Levensmiddelentechnologie - Low Temperature Preservation

What is low T preservation?

increase in shelf life by inhibition but no destruction of MO and enzymes

→ T-increase permits growth of pathogens and increases spoilage

Careful control of cold chain (-1°C to 8°C)

• Chilled storage at production plant

• Refrigerated transport

• Retail chill display cabinets

• Domestic refrigerators

Low T preservation: outline

• Mechanical vapor-compression system

• Cryogenic chilling

• Effect on MO

• Case-study

Low T preservation: Mechanical vapor-compression system

• Refrigerants

• Components of a refrigeration system

• Pressure-enthalpy charts

• Mathematical expressions

• Equipment

Low T preservation: Mechanical vapor-compression system - Refrigerants

Preruiquisites phase behaviour

• High latent heat of vaporization → minimal amount of refrigerant

• Low condensing pressure → avoid heavy construction

• Freezing T < Evaporator T

• High critical temperature → no liquification at T > critical T

Other conditions

• Ammonia → flammable, toxic & corrosive

• CO2 → causes suffocation

• Hydrocarbons → highly flammable

• Chlorofluorocarbons (CFCs) → ozone depletion

• Hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) → no ozone depletion but greenhouse gas (global warming potential)

Low T preservation: Mechanical vapor-compression system - Components of refrigeration system

1. Evaporator (coils)

2. Compressor

• P increase

  • Reciprocating compressor

  • Centrifugal compressor

  • Rotary compressor

• T increases above ambient T

• Heat can be released to environment in condenser

3. Condenser

• Water-cooled

• Air-cooled

4. Expansion valve

• = metering device controlling the flow of liquid to the evaporator

• High pressure saturated liquid → low pressure mixture liquid/vapour

Low T preservation: Mechanical vapor-compression system - Pressure-enthalpy chart

• Characteristic for each refrigerant

• Indicates pressure and enthalpy changes of refrigerant throughout the refrigeration cycle

• Evaporator: constant P and T

• Condenser: constant P and T

• Compresser = constant entropy

• Expansion = constant enthalpy

D = saturated liquid

D’ = D more to left = subcooled liquid

A = saturated vapour

A’ = A more to right = superheated vapour

Mathematical expressions → see exercises

Low T preservation: Mechanical vapor-compression system - Equipment

• Chilling equipment = reduce T of a product

• Cold storage equipment = maintain a defined T

• Temperature monitoring = integral part of quality and safety management

Low T preservation: Mechanical vapor-compression system - Equipment: Chilling equipment

Air chillers

• Forced convection to reduce boundary film → increased rate of heat transfer

Batch chillers

• Trolleys or pallets are placed in a tunnel

Continuous chillers

• Product moves through tunnel

• Speed ~ adequate cooling

Blast chillers

• Loading → chilling → defrosting

• T control: loggers and alarms

Eutetic plate system

• Local distribution

• Salt solution (KCl, NaCl or NH3Cl) → frozen to eutectic T

• Air circulated across the plates

• The plates are regenerated by re-freezing in an external freezer

Vacuum cooling

• Cooling takes place as moisture evaporates from the surface

• Fresh foods with larger surface area: lettuce, mushroom, broccoli

• Vacuum chamber (pressure reduced to 0,5 kPa)

Hydrocooling

• = direct immersion in chilled water or brine

• e.g. fruits and vegetables, poultry, fish, cheese

Plate heat exchanger or scraped-surface heat exchanger

• cool (semi-)liquid foods after pasteurization)

Low T preservation: Mechanical vapor-compression system - Equipment: Cold storage

• Circulation of cold air → mechanical refrigeration system

• T < 5°C

• Control of relative humidity

• Composition of storage atmosphere

• Adequate circulation of air using fans

Low T preservation: Mechanical vapor-compression system - Equipment: Temperature monitoring

Temperature data loggers + temperature sensors

• Air temperature or product temperature

• load test = establish a relation between air and product T

• difficult in open retail cabinets → measure product T

Critical temperature indicators

• Cumulative t,T exposure above reference T

• Specific reactions or growth above reference T

Full history time-temperature indicators

• Continuous integrated t,T history

• overall quality loss

Intelligent packaging

• barcode system

• smart label

Type of indicators

• Liquid crystal coatings: change colour with storage T

• melting of wax releasing a coloured dye

• Diacetylene changes as function of time and T

• Enzymic reaction → pH indicator

Low T preservation: Cryogenic chilling

Solid or liquified CO2

• preferred for chilling

• higher boiling/sublimation point

• Most of the heat capacity in phase change

• Better control over temperature

Liquified nitrogen

• Preferrred for freezing

• Lower boiling/ sublimation point → larger T gradient

• Heat capacity: gas absorbs sensible heat → special gas-handling equipment

Equipment

• Solid CO2 → dry ice-pellets

• Liquid CO2 → injected in air (= snow)

