Lesson 7: Low Temperature Preservation of Foods

Undesirable changes during refrigeration (cool storage)

- chill injury to some fruit/veg - staling of bread - loss of crispness in fruit/veg - change in colour of fresh meat - loss of flavour, nutrient value (vitamins) - oxidation - drip from fish - MO growth

Freezing methods: Cryogenic freezing

- direct contact w/ refrigerating medium - food is sprayed with cryogenic liquids = liquified gases of extremely low boiling points - liquid N2 (-196) and liquid CO2 (-79) - rapid freezing rates - spraying ex. TV dinner, hamburger patties Process: cold temp from evaporation of liquid to gas *see lecture slide #35

Freezing methods: Immersion freezing

- immersion in refrigerant fluid: sugar, slns, salt brine, glycerol - can also use cryogenic liquids (liquid nitrogen, liquid CO2) ex. frozen turkeys, chickens, *ice cream popsicles*

Steps of the freezing and frozen storage?

- removal of heat from the product (sensible heat) - freezing of water (liquid → solid or ice crystal) ↳ latent heat of fusion of crystallization - further cooling to surround temperature *see lecture slide #16

Modification of gas atmospheres

- the packaging material MUST exclude oxygen and moisture - NOT sterilization, no killing of MOs - doesn't extend shelf-life, still needs to be refrigerated Option1: a. remove air b. back-flush with desired gas mixture c. seal the package (MAP) ex. fresh pasta, cured meats, cheese salads OR Option2: completely remove air - vacuum pack ↳ prevents growth of aerobic, psychotrophic MOs

At what temperature can no microorganism grow?

-9.5°C (not killed, just inactivated)

List the freezing methods

1. Air freezing: air as cooling medium (home freezers) 2. Indirect Contact Freezing: ex. plate heat exchanges 3. Immersion & Cryogenic: cooling medium that you immerse your product inside - both involve heat exchange fluid, compressed gas, and refrigeration *?

The rate of freezing depends on:

1. Composition of food: fats (also prot/starches etc) are insulators so it needs a lower temp for higher freezing rate 2. Temperature difference: colder the temp = faster freezing - lowering air temp from -18° to -30° will shorten the freezing time from 40min to 20min 3. Product thickness: thin, flat package has faster heat transfer = faster FR 4. Air velocity: ↑ velocity, better circulation, faster Freezing Rate (FR) 5. Degree of contact: b/w food and cooling medium. More contact = faster FR

Factors affecting cool storage: Gas Atmosphere Storage- CA vs MAP

1. Controlled Atmosphere (CA) - composition is constantly monitored and maintained (through sensors) - for bulk packaging - ex. apples 2. Modified Atmosphere Packaging (MAP) - food is placed in package w/ no access to inside - gas composition inside changes overtime we/ no control once sealed - for packaged products

Freezing methods: Indirect Contact Freezing

1. Plate freezers - uniform shape: fish sticks, fish fillets - contact w/ metal surface cooled by refrigerant - can be single plate, double plate, pressure plate or flash freeze 2. Scraped surface slush freezers - ice cream makers, slushies - used for fluid/liquid products only ex. ice cream: rotator promotes rapid freezing and formation of *small* ice crystals and incorporate *air bubbles* into the freezing mix = formation of a *solid foam*

Factors affecting quality of frozen foods

1. Rate of freezing: fast as possible preferred. The goal is to create smaller ice crystals since the large crystals puncture the cell and damage tissue 2. Final storage temperature: lower the better, commercially -18°C 3. Stability of storage temperature: No fluctuations aka no thawing and re-freezing. This can lead to formation of 'package ice', blocks of peas vs individual 4. Rate of thawing: as fast as possible and as safely as possible. ex. fridge or running cold water NOT room temp - less time for damage due to concentration effects or large ice crystals to form

Name the 2 low temperature preservation methods

1. Refrigeration or cool storage (-2°C to 16°C) - preserve most perishable foods for days or weeks - not killing enzymes or MOs, just lowering temp to control M,E,C, by slowing down these rxns and thus making food last longer 2. Frozen storage (w/ proper packaging) - preserve foods for months or even years - again, not killing enzymes or MOs, just inhibiting the growth

Packaging material requirements for frozen foods

1. Resistant to transfer of *water vapour* - can lead to freezer burn 2. Not shatter in the cold temp - glass tends to shatter and is not flexible 3. Resist formation of pinholes 4. Barrier properties towards light and oxygen

How can we minimize changes in food during freezing/frozen storage + thawing?

