Cooling and freezing


1. Cooling

Bread has to cool down in a proper manner basically for two reasons:

The main cause of microbial infections is (the inevitable) high humidity in the storage area and condensation on the bread if there is no adequate temperature control in the storage area. All storage areas must be kept clean, well aerated and free from foreign odours. Where forced airflow is used, the air intake should ideally be filtered before entering the cooling area.

On removal from the oven, a loaf of bread loses heat to the surrounding atmosphere and simultaneously loses weight due to moisture loss. The temperature of the crust of a bread as it comes out of the oven is somewhere between 150°C and 180°C depending on the baking conditions. At the interface between crumb and crust the temperature is 100°C (see chapter on baking Bakery technology - Baking). The moisture content of the crust is then somewhere between 0 and 3 %. Also as noted in the chapter on baking, the moisture content of the crumb is higher then the moisture content of the dough (± 45 %).

During bread cooling there is a considerable temperature gradient between the crust and the crumb. With progressive cooling this gradient gradually reduces to zero. On the other hand the moisture gradient between crust and crumb is rapidly reduced to zero and doesn't change much during cooling and storage. An obstacle to the rate of moisture loss during cooling is a thin boundary layer of still air which clings to the external surface of the bread. This thin layer impedes the free external diffusion of moisture into the atmosphere.

2. The technological cycle

The quality of fresh bread is often related to its crust (thickness, crispiness, colour and taste) and to the structure of the crumb (flavour, softness, cell wall thickness and cell size). Unfortunately fresh bread is a product with a short shelf life and a number of chemical and physical changes, known as staling, take place during storage. As a result of these changes the bread gradually loses its freshness and crispiness while the crumb firmness and rigidity increase. The pleasant aroma vanishes and off tastes can be detected. So the basic challenge for the baker is to get is product as fresh as possible to the market. This can be done by making sure that the moment of baking is as close as possible to the moment of consumption. For this reason frozen dough and bake-and-serve products were developed. The following picture gives an overview of what I call the "technological cycle".

overview technological cycle

It shows that you can interrupt the normal production cycle at various points. Immediately after moulding for instance. If the dough is frozen at that point, frozen dough is obtained. But the dough can also be frozen after proofing (pre-proofed frozen dough) or after baking or partially baked (so called parbaked frozen bread) etc. Each technique requires specific raw materials and special attention to the process parameters. The baking industry has changed profoundly because of the introduction of sub-zero techniques.

These developments let to the commercialisation of semi-finished (i.e. not ready to eat) baked goods. The more important technologies are:

Refrigeration is used to reduce the activity of yeast and in most cases bakers refer to "retarded" dough. Most of the time the baker will use refrigerated dough to reduce the hours he has to work during the night i.e. he can mix and shape the dough late morning or in the afternoon and keep it in the fridge until the next morning to put in the proofer and bake it in order to have freshly baked bread when the shop opens. The difference between a refrigerator and a retarder, is that the latter has better humidity control. If the humidity in the retarder would get too low, the dough pieces might dry out and a layer of dry dough will be created. Normally dough pieces are stored somewhere around 0 to 4°C. Also negative temperatures are possible but I wouldn't go lower than -2°C. The lower the temperature, the lower the rate of gas production by the yeast but even at negative temperatures some gas production occurs.

In the case of frozen dough temperatures around -20°C are used. In that case yeast activity stops and other physiochemical changes do not occur or occur very very slowly. There is some evidence that even at these low temperatures enzymes show some activity. There is some debate about the final temperature the dough piece should reach. I believe that the temperature in the core of the product should reach at least -15°C. If the temperature of the warehouse is lower, the temperature of the products, which are normally packed in bags and cartons and stacked on a pallet, will only slowly go down. Imagine the warehouse is at -20°C and we put packed products of -15°C in that warehouse it will take days, even weeks before the products reach -20°C. More dangerous for the quality of the products and the so-called freeze burn are temperature fluctuations during storage. These fluctuations cause moisture loss and formation of ice in the bags. The moisture loss in its turn gives after baking, sub-standard products. On top of that this kind of products are disappearing from the market: they require thawing and proofing before they are baked. These steps have to be carried out gradually so the product is completely defrosted in the centre before it is fermented. If not the core of the product will have a tight structure after baking. Defrosting times therefore are very long (overnight in a fridge at 5 - 8°C, is the best way) in order to obtain an uniform temperature in the thawed product. This is also the reason why this technology is used for small products like rolls, buns etc.

