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Lecture notes Food Chemistry, FCH20806
- Instelling
- Wageningen University (WUR)
Lecture notes of the knowledge clips of Food Chemistry (FCH20806) at WUR. For the studies Biotechnology and Food Technology.
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Food Chemistry lecture notesOverview of chemical reactions in food:
Carbohydrates
The sweet taste in carbohydrates can come from monosaccharides; sugars such as glucose, fructose,
or aspartame. For sorbitol, xylitol, and aspartame no insulin is needed to be taken up by our body.
Sweeteners (aspartame) have a higher amount of sweetness, so smaller doses are needed.
Dietary fiber (can’t be completely broken down by the human digestion system): many
polysaccharides (cellulose, hemicellulose, pectin etc.) and non-digestible oligosaccharides (NDO,
galacto-oligosaccharides, fructo-oligosaccharides etc.).
Carbohydrates provide structure to food. There are thickeners, gelling agents, stabilizers, and
humectants. Pectin (in for example strawberries) can make a gel structure in jam. Starch in ‘vla’ will
make it thicker. Starch is also very important for the chewy structure of ‘drop’.
Colour and flavour can occur during reactions (colour is given by reaction products formed), such as
the caramelization and the Maillard reaction. Disadvantages of these reactions are the forming of
toxic compounds, such as acrylamide (formed when asparagine reacts with sugar). Acrylamide gives a
strong brown colouring.
,Monosaccharides can be either aldehydes or ketones, either a
pyranose or a furanose, either an alfa or a beta anomer and either
a hexose or a pentose.
Bonds can be formed (by
binding an oxygen atom) to
create a polysaccharide.
Cellulose (a fiber) is made of beta-glucose (OH-group pointing up)
and amylose (a starch) is made of alfa-glucose (OH-group pointing down). This difference makes a
huge difference in the behaviour of polymers.
Reducing sugars can open their ring and can therefore transform between the alfa or the beta
position for the OH-group. When the ring is opened, the OH-group becomes a double-bonded oxygen
atom and is thus a reactive group.
During the hydrolysis, the number of reducing ends (where a reaction can occur) will increase.
The pH is a big influencer of the types of reactions taking place in food.
Caramelization reactions start with the hydrolysis of saccharose (made of glucose and fructose) to
get the two monomers. Then an enolization reaction, dehydration reaction and
fragmentation/oxidation reaction occur. During this last reaction, the colour and smell of caramel are
formed. Finally, a condensation/polymerization reaction occurs, giving the final deep caramel
colour/pigment and the bitterness to caramel.
Small molecules: flavour, big molecules: colour. For a molecule to have a flavour, it needs to attach to
the receptors in our nose and needs to be volatile. Small molecules are the only molecules which
have flavours because big molecules can’t be volatile. Only small fragments of molecules can become
volatile. The bigger the molecule, the darker the colour.
The carbohydrate content in food can be determined by measuring the reducing sugar content or by
measuring the total sugar content. For the polysaccharide content, only the total sugar content is
measured and for the degree of hydrolysis, both the methods are used.
Analysing reducing sugar content: each reducing end reacts with copper to form Cu 2O which is
measured spectrophotometrically.
Analysing total sugar content: first perform a complete hydrolysis with concentrated sulphuric acid,
then perform a colour reaction and finally measure the colour with a spectrophotometer.
Analysing the polysaccharide content: make of suspension of the food product in water and add 70%
alcohol (makes the polysaccharides insoluble). Then separate the polysaccharides from the soluble
part and measure the total sugar content before and after adding alcohol. The total polysaccharide
content can thus be determined via subtracting (before-after).
Analysing the degree of hydrolysis: measure the degree of hydrolysis by performing an analysis of
100∗reducing sugar
the reducing sugar content and the total sugar content. DE= .
total sugar
Knowledge clips carbohydrates
Clip 1: Carbohydrates (saccharides) can be in for example jam, fruit
and vegetables, milk, and potatoes. The molecules are a ring
structure of carbon atoms and one oxygen atom. Many OH-groups
are attached to this ring. A disaccharide are two rings connected to
each other. A polysaccharide are multiple rings linked together (see
image). Carbohydrates can consist of different residues (glucose,
galactose, fructose and so on).
, Carbohydrates can have different roles in food: sweetener, humectant (binding of water, lowers
water activity), energy supplier, texture (thickener), colour and even more roles exist.
Clip 2: the main monosaccharides in food are glucose and fructose.
Many monosaccharides are not available as individual monomers but
connected into oligosaccharides or other forms. Fructose is a ketone
since it has a double bonded oxygen on the second carbon atom.
Glucose has the double bonded oxygen on the first carbon atom,
called an aldose. A ketose can be found while looking at the closed structure. If an extra carbon atom
is bonded to the anomeric carbon atom (the one next to the oxygen atom), then the structure used
to be a ketone (so ketose). A pyranose is a 6 membered ring and a furanose is a 5 membered ring. A
hexose has 6 carbon atoms, and a pentose has 5 carbon
atoms. An alpha glucose is when an OH-group is pointing
downwards, a beta glucose is when the OH-group is pointing
upwards. To determine this, look at the anomeric carbon
atom from the right side of a molecule.
Clip 3: carbohydrates can have reducing properties. The double bonded
oxygen (in the open ring form, see image above) is reactive and can
undergo different types of reactions. This is called a reducing property,
because the sugar can react with cupper ions (in the open ring) to reduce
the cupper ions. This reducing property is used in the quantification of
carbohydrates in food. The free OH-group in a polysaccharide can open
the ring (red circled in the image) and is called the reducing end. Not all
carbohydrates are reducing sugars, for example cyclic carbohydrates don’t have reducing ends.
Mutarotation (from alpha anomer to beta anomer) is the equilibrium between different anomeric
forms. Only reducing sugars can have this mutarotation property (see image).
Clip 4: a glycosidic bond can connect to monosaccharides in a carbohydrate. When two
monosaccharides connect, a water molecule is also formed. When looking at the glycosidic bonds, a
difference between an alpha or a beta bond can also be
made. An alpha bond is when the OH-group in the left
monomer is alpha configurated and vice versa. The name of a
carbohydrate is based on the position of the glycosidic bond
(1,4). The first carbon atom is the carbon atom on the right of
the oxygen atom in the ring structure of monomers. The second monosaccharide in the name (of
carbohydrates) only has alpha or beta before the name of the sugar, when it is not a reducing sugar.
Clip 5: in a hydrolysis reaction of carbohydrates, the glycosidic bond is cleaved by adding water. The
cleavage can be done by an enzyme called invertase or it can be done chemically, with a high
temperature and acidic conditions. After hydrolysis, more reducing groups are formed. The dextrose
100∗number of reducing ends
equivalent can be measured: DE= . In the lab, the dextrose
total number of glucoseunits
equivalent is determined with the concentration of reducing sugars divided with the concentration of
total sugar.
Clip 6: oligosaccharides are carbohydrates with 2-20 monomers linked together. Oligosaccharides can
be classified as alpha-galacto- (galactose), lactose- (galactose), malto- (glucose), fructo- (fructose) or
cyclodextrins (glucose). Oligosaccharides can have all kind of properties in food. It can be NDO’s
(non-digestible oligosaccharides), which are not degraded in the digestive tract. Good bacteria can
grow on these NDO’s and can function as prebiotic, and they can cause flatulence. Cyclodextrins can
be added to food to stabilise hydrophobic compounds in an aqueous phase. Disaccharides and malto-
oligosaccharides can be added to food as a sweetener.
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