1. blanching first 2. proper temperature control for freezing and frozen storage 3. appropriate packaging

Factors that affect cool (refrigeration) storage

1. controlled temperatures 2. humidity 3. gas atmosphere composition

Preservation principle behind freezing/frozen storage

1. lower temperature 2. lower water activity ↳ both TOGETHER, control M,E,C, activity and inhibit the growth of MOs BUT NOTE: - MOs can survive and resume growth upon thawing (NOT being killed through freezing) - changes can occur (chemical, enzymatic and physical)

% of foods consumed and preservation mehod

1. refrigerated or frozen (55%) 2. thermally processed (30%) 3. dehydrated (5%)

Freezing methods: Air Freezing

1. still air freezers: no circulation, slow freezing (small packaging increases FR) - household freezers 2. air black freezers: cold air at high velocity (-18° to -34°C) - FR is much higher than still air b/c of vigorous circulation and lower temp a. fluidized bed freezers (Individually Quick Frozen IQF) - counter-current flow where food items are frozen as individual pieces ex. corn, peas

Advantages and disadvantages: Indirect Contact Freezing

Advantages: - economical - minimal dehydration (freezer burn) - minimal package bulging Disadvantages: - slow freezing process - product must be of uniform thickness

Advantages and disadvantages: Immersion/Cryogenic Freezing

Advantages: - rapid freezing process - almost no dehydration (freezer burn) - O2 excluded, decreasing oxidative spoilage - indiv. freezing pieces have less freezing damage Disadvantages: - difficult to find suitable freezants - expensive operative cost

Advantages and disadvantages: Air freezers + IQF

Advantages: - economical - freeze various sizes/shapes - IQF has efficient heat transfer to increase FR Disadvantages: - possible excess dehydration (freezer burn) - undesirable bulging of package - non-uniform product can't be fluidized

Thawing and re-freezing and ice crystal size/formation

As the temp of frozen food increases, so too does the amt of unfrozen water content. Therefore, small fluctuation in storage temp causes melting of the ice crystals and re-freezing at the temperature decreases leads to large ice crystals - this is how the formation of 'package ice' or blocks of ice occur with ex. peas - large ice crystals puncture and damage cell tissues - ex. SEM of bovine muscle tissue, freeze/thawed had large intracellular ice crystals that left cavities and formed textural changes whereas non-freeze/thaw had many small ice crystals which increased the quality

List refrigeration load factors and define specific heat capacity

Factors: - initial temp of product (will take longer for hotter product) - volume of product (will take longer for larger product) - specific heat capacity ↳ water content of food ↑ SHC (will take longer for foods with higher water content ex soup vs pasta w/o cheese) Specific Heat Capacity: the amount of energy to raise or lower the temp of each (g) of product by 1°C

Flash freezing vs long freezing

Flash freezing is quick and promotes a lot of small crystals therefore there is minimal physical damage to the product and the quality is higher - quality is maximized if able to form and maintain small ice crystals Long freezing promotes few large ice crystals which can eventually lead to dehydration/freezer burn and/or formation of 'package ice' from large ice crystals

What is the freezing method for peas?

Fluidized bed freezers performing IQF (individually quick frozen. This is a type of air blast freezing under the air freezing method using perforated belts.

Unfrozen phase in freezing and frozen storage

Not all of the water is frozen during frozen storage. Constant increase in concentration of solutes in the *unfrozen phase* exist ↳ the amt of unfrozen water that remains depends on the temperature and the type of product ex of beef -4°C, 70% unfrozen -9°C, 3% unfrozen -18°C, very small amt unfrozen (flash freeze is better than long freeze because not much damage and quality loss)

Factors affecting cool storage: Humidity

Purpose: 1. prevent dehydration as well as excess moisture 2. packaging to help maintain proper humidity within each food

Difference between FR and 'freezing faster'

Rate of freezing ≠ freezing faster ↳ for something starting at a higher temp can freeze at the same rate but takes longer to 'freeze' since the temperature drop is higher

Factors affecting cool storage: Controlled temperatures

Refrigeration load: heat that must be removed from food products to bring the food from its initial temp to the desired storage temp. Based on - initial temp of products - volume of product - specific heat capacity (also water content) Purpose of C.temp: to AVOID undesirable effects ex. chill injury

What is IQF?

