Transcript Water activity in food

Water activity in food
• Water in food which is not bound to food
molecules can support the growth of bacteria,
yeasts and moulds (fungi). The term water
activity (aw) refers to this unbound water.
• The water activity of a food is not the same thing
as its moisture content. Although moist foods are
likely to have greater water activity than are dry
foods, this is not always so; in fact a variety of
foods may have exactly the same moisture
content and yet have quite different water
activities.
Water activity in food
• The water activity scale
extends from 0 (bone dry)
to 1.0 (pure water) but
most foods have a water
activity level in the range
of 0.2 for very dry foods
to 0.99 for moist fresh
foods.
• Water activity is in
practice usually
measured as equilibrium
relative humidity (ERH)
• The typical water
activity
• Fresh meat and fish 0.99
• Bread - 0.95
• Aged cheddar - 0.85
• Jams and jellies - 0.8
• Plum pudding - 0.8
• Dried fruit - 0.6
• Biscuits - 0.3
• Milk powder - 0.2
• Instant coffee - 0.2
The diagram above illustrates the water activity (aw) levels which can support the
growth of particular groups of bacteria, yeasts and moulds. For example we can
see that food with a water activity below 0.6 will not support the growth of
osmophilic yeasts, which can pose a problem in high sugar products. We also
know that Clostridium botulinum, the most dangerous food poisoning bacterium, is
unable to grow at an aw of 0.93 and below.
The risk of food poisoning must be considered in low acid foods (pH > 4.5) with a
water activity greater than 0.86 aw.
Staphylococcus aureus, a common food poisoning organism, can grow down to
this relatively low water activity level. Foods which may support the growth of this
bacterium include cheese and fermented sausages stored above correct
refrigeration temperatures.
Semi-moist foods
• For foods with a relatively high water activity correct refrigeration is
always necessary. These include most fresh foods and many
processed foods such as soft cheeses and cured meats.
• Many foods can be successfully stored at room temperature by
proper control of their water activity (aw).
• These foods can be described as semi-moist and include fruit cakes,
puddings and sweet sauces such as chocolate and caramel.
• When these foods spoil, it is usually the result of surface mould
growth. Most moulds cease to grow at a water activity level below
about 0.8, but since some moulds will grow slowly at this aw, it is
usually recommended that products of this type do not have an aw
greater than 0.75.
• While this will not ensure complete freedom from microbial spoilage,
those few yeasts and moulds which do grow at lower water activities
need only to be considered when special shelf life conditions must
be met.
• For example, a commercial shelf life over twelve months might be
required for confectionery; in these circumstances an aw below 0.6
would be required.
Equilibrium Relative Humidity
(ERH)
• Water activity is in practice usually measured as
equilibrium relative humidity (ERH).
• The water activity (aw) represents the ratio of the water
vapour pressure of the food (p) to the water vapour
pressure of pure water (po) under the same conditions
and it is expressed as a fraction.
• If we multiply this ratio by 100, we obtain the equilibrium
relative humidity (ERH) that the foodstuff would produce
if enclosed with air in a sealed container at constant
temperature.
• Thus a food with a water activity (aw) of 0.7 would
produce an ERH of 70%.
Fundamentals of Water Activity:
Methods of Measurement
• 2 methods of water activity instruments available
• There is no device that can be put into a product
that directly measures the water activity.
• aw is measured with an indirect method.
• Water activity is measured by equilibrating the
liquid phase water in the sample with the vapor
phase water in the headspace of a closed
chamber and measuring the relative humidity of
the headspace.
Fundamentals of Water Activity:
Methods of Measurement
• Two different types of water activity instruments
are commercially available.
• One uses chilled mirror dewpoint technology
while the other utilizes relative humidity sensors
that change electrical resistance or capacitance.
• Each has advantages and disadvantages.
• The methods vary in accuracy, repeatability,
speed of measurement, stability in calibration,
linearity, and convenience of use.
Chilled Mirror Dewpoint
• A sample is placed in a sample cup which is sealed
against a sensor block.
• Inside the sensor block is a dewpoint sensor, an infrared
thermometer, and a fan.
• The dewpoint sensor measures the dewpoint
temperature of the air, and the infrared thermometer
measures the sample temperature.
• From these measurements the relative humidity of the
headspace is computed as the ratio of dewpoint
temperature saturation vapor pressure to saturation
vapor pressure at the sample temperature.
• When the water activity of the sample and the relative
humidity of the air are in equilibrium, the measurement of
the headspace humidity gives the water activity of the
sample.
• The fan is to speed equilibrium and to control the
boundary layer conductance of the dewpoint sensor.
Chilled Mirror Dewpoint
• The major advantages of the chilled mirror dewpoint
method are speed and accuracy.
• Chilled mirror instruments make accurate (± 0.003 aw)
measurements in less than 5 minutes.
• Chilled mirror dewpoint is a primary approach to
measurement of relative humidity based on fundamental
thermodynamic principles.
• Since the measurement is based on temperature
determination, calibration is unnecessary, but running a
standard salt solution checks proper functioning of the
instrument.
• If there is a problem, the mirror is easily accessible and
can be cleaned in a few minutes.
• For some applications, fast readings allow
manufacturers to perform at-line monitoring of a
product's water activity.
Electric Hygrometers
• Other water activity instruments use resistance or
capacitance sensors to measure relative humidity.
• These sensors are made from a hygroscopic polymer
and associated circuitry that gives a signal relative to the
equilibrium relative humidity (ERH).
