The document discusses methods for measuring moisture content, temperature, and relative humidity and their effects on food storage. It describes laboratory and field methods for measuring moisture content including oven drying and resistance/capacitance meters. Temperature measurement methods discussed include liquid expansion thermometers, bimetallic thermometers, and electronic devices. Relative humidity is measured using wet/dry bulb thermometers or whirling hygrometers. High or low temperatures and moisture levels can cause physical or chemical changes in stored foods affecting texture, flavor, and nutrition.

1. Y.Bavaneethan.
Lecturer
Department of Food Technology
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2. • Changes in the moisture content and temperature of exposed
foodstuffs
• fluctuations of temperature and relative humidity of the
ambient air
• especially stored in small area.
• Records of temperature and humidity within stores are;
• useful in assessing their suitability.
• It may also be necessary to determine the moisture content of
foodstuffs,
• Either by standard laboratory methods or, field instruments
(moisture meters).
• No effect - moisture content is already specified and
controlled by impermeable packaging.
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3. Measurement of moisture content
01) Laboratory methods
• Oven moisture determinations should follow an international or
national standard
• Such methods generally involve;
• Grinding then
• Drying a sample of 1–10 g in a ventilated oven (preferably
with fan ventilation), or in a vacuum oven
• at temperatures ranging from 45–130 °C.
• The time required varies according to the method.
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4. 02. Field methods
• Resistance meters and capacitance meters:
• The electrical characteristics of many food commodities vary
with;
• the moisture content
• degree of compaction and temperature.
• There are many moisture meters that measure either the
electrical resistance or capacitance.
• Most are reasonably accurate and reliable
• If used carefully and calibrated frequently against a
standard laboratory method
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5. • Resistance moisture meter
• Function by measuring the electrical resistance between two
electrodes.
• When moisture is present within the Food - electricity flows
more easily.
• Conversely, Food that is becoming dryer will resist electrical
flow.
• Capacitor moisture meter
• The food tested is placed between the capacitor plates
• as the dielectric
• calibration is absolutely essential
• Dielectric constants of many materials,
• have been experimentally determined.
• agricultural materials, such as grain, increase with increasing
moisture content.
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8. Measuring relative humidity
• Relative Humidity Relative humidity is the ratio of two
pressures:
• %RH = 100 x p/ps
• where p is the actual partial pressure of the water vapor
present in the ambient and
• ps the saturation pressure of water at the temperature of the
ambient.
• This commonly consists of two identical mercury-in-glass
thermometers,
• bulb of one covered with muslin wetted by distilled water.
• Movement of air, thermometer bulbs causes water to evaporate
from the wet bulb,
• resulting in a decrease in temperature.
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9. • Standardized conditions,
• the difference between the wet and dry bulb readings provides
a measure of the relative humidity.
• Accurate and more convenient type is the whirling hygrometer
• Wet and dry bulb temperature may also be used, with
hygrometric tables,
• To determine the dewpoint.
• This is the temperature - air would become saturated and
start to deposit water as condensation on surfaces or as a
mist.
• The dewpoint is normally lower than the wet bulb temperature,
• but it as the relative humidity comes close to 100%.
• Foods are then at risk of damage from the condensed water.
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10. • Most commonly used units for humidity measurement are
Relative Humidity (RH).
Dew point:
• is the temperature (above 0oC) at which the water vapor in
a gas condenses to liquid water.
Frost point:
• is the temperature (below 0oC) at which the vapor
condenses to ice.
 Absolute humidity is the total mass of water vapor present in
a given volume of air.
 No temperature consideration
 Absolute humidity is the mass of the water vapor divided
by the volume of the air and water vapor
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11. Hygrometer
• A hygrometer is an instrument used for measuring the
moisture content in the atmosphere.
• Humidity measurement instruments effect on:
 temperature,
 pressure,
 mass or a mechanical
 electrical change in a substance.
• By calibration and calculation, these measured quantities can
lead to a measurement of humidity.
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12. Basic Principle :-
 Due to humidity, several materials experience a change in
physical, chemical and electrical properties.
 This property is used to designed and calibrated to relative
humidity directly.
 Hair hydrometer is a type of absorption hydrometer
 Certain hygroscopic materials such as human hair, animal
membranes, wood, paper, etc., undergo changes in linear
dimensions when they absorb moisture from their surrounding
air.
 This change in linear dimension is used as the measurement of
humidity present in air.
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13. Description :-
 Human hair is used as the humidity sensor.
 The hair is arranged in parallel beam and they are separated
from one another to expose them to the surrounding
air/atmosphere.
 Number of hairs are placed in parallel to increase mechanical
strength.
 This hair arrangement is placed under small tension by the use
of a tension spring to ensure proper functioning.
 The hair arrangement is connected to an arm and link
• link is attached to a pointer and humidity calibrated scale.2/13/2020 Y.Bavaneethan 13

