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ISSN 1392-2130. VETERINARIJA IR ZOOTECHNIKA. T. 17 (39). 2002


Jacek Kondratowicz1, Paulius Matusevi?ius2
1 University of Warmia and Mazury in Olsztyn, Department of Sciences of Commodities of Animal Raw
Materials. M. Oczapowskiego 5, PL–10-975, Olsztyn.
2 Lithuanian Veterinary Akademy, Department of Applied Zootechnology. Tilž s g.18, LT –3022 Kaunas,
tel.: 36 34 14

Abstract. Modern technology knows many methods of freezing. Their application depends on the kind of
product, scale of production, type of packaging, availability and price of refrigerants. One of the newest methods
employed at present for freezing food products is based on using liquid gases – mainly nitrogen and carbon dioxide.
Technologies in which liquid gases are used contribute to the development of the refrigeration industry. Research
results show that in Poland liquid gas freezing may be employed in the meat and forestry industries, fruit and
vegetable processing and drink production.
Keywords: food, methods of freezing, cold storage, quality, durability.


Santrauka. Šiuolaikini? šaldymo technologij? yra daug. Kuri? iš j? pasirinkti, priklauso nuo produkto r?šies,
gamybos apimties, ?pakavimo tipo, šaldytuv? ir kainos. Viena naujausi? maisto produkt? šaldymo technologij?
pagr?sta suskystintu duj? naudojimu. Dažniausiai naudojimas azotas ir anglies dioksidas. Lenkijoje atlikt? tyrim?
rezultatai rodo, kad suskystint? duj? naudojimu pagr?stos šaldymo technologijos tinkamos m sos, vaisi?, daržovi?,
g rim?, miško pramon je.
Raktažodžiai: maistas, šaldymo b?dai, šaltas laikymas, kokyb , patvarumas.

A Review. Looking from a historical perspective,
means of low temperatures, taking into account all
food preservation - aimed at extending its keeping quality
necessary technological operations [20].
– appeared probably much earlier than farming or animal
Food freezing is based upon:
breeding. Problems with finding food resulted in the need
- biose (reversible inhibition of natural life processes
to prolong its storage life. Also the fact that some plants
of products),
and animals were seasonal only made it necessary to find
- anabiosis (maintenance of some life processes in a
a method that would allow to store food products even for
latent form, characterized by a different degree of
several months.
Food storage was from the very beginning connected
- abiosis (irreversible inhibition of all life processes).
with simple forms of its preservation under natural
The papers presented during the International
conditions, such as: drying, smoking, salting, chilling and
Congress on Refrigerating Engineering, devoted to
freezing. With time those techniques developed into
‘Refrigeration in the Food Industry’, emphasized that the
independent scientific disciplines, making use of
process of freezing:
additional processing and enriching procedures.
- facilitates food reserve stocking,
In 1876 the ‘father of refrigerating engineering’
- increases production quality,
Charles Tellier for the first time in history organized
- contributes to the regulation of periodical
intercontinental sea transport of chilled meat from Europe
fluctuations in supply and prices,
to South America (12000 km) and back. The ship’s name
- facilitates international trade in food products,
was ‘Le Frigorifique’. It is also worth noting that in 1978
- reduces storage losses.
we celebrated the hundredth anniversary of opening of the
The global agricultural and fish production in 1997
first machine-cooled cold store in Chicago. Those events
was at the level of 5 165 mln tons [6]. 50% of that are
revolutionized world food economy and provided the
perishable products which require preservation at
basis for the development of a modern ‘refrigeration
individual stages of production. According to the studies
chain’. Cryogenic engineering started to develop on the
conducted by the International Institute of Refrigerating
turn of the 19th century, when Olszewski and Wróblewski,
Engineering, only 25 – 30% of perishable goods are
and Carl von Linde carried out liquefaction of gas.
properly protected with the help of refrigerating
Food freezing is a process of lowering inner
machines. Due to that food losses are still considerable,
temperature of products below the cryoscopic point. The
reaching 20 – 30% of food perishable weight.
freezing effect is achieved through water wintering
The above data indicate that in the future we should
(above 80% of its content). Products are kept in this state
aim not only at increasing production, but first of all at
during storage in order to limit physical, biochemical and
reducing losses. It follows that the development of food
microbiological changes which could decrease their
storage methods will constitute an important task.
quality. Viewed from a broader perspective, freezing
Modern technology knows many methods of freezing.
includes the whole cycle of product preservation by
Their application depends on the kind of product, scale of