• Liquid nitrogen → injected in air (= immediate vaporization) → fans distribute this gas

Continuous cryogenic chilling

• inclined, cylindrical barrel

• Rotates slowly → food tumbles through the cold gas

Disadvantages

• Suffocation

• Cold burns

• Frostbite

• Hypothermia

Low T preservation: Effect on MO

Temperature decrease

→ prolonged lag phase

→ decreased growth rate

→ changes at cellular level: cell membrane structure, uptake of substrate, enzymic reactions

G- bacteria

• most common spoilage MO in chilled foods

• max growth T: 0-3°C

• Contamination by water, bad cleaned equipment or surfaces

• produce slime, off-flavour or off-odors

Yeasts and moulds

• Able to tolerate chill temperatures

• grow more slowly than bacteria

• out-competed unless other environmental factors limit the growth of bacteria

• if bacteria growth is limited → yeasts may cause spoilage

• Yeasts can grow in the absence of oxygen

Pathogenic bacteria

• Some can grow at chilling T

• Some are sufficiently virulent → poisoning after digestion of only few cells

• Some are unable to grow at < 5 °C but may grow if T abuse occurs and then persist in the food

!! Be aware of the psychrophilic and psychrotrophic MO still being alive

Low T preservation: Case study → Fresh foods

cooling → enzymatic changes ↓ → respiration and biological aging is retarded

Factors to control fresh-crops in cold storage

• Type of food and variety of cultivar

• Part of the crop selected

• Condition of the food at harvest

• Temperature during harvest

• Relative humidity of storage atmosphere → influence dehydration losses

• Gas composition of storage atmosphere

Storage life ~ Respiration rate

Climacteric vs non-climacteric

• Climacteric: ripen after harvest + respiration peak (e.g. bananas)

• Non-climacteric: do not ripen after harvest + no respiration peak (e.g. orange, lemon)

Low T preservation: Case study → Fresh foods (Quality losses)

Chilling injury

• storage temperature is reduced below a specific optimum for the individual crop

• Effects

  • Imbalance in metabolic activity → over-production of metabolites → toxic to the tissues

  • Changes in membrane lipid structure, enzyme activity and structural proteins → loss of membrane integrity and leakage of solutes

Transpiration

• Excessive storage time

• Incorrect temperature

• Mechanical damage to drops

• Effects

  • enzymatic browning

  • wilting

  • weight loss

Losses of vitamin C

• Accelerated at higher storage T and longer storage t

• sometimes higher losses at lower storage T

• moisture loss → vitamin C loss

• Especially for leafy vegetables

• Cause: bruising, mechanical injuries and excessive trimming

Influence on carotenoids

• dependent on type of fruit: increase or decrease

→ !! Fresh-cut fruits visually spoil before any significant nutrient loss occurs

Low T preservation: Case study → Fresh foods: meat and meat products

Muscles

• glycogen, creatine-phosphate and sugar phosphate

• used for ATP production by glycolysis

• ATP supply stops → rigor mortis tissue

Anaerobic respiration

Glycogen → lactic acid → pH falls from ~= 7 to between 5.4 and 5.6

• contribute to the flavour

• against contaminating bacteria

• protein denaturation → drip losses

• T-dependent → lower T is required → slow down biochemical reaction

Low T preservation: Case study → Fresh foods: meat and meat products (Quality losses)

Lipid oxidation

• Adverse changes to flavour, colour, texture and nutritive value → production of toxic compounds; oxidized flavour or “warmed-over flavour”

Enzyme activity

• Both positive and negative effects

Loss of nutrients

• Vitamin C, folate → due to chilling time, storage T and oxidation

Low T preservation: Case study → Fresh foods: Processed foods

categorize by degree of microbial risk

• Class 1: containing raw or uncooked ingredients (salad, cheese…)

• Class 2: mixture of cooked and low-risk raw ingredients

• Class 3: cooked food and then packaged

• Class 4: packaged and then cooked

Shelf-life is determined by

• Type of food

• Preservative factors (pH, aw, chemicals)

• Microbial destruction/ enzyme inactivation by unit processes

• Hygienic control during processing/packaging

• Temperature history

Different processes

• “Cook-chill” or “Cook-pasteurize-chill” processes

• “Sous vide” products: vacuum pack → pasteurisation → cooling → cold storage → shelf life of 2-3 weeks

• To replace warm-holding in institutional catering

High-care area

• area with almost no contamination possible

Modified atmospheres

• = extra measure to control growth of MO in chilled foods and to control product quality

• Types

  • MAS = modified atmosphere storage

  • CAS = controlled atmosphere storage

• Mechanism

  • 78% N + 21% O + 1% of CO2, water vapour and other gasses

  • Proportion of oxygen ↓ and/or ↑ proportion of CO2 with specified limits in the atmosphere → maintain the original product quality and extend the shelf life