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Food Chemistry 1 > Water > Flashcards

Water Flashcards

1
Q

Water is crucial to __________ of ______ and is present in all ____A___ systems
- 3 common roles for water in ____A___ systems

A
  • existence of life
  • food systems
    1. carrier of nutrients and waste
    2. reactant and reaction medium
    3. stabilizer of biopolymer configuration (proteins and carbohydrates)
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2
Q

water’s (3) in food products is extremely important from standpoint of _________ and ________

A

1) amount of water
2) location of water
3) interaction of water with chemical components
- acceptability and stability

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3
Q

texture is often associated with _________ (ie freeze-dried orange vs fresh orange)

A

moisture

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4
Q

because water is a major reactant and serves as a reaction ________, many of our food _________ and _________ methods involve the control of _________ content to slow down _________ reactions

A
  • medium
  • preservation and processing methods
  • water
  • slow down deteriorative reactions
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5
Q

what (3) reduce water activity of a system?

A
  • freezing
  • concentration
  • dehydration
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6
Q

what is one of the most abundant molecules on earth?
- also the only common _________ and most abundant _________

A

water
- only common liquid
- most abundant solid

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7
Q

water is the only compound that exists in all _________ _________ under which conditions?

A
  • all 3 states (solid, liquid and gas)
  • under conditions found on this planet
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8
Q

difference between water (____A_____ of oxygen) to other ____A_____ of other atoms (CH4, NH3, HF, H2S, H2Se, HeTe)?

A
  • hydrate of oxygen
  • all other hydrates are gaseous at ambient temperature
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9
Q

relative to other hydrates, water is unusual in all of its physical properties (7)

A
  • melting point
  • boiling point
  • surface tension
  • dielectric constant
  • heat of fusion
  • heat of vaporization
  • heat of sublimation
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10
Q

Oxygen has ___ electrons, ___ of which are valence electrons + 2 H have ___ electrons each
- the 3 ____ orbitals and the ___ orbitals of oxygen atom are combined to form 4 ____ hybrid orbitals
- only 2 of oxygen’s ___ valence electrons are used up in the # covalent bonds that form between H and O
- the net result is that there are 2 oxygen _______ ________ _____ left in 2 of the _____ orbitals

A
  • 8 e- –> 6 are valence + 2 hydrogen have 1 electrons each
  • three 2p orbitals and one 2s orbital –> 4 sp3 hybrid orbitals
  • 2 of 6 valence e- used in the 2 covalent bonds
  • 2 O lone electron pairs left in 2 of the sp3 orbitals
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11
Q

because of its lone electron pairs, O is highly _________ –> hence covalent bond formed with hydrogen is uniform/not uniform and the electron from Hydrogen is drawn away/towards Oxygen
- the result is a(n) increase/lowering of the electron _________ around the hydrogen atom –> giving it a slight negative/positive character (electron-rich/poor)
- the 2 lone e^pairs of O produce 2 regions of _________ character (electron rich/poor) relative to the _________

A
  • electronegative –> bond is not uniform
  • H electrons drawn towards O
  • lowering of electron shield –> positive character (e- poor)
  • negative character (e- rich) relative to hydrogens
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12
Q

water molecules can be viewed as having # electron rich regions and # electron poor regions situated _________ally
- in the presence of other water molecules, these regions of differential electron density will lead to _________ to minimize overall _________ _________ of system

A

2 rich + 2 poor
- tetrahedrally
- association –> overall energy

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13
Q

what spacial arrangements of e- rich and e- poor regions of water promotes ability to form 3D associations or networks?

A

tetrahedral nature

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14
Q

although hydrogen bonds are not very _________ (# kcal/mol) –> _________ can be formed = _________ effect
- hydrogen bonding is the basis for water _________ _________ in atmosphere, leading to formation of _________

A
  • strong (10 kcal/mol)
  • many can be formed –> cumulative effect
  • water vapor accumulation –> formation of clouds
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15
Q

what is the transitional structure of water called in its liquid phase?
- regions of 3D _______-_______ water molecules somewhat similar in structure to _____

A
  • flickering clusters
  • hydrogen-bonded
  • similar to ice
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16
Q

lifetime of flickering clusters?