• Commercially available instruments measure over the
entire water activity range with an accuracy of ± 0.015
aw.
• Since these instruments relate an electrical signal to
relative humidity, the sensor must be calibrated with
known salt standards.
• In addition, the ERH is equal to the sample water activity
only as long as the sample and sensor temperatures are
the same.
• Accurate measurements require good temperature
control or measurement.
• Advantages of capacitance sensors include simple
design and inexpensive implementation.
Colligative Properties
Depend on the number of solute molecules or ions added to the solvent.
• Boiling Point Elevation
• Freezing Point Depression
• Osmotic Pressure
The above are colligative (collective) properties and are used to
determine the molecular weights and to measure water activity
Boiling-Point Elevation
• The boiling point of a solution is the temperature
at which its vapor pressure equals the
atmospheric pressure (1 atm)
• At 1 atm, water boils at 100oC, at 0.5 atm –
reduced pressure – water boils at only 82oC
• Because the presence of a non-volatile solute
lowers the vapor pressure of a solution, it must
also affect the boiling point of the solution
Boiling-Point Elevation
• The boiling-point elevation (ΔTb) is defined as
the boiling point of the solution (Tb) minus the
boiling point of the pure solvent (Tob):
ΔTb = Tb – Tob
• Since Tb > Tob, ΔTb is a positive quantity.
• The value of ΔTb is proportional to the vaporpressure lowering, and so it is also proportional
to the concentration (molality) of the solution.
ΔTb ∞ m
ΔTb = Kbm
• Where m is the molality of the solution of the
solution and Kb is the molal boiling-point
elevation constant. The units of Kb are oC/m
Boiling-Point Elevation
DTb = Tb – T b0
T b0 is the boiling point of
the pure solvent
T b is the boiling point of
the solution
Tb > T b0
DTb > 0
DTb = Kb m
m is the molality of the solution
Kb is the molal boiling-point
elevation constant (0C/m)
12.6
Freezing-Point Depression
• Defined as the freezing point of the pure solvent (Tof)
minus the freezing point of the solution (Tf):
ΔTf = Tof – Tf
Because Tof > Tf, ΔTf is a positive quantity. Again, ΔTf is
proportional to the concentration of the solution:
ΔTf ∞ m
ΔTf = Kfm
Where m is the concentration of the solute in molality
units, and Kf is the molal freezing-point depression
constant. Like Kb, Kf has the units oC/m
Freezing-Point Depression
DTf = T 0f – Tf
T
0
Tf
f
is the freezing point of
the pure solvent
is the freezing point of
the solution
T 0f > Tf
DTf > 0
DTf = Kf m
m is the molality of the solution
Kf is the molal freezing-point
depression constant (0C/m)
12.6
• Ice on frozen roads or sidewalks melts when sprinkled
with salts such as NaCl or CaCl2. This method of
thawing succeeds because it depresses the freezing
point of water.
• Freezing involves a transition from the disordered state
to the ordered state. For this to happen, energy must be
removed from the system. Because a solution has
greater disorder than the solvent, more energy needs to
be removed from it to create order than in the case of a
pure solvent. Therefore, the solution has a lower freezing
point than its solvent.
• Note that when a solution freezes, the solid that
separates is the solvent component.
Osmotic Pressure
• Many chemical and biological processes depend
on osmosis, the selective passage of solvent
molecules through a porous membrane from a
dilute solution to a more concentrated one.
• Semipermeable membrane allows the passage
of solvent molecules but blocks the passage of
solute molecules.
• The osmotic pressure (π) of a solution is the
pressure required to stop osmosis
Osmotic Pressure
• The osmotic pressure of a solution is given by
π= MRT
Where M is the molarity of solution, R is the gas constant
(0.0821 L.atm/K.mol) and T is the absolute temperature.
• The osmotic pressure, π, is expressed in atm.
• Since osmotic pressure measurements are carried out at
constant temperature, we express the concentration in
terms of the more convenient units of molarity than
molality.
Definition
• Mole Fraction (x)
• The mole fraction of a component of a
solution, say, component A, is written XA
and is defined as
Moles of A
• Mole fraction of component A = XA = ----------------------------------•
Sum of moles of all components
• Mole fraction has no units
Definition
Molarity (M)
moles of solute
=
------------------------- mol/L
liters of solution
Molality (m) –
number of moles of solute dissolved in 1 kg (1000 g) of solvent
moles of solute
=
------------------------mass of solvent (kg)
For example, to prepare a 1 molal or 1 m sodium sulphate (Na2SO4)
aqueous solution we need to dissolve 1 mole (142.0 g) of the
substance in 1000 g (I kg) of water.
Example
• Calculate the molality of a sulphuric acid solution
containing 24.4 g of sulphuric acid in 198 g of
water. The molar mass of sulphuric acid is 98.08
g.
• From the known molar mass of sulphuric acid,
we can calculate the molality in two steps. First
we need to find the number of grams of
sulphuric acid dissolved in 1000 g (I kg) of water.
Next we must convert the number of grams into
the number of moles. Combining these two
steps we write:
Moles of solute
Molality =
--------------------------Mass of solvent (kg)
24.4 g H2SO4 1000 g H2O 1 Mol H2SO4
= ----------------X------------X-------------198 g H2O
I kg H2O
= 1.26 mol H2SO4/kg H2O
= 1.26 m
98.08 g H2SO4