Table Of ContentINTRODUCTION TO THE
CHEMISTRY OF FOOD
MICHAEL ZEECE
Professor Emeritus
Department of Food Science
University of Nebraska
Lincoln, Nebraska, United States
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Acknowledgments
I wish to thank my wife, Pauline Davey Zeece, for her comments and
suggestions regarding the contents of this book. Her expertise in develop-
mental psychology contributed to summarizing research regarding food
additivesandhyperactivityinchildren.Iwishtothankourdaughter,Megan
Mclaughlin(9speedcreative.com),fortheartworkonthecoverofthisbook.
I also wish to thank our sons, Michael Zeece and Eric Zeece, for their
ongoing encouragement and support.
j
xi
CHAPTERONE
Chemical properties of water
and pH
Learning objectives
This chapter will help you describe or explain:
·
Water’sstructure
·
Thehydrogenbondanditsimportancetowater
·
Whatafoodacidis,includingexamples
·
pHandtitratableacidity
·
Theimportanceofwatertofoodcolor,taste,andtexture
·
Whyoilisnotsolubleinwater
·
Wateractivityanditsimportancetofoodqualityandsafety
IntroductiontotheChemistryofFood
ISBN:978-0-12-809434-1 ©2020ElsevierInc. j
1
https://doi.org/10.1016/B978-0-12-809434-1.00001-3 Allrightsreserved.
2 IntroductiontotheChemistryofFood
Introduction
Water is the major component of all living things and therefore an
importantpartoffood.Wateraffectsthetexture,taste,color,andmicrobial
safetyofeverythingweeat.Themoisturecontentoffoodisagoodindicator
ofitstexture.Ingeneral,itequateswithasofterfoodtexture.Forexample,
the texture of yogurt, meat, bread, and hard candy decreases in that order
and parallels the respective moisture content of these foods. Water is the
vehiclethat carriestastemolecules to receptorsin themouth. Forexample,
the sweetness of cherries, bitterness of beer, sourness of lemons, saltiness of
pretzels, and pungency of peppers results from compounds (tastants) dis-
solved in water. The method of cooking (wet or dry) affects food flavor
and color. Food cooked using wet methods, such as boiling, are generally
low in flavor and color. In contrast, foods cooked with dry methods, such
as frying or grilling have greater flavor and color. The moisture content of
foods, such as milk, is directly related to its potential for microbial spoilage.
Control of water available to spoilage organisms can be accomplished by
loweringthefood’swateractivitylevel(a )withhumectantsorbydehydra-
w
tion.Botharecommonpracticesinfoodpreservation.Thischapterdescribes
thepropertiesofwaterandchemistryinfood.Italsodescribesthechemical
concepts of acids and their relationships to food safety and spoilage.
These questions will help you explore and learn about water and its
effects on food.
(cid:129) How can surface tension be demonstrated using a cup of water and a
paperclip?
(cid:129) Why did my can of pop explode in the freezer?
(cid:129) Why does it take longer to boil potatoes in Denver than in Chicago?
(cid:129) What is a pKa?
(cid:129) Gee fizz, what makes soda pop so tasty?
(cid:129) Why did the biscuit dough package explode in the refrigerator? Hint:
The answer involves acid-base chemistry.
(cid:129) So, what happens when oil is added to water? Why doesn’t it dissolve?
(cid:129) Whatistheacid-ashhypothesisanddoesalkalinewatermakemybones
stronger?
Structure of water
Beforeconsideringtheeffectsofwaterinfood,itisnecessarytounderstand
itsuniquemolecularproperties.Thephysicalandchemicalpropertiesofwa-
ter directly result from its molecular composition and structure. Water is a
ChemicalpropertiesofwaterandpH 3
Fig.1.1 Watermoleculebondangle.Permissionsourcehttps://alevelbiology.co.uk/notes/
water-structure-properties/.
simplecompoundcontainingonlythreeatoms:oneoxygenandtwohydro-
gens.Hydrogenatomsinwaterarebondedtotheoxygenatomwithprecise
spacing and geometry. The length of the oxygen bond to hydrogen is
(cid:2)
exactly 0.9584 A and the angle formed between all three atoms is
104.45(cid:2). A more visual interpretation of a water molecule’s structure is
shown as a ball and stick model (Fig. 1.1).
The bond between oxygen and hydrogen is a true covalent bond, but
electrons in this bond are not shared equally due to the difference in
electronegativity between oxygen and hydrogen atoms. Oxygen is a
highly electronegative atom and hydrogen is weakly electronegative. As
a result of the difference in negativity, electrons spend more time on
the oxygen end of the bond, giving it a slightly negative charge.
Conversely, electrons spend less time at the hydrogen atom giving it a
slightly positive charge. The asymmetrical distribution of electrons
between hydrogen and oxygen is termed a dipole. Dipoles are noted
by Greek letter delta (d) and indicates a partial positive or negative charge
exists in the bond. The letter d together with the appropriate sign
(positive, dþ or negative, d(cid:3)) indicates the direction of bond polarity.
The dipoles between hydrogen and oxygen atoms are responsible for the
forcethatholdswatermoleculestogether,calledhydrogenbonding.Water
molecules have a V shape, providing optimal geometry for hydrogen
bonding between water molecules. Each water molecule is hydrogen
bonded to four others and this extensive interaction is responsible for its
unique physical properties (Fig. 1.2, Yan, 2000).
Whilewatermoleculesarelinkedbyhydrogenbonding,theirpositionis
not fixed. Water molecules in the liquid state rapidly exchange their
bonding partners.
4 IntroductiontotheChemistryofFood
Fig. 1.2 Hydrogen bonding of water molecules. Permission source Shutterstock ID:
350946731.
