After completing this lesson you should be able to :
• Within proteins, the long-chain molecules may be twisted to form spirals, folded
into sheets, or wound around to form other complex shapes. The chains are held
in these forms by intermolecular bonding between the side chains of the
constituent amino acids.
• When proteins are heated, during cooking, these intermolecular bonds are
broken allowing the proteins to change shape (denature).These changes alter the
texture of foods. and acidified dichromate solution can be used to differentiate
between an aldehyde and a ketone.
• Many flavour and aroma molecules are aldehydes.
• In terms of intermolecular forces, explain the influence of functional groups,
including hydroxyl, carbonyl and carboxyl, on solubility, boiling points and
volatility.
• Antioxidants in foods.
Taste
“The notion that the tongue is mapped into four areas—sweet, sour, salty and bitter—
is wrong. There are five basic tastes identified so far, and the entire tongue can sense
all of these tastes more or less equally.
As reported in the journal Nature this month, scientists have identified a protein that
detects sour taste on the tongue. This is a rather important protein, for it enables us
and other mammals to recognize spoiled or unripe food. The finding has been hailed
as a minor breakthrough in identifying taste mechanisms, involving years of research
with genetically engineered mice.
This may sound straightforward but, remarkably, more is known about
vision and hearing, far more complicated senses, than taste.” - Live Science
The five tastes
are:
1. Sweet
2. Salty
3. Bitter
4. Sour
5. Umami
(savoury)
Are you are super taster?
Experiment 1 – Phenylthiocarbamide
Lick the strip and tell us what you taste.
Phenylthiocarbamide
PTC – Chemistry World podcast
Experiment 2 – Taste test
• Blind-folded pupil (or with eyes closed and no peeking) is given a selection of
jams to taste. The taster should hold their nose and see if they can detect the
flavour of the jam.
• The taster again is then to determine which flavour of crisp they are eating
while another flavour crisp is held under their nose.
Molecular basis of flavour
The role of volatile molecules in flavour can be illustrated by an experiment where
foods are tasted while your nose is blocked. Strawberry jam works well. A blindfolded
taster is fed a small amount of strawberry jam with a teaspoon. While holding his/her
nose, the taster can detect the sweetness of the jam (which is detected with the
tongue) but will be unable to tell the flavour of the jam, which is caused by volatile
molecules detected by the nose. On releasing his/her nose, the taster will be able to
tell the flavour of the jam. Other foods which can be used in this experiment include
apple, parsnip, and even different flavours of crisp. In this variation the taster is given
a plain crisp to taste but, without telling them, a flavoured crisp is held under their
nose. The taster will report that the crisp they are tasting has the flavour of that held
under their nose.
Flavour in Food
Experiment 2 – Taste test
The Chemistry of flavour molecules
Formative assessment 1 – read and answers questions on pages 3-6
Molecules responsible for flavour in vegetables are normally trapped inside the
cell walls. During cooking the cell walls are damaged for two reasons:
• Chemical damage occurs as the cell walls, which are made of cellulose, break
down.
• Physical damage occurs as water inside the cells boils forming steam and the
cell walls break open.
A major issue in cooking is to retain molecules responsible for flavour in the food
– overcooking can result in loss of these molecules. One destination for lost
flavour molecules is in the cooking water. This will occur if the flavour molecules
are water-soluble. If this is the case, many of the flavour molecules will be lost
down the drain when the cooking water is poured away. Referenced article
The Chemistry of flavour molecules
b) Flavour in foodChanges upon heating
Learning intention
Learn how heating proteincontaining foodstuffs leads to a
change in food texture as
intermolecular
forces
are
broken.
Changes in protein structure
on heating
• Within proteins, the long chain molecules may be
twisted to form spirals, folded into sheets, or wound
around to form other complex shapes. The chains are
held in these forms by intermolecular bonding between
the side chains of the constituent amino acids. When
proteins
are
heated,
during
cooking,
these
intermolecular bonds are broken allowing the proteins to
change shape (denature).These changes alter the
texture of foods.
Changes in Food upon heating
Experiment 3 – Cooking meat and the Maillard-Hodge reaction of browning
Maillard-Hodge reaction
Other types of browning
Experiment 4 – Enzymic browning
This reaction differs from the Maillard reaction in that
proteins and amino acids are not involved.
Formative assessment 1 – read and answers
questions on pages 9-13
Enzymic browning