ISSN 1392-2130. VETERINARIJA IR ZOOTECHNIKA. T. 17 (39). 2002

production, type of packaging, availability and price of
freezing started to develop. Their application was based
refrigerants. In practice ca. 90% of freezer rooms are
on using liquid gases, mainly nitrogen (the method of
equipped with an air apparatus, 9% - in a contact one.
Liquid Nitrogen Freezing – LNF) and carbon dioxide (the
Other types of equipment are used sporadically only – in
method of Liquid Carbon Dioxide Freezing – LCO2F) [2,
1% of freezer rooms [20].
As the topic discussed is very extensive, the present
paper will give only general characteristics of the basic
physicochemical properties which enable to satisfy the
freezing methods and machines used. The most popular
basic requirements of the refrigeration industry, such as:
methods, applied worldwide, are:
- short freezing time, ensuring the highest degree of
Ventilation freezing in tunnels, with a longitudinal or
preserving the original product quality,
crosswise forced flow of cold air. This method is applied
- continuous, automated operation of machines,
most often, despite the fact that its thermodynamic and
enabling almost complete elimination of operating
technological indices are worse than those of other
methods. It is characterized by simplicity and
- high efficiency, combined with limited room
universality, it is easy to apply and may be adapted to the
conditions of a given plant. The freezing cycle lasts from
- universality of application due to the fact that the
2 to 40 hours, depending on the type of equipment,
apparatus is characterized by light-weight structure and
product size and kind of packaging. The disadvantage of
may be easily taken from one place to another,
this method is high unit energy consumption (operation of
- high refrigerating capacity.
ventilators, temperature of ammonia evaporation from -40
The presentation of the above methods is connected
to –450C). Moreover, the phenomenon of lumping may
with certain terminological and classification problems.
also be observed in the case of crumbled products.
The technique based on using liquid gases is referred to as
Fluidization method is characterized by 30 – 40 times
cryogenic or cryogenous freezing, while such gases are
higher intensity of heat exchange than the ventilation
known as cryogenic fluids. According to the official
method. In this technique a stream of cold air goes
international terminology, cryogenic techniques are those
through the whole mass of a frozen layer of loose
in which temperatures lower than –1530C are applied.
products placed on a conveyor belt. This allows to avoid
This suggests that it is wrong to use the terms ‘cryogenic
lumping, even of dump products. The freezing process is
freezing’ and ‘cryogenic fluids’ in relation to methods
short, due to very intensive heat exchange. The
based on temperatures of ca. –78.50C (sublimation
disadvantages of this method are relatively high
temperature of CO2). Probably the best solution would be
investment costs and high consumption of power needed
to describe this group of methods as freezing with the use
to drive compressors and ventilators.
of liquid gases.
In both methods the refrigerating medium is ammonia,
LNF is based on pure nitrogen, obtained as the
used in a closed cycle (minimum losses).
product of air liquefaction or natural gas denitriding.
The method of contact freezing is applied rarely.
Nitrogen is chemically inert gas; it does not react with
Products are placed on metal trays. During freezing they
food products. Its evaporation temperature under normal
are in contact with pressure plates. In the past those
pressure is –195.80C.
plates were cooled with brine, now direct ammonia
There are three basic freezing methods:
evaporation is used. It allows to achieve high values of
- immersion – products are put into liquid nitrogen,
- spraying – products are in direct contact with
consumption. The disadvantages of this method are
nitrogen sprayed on them,
problems with work mechanization: it cannot be adapted
- freezing in a stream of cold nitrogen steam.
to continuous running and it is difficult to apply it in the
Each of those methods has both advantages and
case of belt-system production. It is employed mainly for
disadvantages. The immersion method has been almost
fish freezing on factory ships, as well as fish processing
completely eliminated, mainly for two reasons:
on land. However, the method discussed is becoming less
- the freezing process is very quick due to a great
and less popular in world refrigeration.
temperature difference, which causes micro- and macro-
The method of immersion freezing consists in freezing
damages of tissues,
products in air-tight packages in salt solutions or other
- the method uses heat of nitrogen evaporation only.
media. Good conditions of heat exchange result in a
The spraying method is commonly applied for
relatively short time of freezing, with the medium
freezing food products. In freezing tunnels of this type
temperature –200C. An immersion apparatus is extremely
cold nitrogen steam is used for pre-cooling and freezing.
economical (heat losses up to 7% of the total cold
Then products are sprayed with liquid nitrogen. If the
requirement, compared with 20 – 30% observed in
process is carried out until reaching the final product
ventilation and fluidization tunnels). At present this
temperature of -200C, nitrogen consumption is equal to
method is applied, in a limited range, for initial surface
1.0 – 1.5 kg per kg of product.
freezing of poultry carcasses (further freezing is
Liquid carbon dioxide freezing is similar to liquid
conducted in ventilation tunnels).
nitrogen freezing. Because CO2 under atmospheric
In the years 1963 – 1964 in the United States, and in
pressure has the form of gas or a solid body only, those
the years 1967 – 1968 in Europe, new methods of food
methods differ in the range of temperatures and rate of the