A

10^-10 to 10^-11 seconds
- continually form and break up

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17
Q

increase in temperature =
- increase/decrease of number of clusters
- increase/decrease of number of molecules per cluster

A
  • increase in number of clusters
  • decrease in number of molecules per cluster
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18
Q

flickering cluster
- exist at boiling point of water?
- special feature of their structure?
- have been postulated to explain the relatively normal (low) _________ of water

A
  • yes
  • hole in the middle
  • normal viscosity of water
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19
Q

when ionic compounds dissolved in water, they ________ into their respective _______
- due to the _______ nature of water –> this _______ is sufficient/insufficient to break _______ bonds, but can dissociate _______ bonds by allowing multiple water molecules to act as _______ for opposing charged ionic species
- hence, when NaCl is dissolved, each ion is _______ by water which substitutes its polar contributions for the _______ charges of the respective _______

A
  • ionize into their respective ions
  • very polar nature of water molecule –> polarity is insufficient to break covalent bonds but can dissociate ionic bonds
  • act a counterions
  • each ion is hydrated by water
  • for the opposing charges of the respective ion
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20
Q

what forms around each ion to neutralize the charge of the respective ion?
- this forces water into a _______ orientation around each ion, therefore disrupting the normal structure of water

A
  • hydration shell
  • specific orientation
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21
Q

The formation of bulky hydrates generally causes an increase/decrease in viscosity and tends to increase/decrease the freezing point of a system

A
  • increase in viscosity
  • decrease in freezing point of system
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22
Q

water around a hydrogen bonding site on a molecule will associate in a manner that disrupts the natural _______ geometry of water
- overall effect is to _______ up water locally in a similar manner as occurs with _______ compounds
- hydrogen bonding solutes will disrupt water structure –> number of flickering clusters will increase/decrease

A

tetrahedral geometry
- tie up water
- ionic compounds
- number of flickering clusters will decrease

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23
Q

what happens with water in presence of macromolecules that have a large number of hydrogen bonding sites (polypeptides, proteins, carbohydrates, and polysacs)?

A

the water can be completely or extensively immobilized by hydrogen bonding if amount of water present is limited

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24
Q

when low concentrations of nonpolar substances (hydrocarbons, FA, nonpolar side chains of proteins) are dissolved or dispersed in water, _____-_______ _______ are though to be induced
- explain

A

ice-like inclusions
- formation of ice-like structure around a nonpolar group or molecule has been postulated to minimize the free energy state of system when symmetric electron-neutral constituents and asymmetric electron-rich hydrophilic materials are forced to associate

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25
Q

ice-like inclusions are/are not structurally identical to ice, but rather a semicrystalline form is produced called _______ _______
- definition?

A
  • are not structurally identical
  • clathrate hydrates
    def: compounds not formed by action of valence bonds but by molecular imprisonment –> important in stabilizing protein structure around hydrophobic moieties
26
Q

overall, what has a disruptive effect on the structure of water?
- in what way or another, water has to interact with what?
- water activity increased/reduced when solute is added

A
  • any solute! (ionic compounds, hydrogen bonding compounds, nonpolar substances)
  • with any compound dissolved or dispersed in it
  • reduced!
27
Q

when does freezing or crystallization of water takes place?

A

when kinetic energy of molecules is reduced to such an extent that at some point in time, a flickering cluster is able to become large enough to form a minute crystal or nucleus –> the nucleus can grow in size as more water molecules fit into the 3D crystal lattice

28
Q

at 0°C there is an equilibrium btw _______ _______ and liquid water

A

crystal nucleation

29
Q

in ultrapure water, nucleation is easy/difficult and does/does not readily take place –> _______ can occur: what does it mean?

A

difficult and does not readily take place
- supercooling can occur –> water stays liquid well below 0°C

30
Q

supercooled water will freeze almost instantly if what?

A

if disturbed! or if an ice crystal is thrown in –> hence, some site of organization or nucleation is required to start the process

31
Q

what aids the start of nucleation process in normal water?

A

impurities such as dust or container wall surface

32
Q

nucleation process is a function of rate of _______
- slow freezing vs
- fast freezing
- large or small ice crystals + consequences?

A
  • rate of cooling
  • slow freezing –> only a few nuclei are formed and, once formed, will grow at the expense of the formation of new nuclei –> net result is the formation of a few large ice crystals
    consequence of slow freezing: larger ice crystals form, which can become large enough to burst cell walls = physical damage that reduces water holding capacity and activates enzymes previously contained
  • rapid freezing: more nuclei will form at the outset, leading to formation of smaller ice crystals –> small ice crystals uniformly throughout the product (can resuscitate frog more easily)
33
Q

how can solutes interfere with nucleation process?
- leads to increase/decrease of freezing point?