Type of fluidized be freezing under Air freezing. Individually Quick Frozen uses vigorous counter-current flow to individually freeze pieces of corn or peas on perforated belts *see lecture slide #29

Define freezer burn

Type of physical damage from large ice crystals during freezing or frozen storage - aka dehydration - this is when the moisture of the product leaves and condensation of ice crystals occurs on the package - due to improper packaging

Video on corn processing: a. why is corn starch added? b. what is the time/temp combo for blanching? c. IQF process? d. length of frozen state storage?

a. (1) controls the density of the water so that corn kernel sinks to the bottom and they can easily clean the debris out. (2) prevents the leeching of the starch from the kernel b. 85-88°C for 3.5min c. -20°C perforated belt w/ air for 10min d. 2-3 years, it's a seasonal process (not year-round)

Video on ice-cream making a. main steps b. type of freezer w/ temps used c. safety/quality tests

a. first the milk follows clarification, homogenization and pasteurization, after the secondary ing (stabilizers, vanilla, sugars) are added, it is re-pasteurized (then nuts added since solids cannot be pasteurized) b. 2 methods shows: bar: immersion freezing where bars are placed into salt brine solution at -17°C pints: scrape surface under indirect freezing sandwiches: scrape surface and then deep freezing and very low temp -40°C c. taste test, sensory evaluation and chemical tests to test for *coliform* (faecal contamination)

Changes that occur during freezing or frozen storage

as a result of: 1. high solute concentration in the *unfrozen phase* a. oxidative deterioration b. enzymatic rxns (ex. enzymatic browning, or lipolytic rancidity) - this is why you should blanch first 2. physical damage from large ice crystals a. dehydration (*freezer burn*): water is removed and/or improper packaging b. formation of *'package ice'*: fluctuation of freeze/thaw cycles occurs and promote large ice crystals c. textural changes (protein denaturation) - meats become tough (textural hardness) - large ice crystals

Can pathogens grow at 4°C or lower?

at 4°C or lower, most pathogens cannot grow. - some exceptions like psychotrophs, which can grow/reproduce below this temperature (spoilage and pathogenic) ex. Listeria

Freezing/Frozen storage facts

commercial freezing temp: -18°C home freezers: -12°C to -14°C - longer shelf life-extension: weeks, months or even years - MOs CAN NOT grow below -9.5°C (not killed, just inactivated), but some MOs can still grow at temp below 0°C - usually higher nutritional & sensory quality than thermally processed foods

What is chill injury?

damage to plant parts in low temperature environments. fruit and veg are living systems with optimum temp req'mts, cold temperature can damage appearance and quality ex. bananas(dull colour), sweet potatoes (decaying), avocado (internal browning), potatoes (mahogany browning), apples (internal browning, soggy)

Define latent heat of fusion

energy released/absorbed by a body/system during a constant temp process ex. 0°C water to 0°C ice and vice versa

Concentration effect in frozen storage

ex. 10g solute in 100mL of water freezes to 10g solute in 20mL of water (unfrozen phase). ↳ the 10g solute that remains is much more concentrated ↳ 10ml vs 0.1ml - 0.1ml would have a much more severe effect

Refrigeration (cool storage)

from -2°C to 16°C but usually 4°C (at home. durable life dates are based on this value) - short term extension: (days, weeks) shelf life, assuming food initial quality - slows down rate of M,E,C rxns - some foods do not freeze at -2°C (ex water) so this is why this value is considered refrigeration - for every 10°C ↓ ↳ rate of senescence (aging) ↓ 2-3 fold ↳ microbial growth ↓ 3-6 fold

Define freezing point

temperature at which ice crystals are in equilibrium with air