Faster freezing times, which can be achieved by using cryogenic processes,

4. Conclusion 5. Tell me more

lead to better organoleptic characteristics, with water retention capacity
being closely linked to other qualitative descriptors of meat such as Various studies on changes in
About the author
tenderness and juiciness. the state of food products have
shown the impact of the quality A graduate of ENSIA (Ecole Nationale Supérieure des Industries
Table 3 quantifies the losses at freezing and defrosting according to the Alimentaires), Adrien Agoulon has been Director of Agro-Hall since 2010,
of freezing and storage methods
freezing technique used. Here, there are significantly higher losses when after heading up Research & Development at the technology center for
on the organoleptic properties
using a mechanical freezing tunnel (+625% at -20°C and +300% at –40°C) eight years.
of the products after defrosting.
compared to the test carried out using cryogenic freezing.
Freezing rate is the key parameter

Impact of freezing parameters

Based in Evreux (France), Agro-Hall was formed in 1986 to provide
• Dehydration for ensuring optimal freezing. companies with tailored support for their R&D and innovation projects.
Cryogenic freezing is a particularly
During the freezing process, products may suffer high levels of dehydration
good way of preventing organoleptic

on the characteristics of food

due to the sublimation of surface water. This dehydration may lead to a
deterioration (loss of texture and
significant loss of weight, up to 10% in certain cases, as well as affecting
dehydration) in sensitive foodstuffs
the taste of the product. Using cryogenic freezing processes enables a
such as seafood, fruit and meat.
significant reduction of this dehydration (up to 10 times less dehydration)
(Löndahl et al., 1195) compared to mechanical freezing.
The level of dehydration is directly linked to the tangential velocity of Adrien Agoulon
the gas (air, N2 or CO2) in contact with the product. Mechanical freezing
requires a high tangential velocity in order to limit the length of time Director, Agro-Hall
required for the freezing process, resulting in significant dehydration for
certain foodstuffs.

Table 3: Loss of weight (in %) with different freezing/defrosting

processes for ham
Freezing x defrosting Freezing loss Defrosting loss (microwave)

-20°C tunnel x microwave 0.50 0.80

-40°C tunnel x microwave 0.24 0.70

Liquid nitrogen x microwave 0.08 0.58

(1) From Jacquet et al. (1976)

The efficiency of the thermal transfer is also dependent on the difference in

For more information about Air Products or freezing technologies
temperature between the surface of the product and the freezing gas. Since To discuss the best freezing technology for your industrial unit, or for a no
cryogenic freezers work at lower temperatures, a higher tangential velocity obligation analysis of your current freezing process, please contact one of
is not the only way of achieving rapid freezing. the Air Products experts:
In addition, the saturated vapour pressure (pressure at which the gaseous Air Products Air Products
phase and solid phase are in equilibrium) is approximately halved for every 2 Millennium Gate Unit 950, Western Industrial Estate
decrease in temperature of 10°C. Since cryogenic freezers work at a lower Westmere Drive Killeen Road
temperature, the saturated vapour pressure at this temperature is lower Crewe CW1 6AP Dublin 12
and dehydration is consequently less. United Kingdom Ireland
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Abstract :
Various studies on changes in the state of food products have shown
tell me more the impact of the quality of freezing and storage methods on the
airproducts.co.uk/food organoleptic properties of the products after defrosting. Freezing rate
airproducts.ie/food
is the key parameter for ensuring optimal freezing. Cryogenic freezing
is a particularly good way of preventing organoleptic deterioration
(loss of texture and dehydration) in sensitive foodstuffs
© Air Products and Chemicals, Inc., 2012 (EU002348)  332-13-001 -GLB such as seafood, fruit and meat.
1. Introduction 2. The freezing process Table 2: Effect of freezing rate on the sensory properties of green beans • Mechanical alteration

(Extract from the FICUR booklet)