ISSN 1392-2130. VETERINARIJA IR ZOOTECHNIKA. T. 17 (39). 2002

process. The effective freezing rate of portioned meat by
In the case of delicate food products (e.g. berries), the
the LCO2 method at the temperature of -700C may vary
amount of drip in the thawing process is treated as a
between 5 - 8 cm of the frozen layer per hour. In the case
synthetic index of the product quality. It is also assumed
of liquid nitrogen, at the temperature of –1100C, it is
that the amount of drip during thawing under standard
higher than 10 cm per hour. Consumption of both gases
conditions may reflect the degree of tissue structure
is similar.
damage in the freezing process. For example, in the case
Freezing of food products causes changes in their
of strawberries the drip amount may be limited
quality, connected directly with this process and further
considerably if freezing lasts for 10 – 12 minutes.
cold storage [5]. Freezing does not inhibit completely the
However, its further acceleration (usually connected with
bio-physicochemical processes taking place in food; it
a significant increase in costs) produces a marginal effect
only limits them or changes their course. The effect of
freezing on food quality varies, depending on individual
As regards beef, the correlation between the drip
products. It is generally believed that it is the most
amount and freezing rate is slight, compared with fruits.
significant in the case of fruits and vegetables [11, 12],
It should be kept in mind that if the freezing rate is
and slighter in that of meat and meat products [13, 14].
very high (above 10 cm/h), as in the case of liquid
An important role is played here by the rate of freezing
nitrogen, considerable mechanical stresses may appear in
and formation of ice crystals.
the surface layer of frozen products. This can result in
Structural Changes. Research results confirm the
breaking and damaging of tissue structure. The
destructive effect of freezing on the tissue structure of
phenomenon discussed is especially well visible while
food products [9, 22]. Good quality of frozen products is
freezing food products which contain large amounts of
connected with the size of ice crystals. When the
water (e.g. tomatoes) and meat elements whose thickness
temperature falls below the cryoscopic point, the so called
exceeds 10 cm. It follows that cryogenic methods of
‘crystallization centers’ are formed, surrounded by
freezing should be quick enough to prevent the
numerous ice crystals. The more quickly the temperature
falls, the more crystallization centers appear and the
microbiological processes, particularly at the initial stage
smaller ice crystals are. This is directly related with less
of freezing.
serious damage of the structure of frozen products.
As concerns sensitivity to the rate of freezing, food
In our studies on the ultrastructure of pig’s muscular
products may be divided into four groups:
tissue frozen employing the ventilation method [9],
1. products whose quality is not affected by the rate of
damages of muscle fibers were observed. They included
freezing (green pea, blueberries),
fiber breaking, in some cases rather extensive. Complete
2. products not sensitive to changes in the freezing
disintegration of the sarcoplasmic reticulum and
rate, except for the rate lower than 0.3 cm/h (e.g. fish),
mitochondria was noted, as well as glycogen structure
3. products whose quality improves significantly if the
‘blurring’. Also the sarcoplasmic, basement and nuclear
freezing rate increases to the level of 5 – 8 cm/h with the
membranes suffered serious damage and disintegration.
application of liquid gases (berries, portioned meat, some
Large infiltration zones were observed both within and
vegetables, mushrooms, forest fruits),
beyond muscle fibers.
4. products which crack when subjected to very quick
The ultrastructure of muscles frozen by means of
freezing – above the level of 10cm/h (animal carcasses,
liquid nitrogen was characterized by better preservation of
large-size products).
the sacroplasmic reticulum, structure and uninterrupted
When Poland joins the European Union, we will have
sequence of the sarcoplasmic and basement membranes.
to apply the methods of food evaluation and quality
Most infiltration zones were found within muscle fibers,
standards which are binding in the Union countries. One
and only few beyond them.
of the ways of improving the quality of chilled and frozen
Therefore, our research confirmed the opinion that the
food is the use of liquid gases in the technology of low
slower the freezing process is, the more serious damage is
temperatures. This may facilitate the obtaining of the
observed in fibrous and non-fibrous elements of muscular
certificates of the ISO 9000 series, and the introduction of
the HACCP system.
The phenomenon of the so called ‘recrystallization’
Quality of frozen food is an ambiguous term, which
may occur during cold storage of food. This means that
includes different groups of distinguishing features,
bigger ice crystals, distributed between muscle fibers, still
determining the product quality. We can talk about
increase in size, contrary to smaller ones located in fibers
sensory quality (positive features of a given product,
[4]. The process of recrystallization, caused by
required by customers, that may be evaluated by the
senses), consumer (degree of satisfying customer’s
disappearance of differences between the quality of
nutritional needs) and health (lack of pathogenic
products frozen quickly and slowly. Due to that, although
microorganisms and harmful substances). According to
specialists agree that quick freezing brings better results
world trends, an organoleptic evaluation should be
as concerns quality, the freezing process is not considered
combined with objective instrumental and analytical
the most important factor among those influencing the
methods [8, 17].
final quality of frozen products. The primary role is
Colour is the feature of food products that is visible
played by storage conditions and ways of thawing.
and evaluated at first sight, and its changes are the first

