 0.95
Produces Emetic toxin (12 - 37°C)
|Bacillus subtilis9,15||0.91||11 - 52°C||-||Causing “ropy” bread > aw 0.95
Produces Subtilisin toxin
|Clostridium botulinum 2,9,13||0.94||6 - 40°C||4.5 - 7||Can only survive without O2 (anaerobic)
Can produce botulin toxin
|Clostridium perfingens26||0.93||10 - 54°C||5.1 - 9.7||Alpha, beta, epsilon and iota toxin production|
|Escherichia coli2,9,13||0.95||7 - 46°C||4.4 - 9||Shiga toxin production in guts of people|
|Listeria monocytogenes2,13,27||0.92||0 - 45°C||4.3 - 9.6||Lysteriolysin (LLO) toxin production|
|Salmonella spp.2,9,13||0.95||5 - 46°C||4 - 9.5||Endo** - and enterotoxin production (not in food)|
|Shigella spp.2||0.96||6 - 47°C||4.8 - 9.3||Shiga toxin production|
|Staphylococcus aureus (aerobic)9,13||0.86||7 - 48°C||4.2 - 9.3||Enterotoxin A production > aw 0.88 (pH 4.5)
Enterotoxin B production > aw 0.97 (pH 4.5)
|Staphylococcus aureus (anaerobic)9,13||0.90||7 - 48°C||4.2 - 9.3||Enterotoxin A production > aw 0.88
Enterotoxin B production > aw 0.97
|Pichia burtonii2||0.91||-||1.5 - 8.5||Produces chalk molds which causes flavor defects due to the formation of ethyl acetate|
|Saccharomyces billii2||0.80||-||1.5 - 9.5||Alcoholic/essence odor - and flavor abnormality due to fermentation|
|Saccharomyces bisporus2||0.69||-||1.5 - 9.5||Alcoholic/essence odor - and flavor abnormality due to fermentation|
|Zygosaccharomyces rouxii2||0.60||-||1.5 - 10.5||Alcoholic odor - and flavor abnormality due to fermentation|
a Water activity (aw) for minimal growth
b Temperature range for minimal growth
c Degree of acidity (pH) range for minimal growth
* A toxin tend to be produced by gram-positive bacteria (thick peptidoglycan layer) within the membrane that is specific for intestinal cells and causes vomiting and diarrhea associated with food poisoning
** A toxin associated with the outer membranes of certain gram-negative bacteria (thin peptidoglycan layer), released upon destruction of the bacterial cell.
More about shelf life:
Leading image: By Robert Kneschke/Shutterstock.com