Time required for centre Sensory characteristics Slow freezing techniques, where the
Sales of frozen foods are on the Contrary to what is widely thought, frozen food is not entirely stable.
 Ice crystals to reach -20°C of cooked product Type of freezing freezing front advances at a rate of
increase globally. Freezing protects Although it is microbiologically stable, the product remains susceptible
less than 0.2 cm/h, impacts on the
the quality of food at a competitive to physical and biochemical reactions, which may compromise its  Cell nuclei No loss of firmness, Cryogenic freezing
< 30 minutes size and number of crystals. Figure 3
price. The freezing technique itself, organoleptic qualities. This is more easily understandable if we look at the colour and taste as normal (liquid nitrogen)
 Cell walls illustrates the impact of the freezing
just like the frozen food market, proportion of frozen water that we find in various frozen foods. Let us take
Loss of texture, Ventilated rate on cells, and consequently on
is developing to become faster, animal products as an example, as in Table 1. 30 minutes-2 hours
colour and taste as normal mechanical freezing organoleptic properties.
more efficient and more cost-
Table 1:
effective. In order to continue to A-Slow freezing B-Fast freezing Loss of texture, Non-ventilated The large crystals that form under
Influence of temperature on the percentage of frozen water in meat > 12 hours
serve the interests of consumers, Figure 1: Illustration of crystal formation according to freezing kinetics colour and taste deteriorated mechanical freezing slower freezing conditions generate
who demand high-quality Frozen water (%) mechanical pressure and sharp
CJ Kennedy, GP Archer, 1999. Maximising Quality and Stability of Frozen Foods, Report 2, 14.
products, both in terms of taste and Temperature (°C) Poultry (muscle) 1
Lean beef 1,2
Fish (cod) 1,2 edges that damage all cellular
Loss of texture, resulting from the loss of cell turgidity, occurs first. Due
nutrition, but who are willing to
-5 74 74 77
3. Impact of freezing on food structure to the rapidity of cooling in cryogenic freezing, freeze-concentration
components (e.g. organelles, cell
spend less and less time preparing membranes etc.). This damage
meals, we need to have a better 3.1. Characteristics of the different freezing technologies is limited and ice crystals are formed in both the intracellular and
results in bringing together
-10 83 82 84
understanding of the fundamentals extracellular spaces. This is because the amount of cold transferred cannot
Before looking in more detail at the impact of freezing technologies on food structure, it is necessary to recall the enzymes and substrates that were
of this process and its effect on -15 85 87 be absorbed by the formation of crystals in the extracellular spaces alone,
principal characteristics of the technologies used at an industrial level. previously separated, and therefore
food. This article aims to present so nuclei form in a homogeneous manner throughout the matrix (cf. Figure
-20 88 87 89 – Mechanical cooling: this is the term used to describe traditional vapour compression cycle freezers, which leads to differing enzymatic
the existing scientific data on the 1). The homogeneity of the nucleation sites therefore limits the effects of
normally operate at a temperature of between -20°C and -50°C. reactions. The organoleptic qualities
impact of freezing technologies on -25 osmotic pressure which would cause loss of texture.
– Cryogenic freezing is achieved using cryogenic liquids such as nitrogen or carbon dioxide, and normally of the meat that are affected by the
the organoleptic characteristics -30 89 88 91 operates at under -70°C. Deterioration of colour and taste varies widely according to storage freezing method are essentially the
(e.g. taste, texture, smell and (1) From Reidel (1957), cited by Desrosier and Tressler Fundamentals of Food Freezing, ix, 629p., conditions and the nature of the frozen product. Nevertheless, it is clear tenderness, the juiciness and the
These notable differences in temperature result in very different freezing kinetics between the two technologies.
appearance) of sensitive foodstuffs AVI Pub. Co. (1977). that the presence of enzymes and their substrates in a confined volume water retention capacity.
(2) IIF (1986) Figure 2 shows the typical freezing time (the time required for the temperature at the center of the product to
such as meat, fruit and vegetables or of liquid creates conditions for fast-acting, damaging reactions, despite
(3) Leistner and Rodel (1976), cited by Daudin, Technologie de la viande et des produits carnés, drop from -1°C to -7°C) of a steak of beef (200 g, 2 cm thick) according to the freezing technique used.
even seafood. Chapitre 1 (1988) the low temperatures. Rapid freezing (i.e. cryogenic freezing) stabilizes
The large temperature difference between product and freezing medium of the cryogenic technique results the environment by limiting the flow of matter through the tissue. Liquid
In a product such as meat, at a stabilized temperature of -20°C, the in significantly shorter freezing times than mechanical freezing. These cooling kinetics are to a large extent phases therefore retain lower concentrations of dissolved substances,
percentage of non-frozen water is still over 10% of the total water responsible for the differences in levels of deterioration seen in the finished product when we compare the impact resulting in slower enzymatic deterioration (oxidation, hydrolysis and
content of the product. This liquid portion has specific characteristics. of the freezing technology with the quality of the food. decarboxylation, for example).
It is progressively enriched with various dissolved substances as more
The acceleration of these enzymatic reactions in slow cooling also leads to
and more of the water forms ice crystals. This is referred to as the freeze-
a second alteration mechanism based on the mechanical pressures caused
concentrated phase, during which a high number of changes occur
by the ice crystals.

Typical time (min)

as frozen products age. Essentially, this phase allows the regrouping
of enzymes and their substrates within a restricted volume of liquid,
which can accelerate certain reactions despite the retardant effect of low
temperatures. (c)
Fast freezing
Cracks
The kinetics of the decrease in temperature during the freezing
process (and therefore the freezing technique used) will influence the
characteristics (number and size) as well as the growth of crystals:
freezer –14°C freezer –20°C cryogenic tunnel cryogenic tunnel Temperature fluctuations
• Nucleation rate. This is the number of nuclei formed per unit of time. –40°C –78°C
(b)
This parameter increases with faster cooling rates. For example, each Intermediary speed

degree of subcooling multiplies the nucleation rate by 10.

• Growth rate of crystals. The growth of crystals is linked to the capacity Figure 2: Typical freezing time of a steak of beef according to freezing technique
to remove heat where the ice crystals are forming (related to the Drip loss
characteristics of the product and temperature of the medium).
3.2. Impact of freezing on food quality
Muscle Meat (a)
• Size of crystals. This depends on the two previous factors. The • Freeze-concentration Slow freezing
Temperature fluctuations
formation of a large number of nuclei, as well as a rapid growth rate,
Slow freezing (freezing front advances at a speed of less than 0.2 cm/h) leads to the formation of ice crystals
encourages the formation of small crystals.
in the less concentrated extracellular space. The progressive concentration of the extracellular space (freeze-
In many cases, deterioration caused by ice can be explained by the rigidity concentration) results in the dehydration of the cells via osmosis. The leaking of intracellular liquid results in a
and the size of crystals within the cellular structure of the food. This weakening of the tissue and a loss of turgidity in plant tissue. Table 2 shows the qualitative differences observed
results in mechanical pressure that can damage the cellular structure of between cryogenic freezing with liquid nitrogen, ventilated mechanical freezing and non-ventilated mechanical Drip loss
Juiciness↘:  Tenderness↘ ?
products such as meat, fruit and vegetables. Figure 1 shows the size and freezing.
distribution of crystals depending on the freezing kinetics.
In the case of green beans, deterioration, which was very minor under cryogenic freezing, was most noticeable Figure 3: Illustration of the impact of the freezing rate on the quality of meat
when the time required for the centre to reach -20°C exceeded 30 minutes. Illustration taken from Congélation et Qualité de la Viande, Claude Genot, INRA Edition, 2000.