[SAR HS 230] - Midterm Exam Guide - Ultimate 17 pages long Study Guide!

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SAR HS 230
MIDTERM EXAM
STUDY GUIDE
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Lecture 1
Water
- Importance:
- Growth of microbes
- enzymatic reactions
- Chemical reactions
- Stability of macromolecules
- Physical properties
- Water in processing
- Majority of processing techniques include:
- Freezing
- Drying
- Water directly affects food shelf life
- Liquid water-water interaction
- Hydrogen bonds between molecules are short lived
- Formations of bonds are rapid
- Flickering clusters
- Never as single molecules
- Specific heat and latent heat
- Specific heat: energy required to raise the temperature of 1g of water by 1ºC
- Water evaporation
- As water temperature raises 0ºC-100ºC
- Distance between nearest neighbors increases
- Thermal expansion
- Vapor pressure: the pressure by gas above a liquid
- Substance with a high VP -> liquid evaporates easily; lowers boiling point
- VP increases with increase in temp
- VP decreases with addition of solutes
- When VP equals external pressure, water boils
!
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Water!and!salt:!have!to!break!bonds!between!the!salt!molecules!and!then!the!H!bonds!in!order!
to!evaporate,!takes!longer!
!
Ice/Freezing!
Liquid!water!–!lots!of!energy!
o Moving!around!
o Vibrating!
o Rotating!
In!order!to!freeze,!must!remove!enough!energy!to!stop!motion!
o Allows!water!molecules!to!come!together!and!form!a!nuceli!
Once!a!nuclei!is!formed,!other!water!molecules!attach!themselves!forming!ice!crystals!
o Each!water!molecule!is!bonded!to!4!other!water!molecules!
The!size!of!ice!crystals!in!food!products!are!dependent!upon!the!rate!and!temperature!
of!the!freezing!process!
o Flash!freezing!!
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§ Extremely!rapid!method!of!freezing!resulting!in!small!crystals!
o Household!freezing!
§ Slow!method!of!freezing!resulting!in!larger!ice!crystals!
o Commercially!prepared!frozen!products!are!subjected!to!flash!freezing!
§ !!
o Household!freezers!may!not!be!as!controlled!as!commercial!freezers!
§ Over-packed!
§ Lower!quality!
o Household!freezers!go!through!a!thaw!cycle!
§ Temperatures!may!reach!as!high!as!7ºC!
§ Occurs!around!every!6-12!hours!
§ At!thaw!cycle!food!begins!to!melt!
Ice!crystals!may!collide!to!form!one!giant!ice!crystal!
o Accretion!
§ Surviving!ice!crystals!increase!in!size!once!temperature!returns!to!
freezing!
Water!would!rather!bind!to!pre-existing!crystals!than!form!new!
ones!
§ After!a!period!of!time!ice!crystals!are!so!large!you!can!feel!them!in!your!
mouth!
One!type!of!“freezer!burn”!!
§ As!crystals!begin!to!melt,!sugars!precipitate!out!of!the!ice!cream!and!
becomes!gritty!
o If!fresh!meats!are!not!used!right!away!people!may!freeze!them!in!their!feezer!
§ Responsible!for!“meat!juice”!
§ Large!ice!crystals!form!and!rupture!de!the!membrane,!creating!them!to!
come!out!as!liquid!
Water!activity!
o Water!content!is!linked!to!foods!perishability!
o Foods!with!similar!water!content!completely!differ!in!perishability!
o Water!Activity!(AW)!vs.!%!Moisture!
§ Food!scientists!are!more!concearned!with!!
o Difference!of!“free!water”!vs!“bound!water”!
§ “Free!water”:!is!the!amount!of!water!within!a!food!product!that!can!act!
as!liquid!water!
Can!react!in!chemical!reactions!!
Is!accessible!to!microorganisms!
§ “Bound!water”:!may!be!bound!to!any!component!within!a!food!
Cannot!act!as!solvent!
Has!hindered!mobility!
Cannot!take!part!in!chemical!activities!
Is!not!readily!accessible!to!microorganisms!
Cannot!be!removed!(only!by!incineration)!
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