A

once nucleation has taken place and water diffuses to the surface of the crystal, solutes become more concentrated as the freezing process takes place –> leads to depression of freezing point: as the solutes become more concentrated, lower temp required for ice to form

34
Q

what is the eutectic point?
- what temp range?

A

when at a low enough temperature, the solution can become concentrated to a point at which remaining water will co-crystallize with the solute crystals to start to form and grow
- lies between -55 and -70°C

35
Q

when the eutectic point has been reached, all the water in system is ____________

A

immobilized

36
Q

conventional frozen storage temperature?
- above or below the eutectic point? consequence?

A

“-18°C”
- above the eutectic point –> not all water in frozen food product is immobilized (around 98%) –> total stability is not attained –> water is still available to act as a reactant + reactant medium is very concentrated –> even if temp is low, all types of reactions can still take place, especially enzymatic reactions bc the substrate and enzyme may mix due to tissue disruption (from slow freezing)

37
Q

_______ _______ and ___ changes can denature proteins –> can affect water _______ _______

A

ionic strength and pH
- water holding capacity

38
Q

why do frozen foods not have a perpetual shelf life?
- so what is done to fruits and veggies?

A

because deterioration (enzymatic reaction) can still take place from the water that is not 100% immobilized
- fruits and veggies have to be blanches (mild heat treatment) to inactivate enzyme before being frozen

39
Q

all reactions speed up when the _______ process is started –> much of the _______ is lost rather than during frozen storage

A
  • thawing
  • quality
40
Q

water in food present in variety of ways –> 2 extremes?
- present as as _______ –> explain

A
  • bound water and free water
  • as a continuum –> being relatively mobile (free water) to being effectively immobile (bound water)
41
Q

how to assess mobility of water?
- formula
- ranges from what to what?

A
  • water activity
  • Aw = Partial pressure of solution (food matrix)/ partial pressure of pure water at a specific temperature
  • 0 to 1.0 (pure water)
42
Q

how to calculate partial pressure of solution?

A

partial pressure = mole fraction of water in the solution * vapor pressure of pure H2O

43
Q

would 25% NaCl solution lead to a significant reduced partial pressure? explain

A

yes! significant!
- salt, being solvated/dissolved, ties up some of the water, restricting its freedom of movement (activity) or mobility

44
Q

water activity ranges:
- Type 4
- Type 3

  • microorgs?
  • reactions?
  • freezing point depressed?
A

Type 4:
- Aw = 1.0
- this is pure water and does not exist in food systems
Type 3:
- Aw = 0.99-0.80
- water is physically entrapped in the tissue matrix (ie water present in macrocapillaries, > 1 um in diameter
- contains more solutes but not enough to immobilize water in any major way
- many microorgs are capable of growth, with some bacteria and yeasts inhibited at lower end of range
- most hydrolytic, oxidative and enzymatic reactions proceed readily in this water activity range
- freezing point NOT depressed significantly

45
Q

Water activity ranges:
- type 2
- type 1

  • microorgs?
  • reactions?
  • freezing point depressed?
A

Type 2 water:
- Aw = 0.80-0.25
- from water in microcapillaries (<1 um in diameter) to outer layers of bound water (water hydrogen-bonded to solutes)
- most microbial growth is halted, with the exception of some molds at upper end of this range
- freezing point of water is significantly reduced and most enzymatic reactions requiring water are slowed significantly
- non-enzymatic browning takes place readily with heat at the upper end of this range –> occurs without heat but over time at lower end of this range
Type 1 water:
- Aw = 0.25-0
- bound monolayers of water directly hydrogen-bonded to molecules (protein/carbs)
- water is very strongly H-bonded to molecules and an integral part of molecular structure –> essentially immobile
- very difficult to remove and cannot be frozen (effectively locked in place)
- most reactions are minimal with exception of autoxidation

46
Q

why can food absorb water from environment and desorb water?

