[ Pobierz całość w formacie PDF ]

environments, with different physicochemical properties, can be problematic for the
food analyst trying to accurately determine the moisture content of foods. Many
analytical procedures developed to measure moisture content are more sensitive to
water in certain types of molecular environment than to water in other types of
molecular environment. This means that the measured value of the moisture content of
a particular food may depend on the experimental technique used to carry out the
measurement. Sometimes food analysts are interested in determining the amounts of
water in specific molecular environments (e.g., physically bound water), rather than
the total water content. For example, the rate of microbial growth in a food depends on
the amount of bulk water present in a food, and not necessarily on the total amount of
water present. There are analytical techniques available that can provide some
information about the relative fractions of water in different molecular environments
(e.g., DSC, NMR, vapor pressure).
3.3. Sample preparation
Selection of a representative sample, and prevention of changes in the
properties of the sample prior to analysis, are two major potential sources of error in
any food analysis procedure. When determining the moisture content of a food it is
important to prevent any loss or gain of water. For this reason, exposure of a sample to
the atmosphere, and excessive temperature fluctuations, should be minimized. When
samples are stored in containers it is common practice to fill the container to the top to
prevent a large headspace, because this reduces changes in the sample due to
equilibration with its environment. The most important techniques developed to
measure the moisture content of foods are discussed below.
3.4. Evaporation methods
3.4.1. Principles
These methods rely on measuring the mass of water in a known mass of
sample. The moisture content is determined by measuring the mass of a food before
and after the water is removed by evaporation:
35
Here, M and M are the mass of the sample before and after drying,
INITIAL DRIED
respectively. The basic principle of this technique is that water has a lower boiling
point than the other major components within foods, e.g., lipids, proteins,
carbohydrates and minerals. Sometimes a related parameter, known as the total solids,
is reported as a measure of the moisture content. The total solids content is a measure
of the amount of material remaining after all the water has been evaporated:
Thus, %Total solids = (100 - %Moisture). To obtain an accurate measurement
of the moisture content or total solids of a food using evaporation methods it is
necessary to remove all of the water molecules that were originally present in the food,
without changing the mass of the food matrix. This is often extremely difficult to
achieve in practice because the high temperatures or long times required to remove all
of the water molecules would lead to changes in the mass of the food matrix, e.g., due
to volatilization or chemical changes of some components. For this reason, the drying
conditions used in evaporation methods are usually standardized in terms of
temperature and time so as to obtain results that are as accurate and reproducible as
possible given the practical constraints. Using a standard method of sample preparation
and analysis helps to minimize sample-to-sample variations within and between
laboratories.
3.4.2. Evaporation Devices
The thermal energy used to evaporate the water from a food sample can be
provided directly (e.g., transfer of heat from an oven to a food) or indirectly (e.g.,
conversion of electromagnetic radiation incident upon a food into heat due to
absorption of energy by the water molecules).
Convection and forced draft ovens. Weighed samples are placed in an oven
for a specified time and temperature (e.g. 3 hours at 100 oC) and their dried mass is
determined, or they are dried until they reach constant mass. The thermal energy used
to evaporate the water is applied directly to the sample via the shelf and air that
36
surround it. There are often considerable temperature variations within convection
ovens, and so precise measurements are carried out using forced draft ovens that
circulate the air so as to achieve a more uniform temperature distribution within the
oven. Samples that contain significant quantities of carbohydrates that might undergo
chemical changes or volatile materials other than water should not be dried in a
convection or forced draft oven. Many official methods of analysis are based on forced
draft ovens.
Vacuum oven. Weighed samples are placed under reduced pressure (typically
25-100 mm Hg) in a vacuum oven for a specified time and temperature and their dried
mass is determined. The thermal energy used to evaporate the water is applied directly
to the sample via the metallic shelf that it sits upon. There is an air inlet and outlet to
carry the moisture lost from the sample out of the vacuum oven, which prevents the [ Pobierz całość w formacie PDF ]

zanotowane.pl

doc.pisz.pl

pdf.pisz.pl

skierniewice.pev.pl