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Hippie3 (Admin)
Posted on Thursday, September 11, 2003 - 02:24 pm:Edit Post Quote Text Delete Post Print Post Move Post (Moderator/Admin Only)

Water Activity and Microbial Growth

Water Activity (aw) - a measure of the water that is available for biological and chemical reactions

perhaps the most important aspects of food affecting preservation and safety

most bacteria will not grow below aw of 0.91

microbial tolerance to reduced aw - generally, bacteria 0.98
cooked meat, bread 0.91 - .095
cured meats, cheeses 0.91 - .095
fermented sausages (some) 0.83 - 0.87
jams 0.75 - 0.80
honey 0.75
dry cereals (some) 0.65 - 0.75
pastry fillings 0.65 - 0.71
candies 0.60 - 0.65
sugars, syrups 0.60 - 0.75
cake and pastries 0.60 - 0.90
dried fruits 0.60 - 0.75
powdered milk, dried pasta, spices 0.20 - 0.60



Controlling aw in foods

equilibration with atmosphere of known relative humidity

water removal (e.g., dehydration)

addition of solutes (humectants)

sugars

NaCl

polyhydric alcohols (glycerol, sorbitol)

propylene glycol

loss or gain of moisture in packaged foods



Determination of aw in foods

Calculation (estimate)

based upon Raoult's Law for ideal solutions

does not take into consideration solute interactions with water and other components

accuracy of estimation is greater for dilute solutions
aw = p/po = n1/n1 + n2

where:

n1 = moles of solvent (water)

n2 = moles of solute

p = vapor pressure of solution

po = vapor pressure of pure water at the same temperature


Norrish Equation - accounts for water binding properties of the solute and is more useful for large concentrations of solute
log (aw/xw) = -k(1-xw)2 or low aw = log xw - k(1-xw)2

where:

xw = mole fraction of water (i.e., n1/n1 + n2)

k = constant



Solute
Molecular weight k-value

sucrose
342.30
2.7
glucose
180.16
0.7
fructose
180.16
0.7
invert sugar
180.16
0.7
sorbitol
182.17
0.85
glycerol
92.09
0.38
propylene glycol
76.09
-0.12
sodium chloride
58.45
15.8 (for x2 0.02)
(x2 = mole fraction of solute)



for foods containing more than one solute:
aw = (aw1)(aw2)...(awn)

Measurement

electric hygrometers (capacitance sensors - based upon ERH)

consist of potentiometer, sample holder, and sensor with immobilized electrolyte (e.g., lithium chloride)

changes in ERH are reflected in changes in conductance of current through sensor

examples: Beckman, Rotronic

typically slow

requires routine calibration with standards

dew point instruments (chilled mirror theory)

use a cooled mirror as condensing surface

mirror is cooled --> condensation occurs --> temperature = dew point; ERH is derived from psychrometric chart (automatically)

very fast and accurate

calibration is not needed

example: Decagon CX-series


Microbial Growth at low aw

most bacteria require aw » 0.90 (see figure)

grow at low aw

halophilic bacteria

osmophilic (osmotolerant) yeasts

xerophilic (xerotolerant) molds

Organism Minimum aw
Bacteria
Staphylococcus aureus
0.86

halophilic bacteria
(Halobacterium spp.)

0.75

Molds

Aspergillus flavus
0.78

Chrysosporium fastidium
0.69

Xeromyces bisporus
0.61

Yeasts

Debaryomyces hansenii
0.83

Torulopsis spp.
0.70

Zygosaccharomyces bailii
0.80

Zygosaccharomyces rouxii
0.62


cause spoilage of high-salt and high-sugar foods (pickles, jams & jellies, confectionary, soft-centered candies, etc.)

slow growth at reduced aw

spoilage --> ethanol, organic acids, CO2

positive aspects - food fermentations (e.g., soy sauce)



Mechanism of Tolerance/Growth

accumulation of "compatible solutes"

increase internal osmotic pressure

"replace" cellular water

compatible with normal metabolic processes

must be retained within the cell

Compatible solutes

bacteria

K+ ions, glutamate, glutamine, proline, alanine, glycine betaine, sucrose, trehelase, glucosylglycerol

fungi

polyhydric alcohols

glycerol

arabitol

Production vs. retention

S. cerevisiae (non-osmophilic) - increases production without retention (metabolic level)

Z. rouxii - increased accumulation without increased production (permeation/transport level)

role of plasma membrane?

membrane lipids

Z. rouxii

increased lipid content at reduced aw

rich in C18:2 fatty acid (none in S. cerevisiae)

reduced aw leads to increase in saturation

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