14. Hair Hygrometer
Human hair has a property that its length increases when it
is wet and its length decreases when it goes dry .
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15. Operation :-
 When the humidity of air is to be measured,
• The hair arrangement absorbs the humidity from the
surrounding air and expands or contracts in the linear
direction.
• This expansion or contraction of the hair arrangement
moves the arm & link
• And thus the pointer to a suitable position on the
calibrated scale and thus indicating the humidity present
in the air/atmosphere.
 Precaution : These Hair hydrometers are called membrane
hydrometers when the sensing element is a membrane.
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16. Methods for measuring temperature
• A variety of devices are available for use in stores, ranging from;
• simple liquid expansion
• bimetallic thermometers
• electronic thermocouples and thermistors
• Continuous recordings of temperature at a number of points within
a store or grain silo.
01) Liquid expansion thermometers
• For the measurement of diurnal air temperature fluctuations within
stores,
• maximum and minimum thermometer is useful.
• The thermometer contains small steel bars which rest on the
columns of mercury or alcohol
• rise to the maximum and minimum temperatures as the length of
the column changes.
• If the temperature falls, the bars remain at the highest point.2/13/2020 Y.Bavaneethan 16

17. 2.Bimetallic Thermometer
In anindustry,there is alwaysaneedto measureand
monitor temperature of aparticular spot, field
The industrial namesgiven to suchtemperature
sensorsare TemperatureIndicators(TI) or
TemperatureGauges (TG).
All these temperaturegauges belong to the class of
instrumentsthat are knownasbimetallicsensors.
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18. Bimetallic Thermometer
Two basic principles of operationis to befollowedin
the case of abimetallicsensor.
1) A metaltendsto undergo avolumetric dimensional
change (expansion/contraction),according to the
change in temperature.
2) Different metals have different co-efficient of
temperatures. The rate of volumetric change
dependsonthis co-efficientof temperature.
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19. Structure
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20. Moisture, Relative humidity and Temperature
Effect on Food Storage
• All foodstuffs –
• not subjected to infestation by insects and microorganisms
• But changes in texture, colour, flavour or nutritive value
• Resulting from the effects of temperature and moisture
during prolonged storage.
• Labelling of non-perishable packaged foods with a shelf-life
(best before or expiry date) is
• usually loss of quality - by physical or chemical changes.
• Certain temperatures and humidity of storage are effect the
• Physical and /or chemical changes
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21. 01. Physical Changes
• At high temperatures,
• fats products such as canned meat or dried oily fish may
melt.
• At low temperatures,
• sugars present foods - condensed milk may crystallize from
solution
• giving a gritty texture to the food.
• The absorption of moisture hygroscopic foods can also cause
undesirable physical changes.
• Sugar absorb water from atmosphere when RH exceeds 86%
• Caking occur in milk powder;
• absorbed moisture from the atmosphere.
• allows crystallization of the lactose in the milk powder
• these crystals - very hard mass
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22. 02. Chemical changes
• Chemical changes in stored food;
• rates governed by the temperature
• moisture content
• The effect of temperature is such that a 10 °C rise,
• causes an approximately twofold increase in reaction rates.
• Cool storage retards
• changes such as fat oxidation, vitamin loss and container
corrosion.
• Dried milk - small reduction in storage temperature,
• discoloration (browning) and the associated off-flavours.
• Keeping such foods cool and dry
• reduce the rate of development of brown
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