ISSN 1392-2130. VETERINARIJA IR ZOOTECHNIKA. T. 17 (39). 2002

indicators of quality deterioration [1]. Very quick
losses are observed in beef, lower in veal and lamb, the
freezing (e.g. with the use of liquid gases) causes the
lowest – in pork.
formation of tiny ice crystals also on the product surface,
The data on thawing drip from fruits and vegetables
which produces an optical effect of its ‘whitening’. From
differ, as regards both its amount and method of thawing.
the consumers’ perspective, this effect may be both
For example, drip from strawberries may reach the level
positive, e.g. in the case of poultry, and negative, e.g. in
of 34 – 36% and depends on the rate of freezing.
that of red meat. It should be noted, however, that the
Protein denaturation and biochemical changes of fats
‘whitening’ of products frozen cryogenically is a purely
and lipoproteids take place during long thawing of meat
physical, reversible phenomenon, which disappears in the
products. Considerable losses of vitamin C are observed
process of thawing.
in fruits and vegetables. They are the highest (60 – 70%)
An extreme form of colour changes observed in frozen
in species susceptible to enzymatic browning. The rate of
food may be the so called ‘frost scald’. This
microbiological changes noted in food products in the
phenomenon, which constitutes a particular form of
process of thawing is much faster than in the same
dehydration of the surface parts of frozen products
products, characterized by similar initial microflora, not
(caused e.g. by slow freezing and long cold storage), may
subjected to freezing. It follows that biochemical and
occur not only in plant and animal tissues, but also in fruit
microbiological changes in products caused by their
juices or mushrooms.
improper thawing may result in making food unfit to eat.
Freezing by means of liquid gases has a positive
It should be assumed that the optimum quality effect of
influence on the sensory quality of frozen food.
thawing is achieved when the period of time needed for
Systematic studies were conducted on meat products of
freezing and thawing of a given product is similar.
different composition, frozen applying the ventilation
At present the possibility of using liquid gases for
method, liquid nitrogen and carbon dioxide [9, 10].
food freezing in Poland, the application of certain
Products subjected to very quick freezing were
technical solutions and prospects for the implementation
characterized by significantly better quality, palatability
of very quick freezing methods depend on the economic
and texture, both directly after freezing and after several
efficiency of freezing. The question of employing new
months of cold storage. Beef frozen in this way showed a
preservation technologies in the agricultural and food
lower degree of fat oxidation, brighter color and better
industry in Poland seems to be very complex. The
water-holding capacity, compared with ventilation
problems in this sphere include:
freezing. No considerable differences in the above
- lack of relevant technological and economic
indices were found between freezing in liquid nitrogen