Physical properties of water
Surfacetensionisasurfacepropertyofliquidsthatallowsresistanceto
external forces. Water’s surface tension results from the attractive forces
(hydrogenbonding)betweenmolecules.Surfacetensionalsoenablesinsects
(e.g., water spiders) to walk on water and unusual objects to float on the
surface of water (Fig. 1.3).
How can surface tension be demonstrated using a cup of
water and a paperclip?
Floating a metal paperclip on the surface of water is often used to demon-
strateitssurfacetensionproperties.Addingadropofdishwashingdetergent
tothewaterimmediatelycausesthepapercliptosink.Theexplanationforits
Fig. 1.3 Water Strider Insect walking on water Permission source Shutterstock ID:
276367415.
ChemicalpropertiesofwaterandpH 5
sinking is that detergents are surfactants that disrupt hydrogen bonds
between water molecules.
Surfactants: Surfactants are substances containing both polar and
nonpolarproperties.Theydisrupthydrogenbondingbetweenwatermole-
cules and destroy its surface tension. Droplet formation is another example
ofwater’ssurfacetensionproperty.Waterexitinganeyedropperorsprayer
forms discrete spherical droplets because molecules near the surface have
fewer hydrogen bonding partners. Those in the interior have greater
hydrogen bonding. Water molecules are thus pulled to the center of the
droplet, resulting in a spherical shape (Labuza, 1970; Yan, 2000).
Specificheatcapacity:Theamountofenergyrequiredtoraisethetem-
peratureofonegramofwater(onedegreecentigrade)isknownasthespe-
cific heat capacity. The specific heat of water is higher than other similarly
sized molecules (e.g., methane), due to extensive hydrogen bonding. The
highspecificheatcapacityofwaterenablesittoabsorborloselargeamounts
of heat without undergoing a substantial change in temperature. For
example, the temperature of water is slow to increase as it is heated, until
it reaches 100(cid:2)C. Water’s specific heat capacity regulates the temperature
of the planet because large bodies of water act as a buffer to changes in air
temperature. Water’s specific heat explains why the temperature in Hawaii
stays within a relatively small range.
Phase changes of water
Waterundergoesreversiblestatetransitionsfromsolidtoliquidtogas
depending on conditions of temperature and pressure. The structure and
mobility of water molecules differ in these states. In the gas state, water
molecules have the highest mobility because the hydrogen bonding force
weakens as temperature increases. Conversely, the mobility of water
moleculesislowerinliquidandsolidstatesbecausethestrengthofhydrogen
bonding is higher at lower temperature. Water’s physical properties are
uniquecomparedtomoleculesofsimilarsize.Waterexistsinthesolidstate
(cid:2)
(ice) at 0 C and below. It melts and transitions to the liquid state as the
(cid:2) (cid:2) (cid:2)
temperature increases from 0 C to 100 C, above 100 C water exists in
the gas state. In contrast, methane is a molecule of similar size and weight.
However, the melting and boiling points of methane are very different
from water. Methane exists in the solid state at (cid:3)182.6(cid:2)C and transitions
to the gas state at (cid:3)161.4(cid:2)C (Table 1.1).
6 IntroductiontotheChemistryofFood
Table1.1 Physicalpropertiesofmethaneandwater.
Physicalproperty Methane(CH ) Water(H O)
4 2
Molecular Weight 16.04 18.01
Melting Point (cid:3)182.6(cid:2)C 0(cid:2)C
Boiling Point (cid:3)161.4(cid:2)C 100(cid:2)C
Water as a solid
(cid:2)
At 0 C, water becomes a solid (ice) with structural and physical
propertiesthataresubstantiallydifferentfromtheliquidstate.Freezingwater
is an exothermic (heat liberating) process. While that statement may seem
incorrect, heat is removed during the transition from liquid to solid state.
(cid:2)
At0 Cwaterexistsascrystallinelattice,variablycomposedofninedistinct
forms.Thebondanglebetweenoxygenandhydrogenatomsisdifferentfor
watermoleculesintheliquidandsolidstates.Specifically,theangleincreases
(cid:2) (cid:2)
from104.5 (liquidstate)to106.6 inice.Thethermalconductivityoficeis
greater because water molecules in the liquid state absorb some energy
through their motion.
Why did my can of pop explode in the freezer?
When water forms a crystal lattice, the space between molecules becomes
larger and its density is lowered. The increased bond angles and greater
distancebetweenwatermoleculesinicemeansthatagivenamountofwater
occupies a larger volume as ice and thus has lower density. Water expands
about9%involumeinthefrozenstate.Thischangeinvolumeisthereason
why a can of pop left in the freezer looks like it is about to explode.
Melting point of water: When ice melts, heat is absorbed from the
environment. This transition is an example of an endothermic process.
Approximately 80 calories of heat are absorbed per gram of ice as it melts.
Thetransferofenergyinmeltingiceisknownasthelatentheatoffusion.It
is a measure of the amount of heat required to convert a solid to a liquid.
Makingicecreamathometakesadvantageofwater’shighlatentheatoffusion.
Theicecreammixisplacedinabucketoficetowhichsaltisadded.Saltcauses
icetomeltandtheresultingendothermicprocessabsorbsheatfromtheliquid
icecreammixcausingittosolidify.Latentheatoffusioncanbeobservedwhen
iceisaddedtoaglassofpop.Thetemperatureofthebeverageisloweredto
(cid:2)
about0 C and remains steady until the ice is melted.
Water as a gas
Water has a high boiling point compared to molecules of similar size
andcomposition(e.g.,methane).Thereasonforwater’shigherboilingpoint
