Meat preservation part 3

Dr. Mohamed Abdelfattah Maky

Food Hygiene Department

Freezing

The aim of freezing is to preserve meat for a long duration

Freezing means lowering the temperature of meat below the freezing point of

meat which is -1 to -1.5 oC.

It's a bacteriostatic method of preservation

Methods of Freezing 1-Slow freezing:

By slow freezing for long time a bigger , also macroscopic visible ice crystals

which partly in the muscle fiber and partly in the intercellular spaces are formed.

They lead to changes of structure of muscle fibers by thawing, the frozen water

(big crystals) could not completely reabsorbed and thus drip down or sweet.

2-Quick Freezing

Rapid freezing at -46 oC for 30 minutes.

Used for minced meat, poultry, fish, meat slices and small products of meat.

Used for freezing of small size meat or meat products.

Formed ice crystals are of small size (ultrasturctured) and within muscle fibers ,so,

quantity of weep is small (actually has no drip).

The amount of weep is inversely related to fat content so weep in veal is more than

beef more than sheep.

Advantages of quick freezing:

1- There is relatively little denaturation of protein and little dehydration as the ice

crystals are small that they cause little rupturing of cell wall thus when the product

thawed most of liquid resulting from the small crystals is reabsorbed and the

cooked product possess an excellent favour and texture.

2-The products are quickly cooled below the temp at which bacteria, moulds, and

yeasts growth occurs thus preventing decompsotion during freezing

Zone of maximum ice formation (ZMIF)

It is the range of temperature at which the maximum amount of ice is formed

and at which the size of ice crystals is the largest size.

This range of temperature is -1.5 to -5 oC

The effect of such destruction appear after thawing in form of drip.

During the freezing process escape of water from the muscle fiber, which

convert to ice crystals of large size between the muscle fibers during the

thawing, these large crystals lead to destruction of cell membrane of muscle

fiber, appear in form of drip.

Weep or drip consists of escaped water, minerals, colloids, albumin and

destroyed myoglobin,

Zone of maximum ice formation (ZMIF)

Durability of frozen meat:

Beef, if stored at -18 oC, one year. In Egypt it is 9 months

Lamb and veal at -18 oC, 8-9 months

Pork at -18 oC , 6 months

Poultry, 8 months

Offal 4 months (liver for 7 months at -20 oC).

By decrease of temperature, the water will be evaporated and converted to

ice. Avoid change of all water of meat into ice because that lead to

irreversible change in protein (denaturation) and salting out, which become

unable to tie the water inside the meat, so increase of weep.

Changes in frozen meat1-Changes before freezing: Bone taint

2- Physical changes:

2.1. The physical state of the muscle protein

2.2. Ice crystal formation

2.3. Loss in weight

2.4. Darkening in color

Changes before freezing (Bone taint)

The muscles around bones as well as near the joints (scapula and pelvic

bone ) appear dark brown in colour an lacerated.

These changes occurs due to escape of microorganisms from gastrointestinal tract

to these parts as a result of slaughtering of unhealthy animals,

stressed animals or delaying of evisceration.

The causative agent is anaerobic bacteria (Clostridium sporogenes)

It is unfavorable condition and condemnation should be occurring.

It is detected by steel tryer.

Loos in weight the approximate loss per 4 weeks of unwrapped meat has

been found to be 0.2% at -30oC and 1.5% at -8oC. Wrapping of meat in

polyethylene reduce the loss in weight per 4 weeks frozen storage to 0.05% at – 30

C and to 0.16% at 8oC

Freezer burns

Definition: It is tiny, dry, wrinkled area of button shape like resulting from

evaporation of ice crystals that leads to irreversible changes in meat protein, so

freezer burns. After thawing of such meat, it becomes unacceptable, repulsive and

of disagreeable taste and odour especially if this defect is diffused.

Freezer burns

Mechanism:

When meat is stored in frozen condition in an atmosphere of low relative humidity,

the exposed tissue dries rapidly , as drying progress , the colur of the surface tends

to change to a pale amber and the consistency of the surface tissue become dry

and shriveled. This change is caused by evaporation of ice crystals leaving behind

tiny air pockets which tend to scatter the incident light and cause the tissue to

appear lighter in colure

Freezer burns

Judgment : It's unfit for consumption.

To overcome this defect

a. meat should be covered by material not allow to escape of water, as the

evaporation may be arrested and freezer burn avoided if the offals are

enclosed in parchment paper before being paked in boxes.

b. freezer burn in frozen poultry can be avoided if the birds are paked in boxes

lined with two layers of aluminum foil and grease prof paper.

c. The tight-fitting , moisture proof packing and stbale storage temp are the

most important requirements for preventing freezer burn.

Freezer burns

Brine sucking or stainingCalcium chloride is now more commonly used in the brine circulating system for

chilling or freezing of meat than brine . so the term brine staining is therefore

applied to contamination by either of these liquids, meat stored in refrigerating

chambers with imperfect joints or leakage in the circulating system may become

covered or partly covered with brine (calcium chloride or ammonium hydroxide)

which produce a characteristic dull or pale greenish colure on the carcass surface.

The staining frequently penetrate and darkens the muscular tissue which acquires

an extremely bitter taste.

Judgment:

Carcass or meat affected with brine satin may safely be released after trimming ,

If sucking of meat is extensive, meat becomes unfit for consumption

Microbial changesCauses of mould formation in frozen meat:

a) Temperature flactuation

b) High humidity and bad ventilation of storage freeze chambers help

moulding

c) Moulds can grow slowly on frozen meat at temp of - 7oC to – 9 oC or even

lower. Yeasts grow at -9 oC and above.

Mould growth in frozen meat are

1.Black spots is due to Asprigellus niger and Cladosporium herbarum.

Cladosporium herbarum is more dangerous because it is microaerophilic and

able

to penetrate the meat surface and grow at depth of 0.5 cm from the meat

surface.

2.White spots due to Sporotrichum

3.Bluish green spots due to Penicillium species and Aspergillus species.

Weeping or drip

Weeping denotes the presence of a watery blood-stained fluid which

escapes from

frozen meat when it is thawed and consist mainly of water, together with

salts,

extractives, protein and damaged blood corpuscles. The latter are

responsible for

the pink coloration of the fluid and are readily recognizable on microscopical

examination. Weeping is an undesirable feature and is caused partly by the

rupture

of the muscle cells and tissues by crystals of ice.

Effect of freezing on spoilage, pathogenic microorganisms and parasites

During freezing , micrograms suffer multiple damage which may cause their

inactivation immediately or later. Freezing leads to temperature shock or may

cause metabolic injury or damage to the plasma membrane.

Gram negative bacteria, particularly mesophiles appear to be more susceptible to

this cold shock than gram positive bacteria.

Some bacteria are destroyed by freezing , but other low temp merely inhibit their

growth and multiplication until conditions favorable to their growth .

Freezing is therefore of no great value in rendering a carcass affected with

pathogenic bacteria safe for human consumption .

Freezing has no great effect on pathogenic bacteria, for examples

o Anthrax bacilli can withstand a temperatureof-130 0C

o Salmonella can withstand exposure to-175 0C for 3 days,

o Tubercle bacilli have been found alive after 2 years in carcasses frozen at-

100C.

o The virus of foot-and-mouth disease can remain viable for 76 days

o Under similar conditons the virus of swine fever may remain infective in

bone marrow for at least 73 days and the virus has been shown to be viable

in frozen pork for 1500 days.