information in the environment of home food producers
and carbon dioxide [18].
and processors,
To sum up, we can confirm a beneficial effect of
- lack of home producers of cryogenic machines,
freezing in cryogenic fluids on the quality of numerous
- high prices of imported equipment, which makes it
products of plant (fruits) and animal (cured meat, poultry)
impossible for small entrepreneurs to buy it.
origin, resulting from high freezing rate and influence of
Due to an increase in the production of liquid
neutral atmosphere. This kind of influence is reflected by
nitrogen, which is a by-product of oxygen production, the
inhibition of bacterial infections, extended storage life
only real possibility of making use of its surplus is food
and increased salubrity of food. The bacteriostatic and
fungistatic activity of carbon dioxide (which decreases the
Despite the difficulties mentioned above, the results of
level of pH in frozen food) should also be emphasized.
research works conducted at the University of Agriculture
Biochemical and microbiological processes moderated
and Technology in Olsztyn, obtained in the years 1975 –
or inhibited during freezing are ‘accelerated’ in the course
1999, enable to state that under Polish conditions the
of thawing [9]. It may be assumed that biochemical
technologies based on using liquid gases may be
changes connected with water wintering in food products
employed in:
take place during thawing as well. However, their range
1. the meat industry [13, 14]
can be different. Apart from enzymes resistant to freezing
- gaseous carbon dioxide may be applied for stunning
and storage at temperatures lower than cryoscopic, there
animals before slaughter, which has a significant, positive
are also less resistant ones whose activity decreases.
effect on bleeding and meat quality,
Moreover, enzymes may be additionally released from
- for freezing deboned meat (beef, pork, horseflesh)
cell organelles damaged in the process of freezing
which is to be exported or stored for a long period of
(especially slow). They are often more active in the new
environment than previously. For instance, improper
- for freezing portioned meat (beef, pork, horsemeat)
meat freezing directly after slaughter (or after 24 hours of
according to the two-stage method, i.e. pre-freezing to the
chilling) may result in the so called ‘thawing contraction’.
temperature of -30C in liquid gas and further freezing to
This leads to irreversible deterioration of meat quality,
the temperature of -200C in a ventilation tunnel. This
especially as concerns its technological properties.
method allows to maintain high quality of meat,
An index of reversibility in the freezing process is the
comparable with one-stage cryogenic freezing, and reduce
amount of thawing drip. With this drip food loses many
the consumption of liquid gases by ca. three times,
valuable ingredients, such as soluble proteins, peptides,
amino-acids, vitamins and mineral salts. The highest


ISSN 1392-2130. VETERINARIJA IR ZOOTECHNIKA. T. 17 (39). 2002

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