Water activity and pH are the two most important intrinsic factors that determine if a product will support the growth of a spoilage microorganism. Water activity and pH work synergistically, with their combined effects being more powerful at control than their individual effects. This synergistic effect is described in detail by hurdle technology for microbial control and is an intricate part of the FDA’s definition of potentially hazardous foods.
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Here’s how you can use the combined power of water activity and pH to increase microbial control using milder preservation techniques, which may result in improved product texture and quality.
How water activity prevents microbial growth
Like all organisms, microorganisms rely on water for growth. They take up water by moving it across the cell membrane. This water movement mechanism depends on a water activity gradient—on water moving from a high water activity environment outside the cell to a lower water activity environment within the cell.
When water activity outside the cell becomes low enough, it causes osmotic stress: the cell cannot take up water and becomes dormant. The microorganisms are not eliminated, they just become unable to reproduce. Different organisms cope with osmotic stress in different ways. That’s why there are different growth limits for each organism. Some types of molds and yeasts have adapted to withstand very low water activity levels.
Each organism has a specific water activity at which it will stop growing. As long as product developers keep the water activity below this limit, the microbe in question won’t replicate to high enough levels to cause infection or illness. See Table 1.
Table 1. Water activity growth limits for many common microorganisms
aw Bacteria Mold Yeast Typical Products 0.97 Clostridium botulinum E
fresh meat, fruits,
vegetables, canned fruit, canned vegetables
0.95 Escherichia coli
low-salt bacon, cooked sausages,
nasal spray, eye drops
0.94 Clostridium botulinum A, B
Stachybotrys atra 0.93 Bacillus cereus Rhizopus nigricans some cheeses, cured meat (ham)
evaporated milk, ral liquid
suspensions, topical lotions
0.92 Listeria monocytogenes 0.91 Bacillus subtilis 0.90 Staphylococcus aureus
Trichothecium roseum Saccharomyces
0.88 Candida 0.87 Staphylococcus aureus
0.85 Aspergillus clavatus sweetened condensed milk, aged cheeses (cheddar), fermented sausage (salami), dried meats (jerky), bacon, most fruit juice concentrates, chocolate syrup, fruit cake, fondants, cough syrup, oral analgesic suspensions 0.84 Byssochlamys nivea 0.83 Penicillium expansum
Deharymoces hansenii 0.82 Aspergillus fumigatus
0.81 Penicillium Penicillium cyclopium
0.80 Saccharomyces bailii 0.79 Penicillium martensii 0.78 Aspergillus flavus jam, marmalade, marzipan, glace fruits, molasses, dried figs, heavily salted fish 0.77 Aspergillus niger
0.75 Aspergillus restrictus
0.71 Eurotium chevalieri 0.70 Eurotium amstelodami 0.62 Saccharomyces rouxii dried fruits, corn syrup, licorice, marshmallows, chewing gums, dried pet foods 0.61 Monascus bisporus 0.60 No microbial proliferation 0.50 No microbial proliferation caramels, toffees, honey, noodles, topical ointments 0.40 No microbial proliferation whole egg powder, cocoa, liquid center cough drop 0.30 No microbial proliferation crackers, starch-based snack foods, cake mixes, vitamin tablets, suppositories 0.20 No microbial proliferation boiled sweets, milk powder, infant formula
Microbial growth limits make water activity an excellent tool for assuring the safety of food products, and water activity measurement can be used as a critical control point in HACCP plans.
Opportunities for synergy
The growth limits in Table 1 assume that all other conditions (pH, temperature, etc.) are optimal for the growth of the organism. If the growth-limiting effects of lowered pH are combined with water activity control, however, microbial growth can actually be controlled at a higher water activity than shown on the chart.
What is pH?
pH is a measure of the degree of acidity or alkalinity of a solution. Values between 0 and 7 indicate acidity; values between 7 and 14 indicate alkalinity. Distilled water, which is neutral, has a pH of 7. Foods tend to be either neutral or acidic.
Microbes have pH growth limits
Just as with water activity, microorganisms have pH limits below which they will not grow. Table 2 shows the minimum pH limits for the growth of different types of microorganisms. All microorganisms prefer a neutral pH for optimum growth, but they can grow in more acidic pH values. Most of them stop growing at a pH of 5.0. Some microorganisms can go as low as 4.6 and even down to 4.4. Historically, a pH of 4.6 was considered to be the lower growth limit, but portions of the food code were changed when it was discovered that some problematic microbes can grow in pH levels as low as 4.2.
|Clostridium perfringens||5.5 - 5.8||7.2||8.9|
|Racillus cereus||4.9||6 - 7||8.8|
|Campylobacter spp.||4.9||6.5 - 7.5||9|
|Vibrio parahaemolyticus||4.8||7.8 - 8.6||11|
|Clostridium botulinum toxin||4.6||8.5|
|Clostridium botulinum growth||4.6||8.5|
|Staphylococcus aureus growth||4||6 - 7||10|
|Staphylococcus aureus toxin||4.5||7 - 8||9.6|
|Enterohemorrhagic Escherichia coli||4.4||6 - 7||9|
|Salmonella spp||4.21||7 - 7.5||9.5|
Uses for pH adjustment
Because of microbial growth limits, lowering pH is an effective way to preserve foods and prevent the growth of microorganisms and can also be used as a critical control point in HAACP plans. Additionally, some manufacturers adjust pH to change flavor. This is often done through pickling or fermentation, which use microbial action, enzymatic reactions, or acids such as vinegar to induce the production of lactic acid. Many chemical reactions are pH dependent and can be prevented or controlled by adjusting pH.
Water activity and pH—more powerful together
The effects of water activity and pH can be combined through hurdle technology to control microorganisms more effectively. In the case of water activity and pH, the combined effect of both hurdles is greater than the effects each hurdle alone. This means you can have effective microbial control at levels that would typically be considered unsafe for either pH or for water activity alone. The currently valid 2013 food code contains pH and water activity interaction tables, shown in Tables 3 and 4, that can be used to determine whether or not a food requires time and temperature control for safety (TCS).
|aw Values||pH: 4.6 or less||pH: >4.6 - 5.6||pH: >5.6|
|0.92 or less||Non-TCS food*||Non-TCS food||Non-TCS food|
|>0.92 - 0.95||Non-TCS food||Non-TCS food||PA**|