A

because foods will naturally equilibrate to a point of equilibrium with the environment

47
Q

desorption vs absorption

A

desorption: occurs when a wet food is placed in a dry environment
- analogous to dehydration; but not the same
- slow process, with moisture gradually decreasing until food is in equilibrium with its environment
- Aw decreases
absorption:
- when a dry food is placed in we environment
- Aw increases

48
Q

difference between desorption and dehydration

A

desorption: implies that food is moving toward equilibrium with environment (ie a package)
dehydration: permanent loss of water from a food
- in both cases, Aw decreases

49
Q

the term hygroscopic is used to describe foods or chemicals that ___________ moisture: a real problem in food industry

A

absorb

50
Q

food scientist is often interested in ___________ ___________ profile of food product –> tells a lot about its potential ___________ and how it would lose or gain ___________ as a function of ___________ ___________
- this is done by deriving a ___________ ___________ ___________

A
  • water activity
  • stability
  • lose or gain moisture as a function of relative humidity
  • moisture sorption isotherm
51
Q

how to derive moisture sorption isotherm?
- goal?

A

food product is dried completely (a relative term) and then placed into an enclosed chamber above a saturated salt solution –> wide variety of salts are available which generate a specific Aw
- goal: tells us what package we need in terms of moisture permeability characteristics

52
Q

Aw is directly related to ___________ ___________ ___________, which is much easier to measure
- formula?
- how to measure?

A
  • equilibrium relative humidity (ERH)
  • Aw = ERH at constant temperature
    1) samples of dried food product are placed in a series of hermetically sealed chambers of known equilibrium relative humidity (ERH) at a constant temp –>
    2) after samples have come to equilibrium, they are weighed to determine amount of moisture absorbed (g H2O/g dry weight) = equilibrium moisture content (EMC)
    3) plot of EMC vs Aw or EHR provides the moisture sorption isotherm –> plot illustrates how food product would behave as a function of relative humidity in terms of water absorption or loss at a given temp
53
Q

can we extrapolate data from moisture sorption plots from one temp to another?

A

no! moisture sorption plots are temperature dependent

54
Q

___________: moisture sorption plot curves is different depending on whether product starts out dry or moist
- differences are attributable to what?
- ex.?

A
  • hysteresis
  • attributable to permanent changes in product (structural/chemical) that have taken place due to changes in moisture content
  • ex.: when a steak is dried, numerous irreversible chemical and physical changes take place, which permanently change its moisture sorption behaviour
55
Q

moisture sorption curves will show ___________, except in very simple systems

A

hysteresis

56
Q

moisture sorption information defines how a product does what?
- useful for determining final ___________ ___________ that can be obtained in a drying operation based on ___________ and relative ___________ of the drying air

A
  • how a product picks up or loses moisture under specific conditions (relative humidity)
  • final moisture content
  • based on temperature and relative humidity
57
Q

moisture sorption data provides important information about relative stability of a product in regards to (2)
- also allows evaluation of effects of compounds that can be used to modify ___________ ___________ profile of a product

A
  1. type of microbial growth possible
  2. types of reaction that may predominate in a food system
    - water activity profile of a product (ie: addition of sorbitol)
58
Q

water activity is an important variable that is of major concern in 5 types of reactions

A
  1. enzymatic reactions
  2. lipid oxidation
  3. hydrolytic reactions
  4. non-enzymatic browning
  5. various forms of microbial growth
59
Q

4 generalizations about the reactions that occur from 0 to 1.0 water activity (interpret graph)
1. most reactions are ___________ functions of Aw –> implies what?
2. breakpoint for many reactions lies at around Aw = ?
3. Aw has to be reduced down to ____-____ to reach a minimum for most deteriorative reactions
4. is there a region where all reactions are stopped?

A
  1. most reactions are exponential functions of water activity: implies that even a slight reduction of water activity will have a significant effect in reducing the rate of a reaction
  2. Aw = 0.8
  3. reduced down to 0.3-0.4
  4. there is no water activity value where all reactions are stopped –> however, optimum appears to be around Aw = 0.3-0.4
60
Q

3 ways to control water activity + details

A
  1. remove water (dehydration or concentration) –> can change physical nature of food + alter its color, texture or flavor
  2. convert free water to bound water by addition of sugars, salts or other water=soluble agents
  3. freeze food –> immobilizes water (and lowers Aw) but not all foods can be or should be frozen –> frozen foods will eventually be thawed and the problem persists
61
Q

is the optimization of Aw the only solution to preservation of foods?

A

no! optimization of Aw is rarely a preservation solution in its own right –> need to consider quality factors as well

62
Q

control of water activity is often ONE component of preservation processed using the ____A______ concept
- what are 3 other barriers/______A_____

A
  • hurdle concept!
    1. pH
    2. ionic strength
    3. modification of atmosphere

Food Chemistry 1 (22 decks)

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