In order to counteract the setbacks of frozen dough, (pre-)proofed frozen dough was developed. The dough pieces are fermented before they are frozen. The freezing is best done in two stages. First stage is rather cooling down the fermented dough pieces and once they have reached a temperature let's say below 10°C, they can really be frozen using air temperature around -30°C. Note that also the air speed has an important effect on the quality of the finale product. It is advised to use airspeeds lower than 5 m/s. This technology allows that the product is being baked after a relatively short period of acclimatisation at room temperature. This technology works particularly well for laminated products such as croissants and Danish pastry.

For parbaked products, the main challenge is to ensure that sufficient heat reaches the centre of the product in order to obtain a stabilised crumb. It is therefore necessary to bake the dough for a sufficient long period. Crust colour can be white or yellow or pale brown. But that depends mainly on the baking temperature. The darker the crust, the bigger the risk that the crust will flake during the freezing process or during storage at negative temperatures. This can be avoided by leaving the products at room temperatures before freezing, so an equilibrium between the crust and the crumb can be reached before freezing. Please see also the chapter on baking.

For all these technologies, the mixing is performed in the same way as for fresh bread. The dough is best scaled immediately after mixing, moulded and frozen before commercial distribution. It should be completely developed in the mixer, but have a minimum of yeast activity and gas generation before freezing. A dense dough has the best heat conductivity to facilitate rapid chilling. For these reasons, a rapid or "no-time" process of dough development and maturing is most suitable for frozen or retarded dough. This means a process in which the dough is fully developed in the mixer in the presence of rapid-acting oxidants, such as ascorbic acid. The finished dough temperature should be relatively low, but remember that below 16°C, it is difficult to develop a good strong gluten network. A finished dough temperature in the range of 20°C to 23°C is most suitable.

Dividing and moulding of dough pieces are carried out as in normal bread making. After the final moulding, freezing of the dough pieces should commence without delay. Yeast stability in frozen dough is inversely related to the time of active fermentation before freezing. Cells in active fermentation have a thinner plasma membrane than dormant cells and therefore they become more susceptible to cell damage. Besides, fermentation products such as ethanol and other volatile compounds, have a negative effect on the fermentative activity of living cells. To compensate the loss of yeast activity, a greater amount of yeast is used in the preparation of frozen dough. For this reason one can use the technique of delayed addition of the yeast to the dough during mixing. Add the yeast as late as possible to the mixer taking care of course that it still gets evenly dispersed through the dough.

In the bake-off shops the pieces are thawed, fermented and baked on demand. The disadvantage of this system is that it takes quite a while to thaw and ferment the dough. Depending on the size of the product is easily takes 3 to 5 hours before the product is ready to be sold. Another draw back of this system is that the products must be maintained at all times at -18°C. In the production plant itself that is not so difficult to achieve but during transport and at the point of sale it is not so easy. Freezers are too small or are to be used for products with a higher margin etc.

The important points relating to ingredients and the formula for frozen dough are:

  1. Adequate oxidation is essential for the "no-time" dough system to mature the dough fully; reducing agents, which are commonly used in bread making to soften the gluten and assist dough clearing, may shorten storage life of frozen dough.
  2. Yeast content of the dough should be higher than normal to allow for some loss of activity during freezing and storage, and for any inadequacy in proofing conditions. The inclusion of four to four and one-half percent compressed yeast is usually satisfactory. Active dried yeast of the type intended for direct addition in dough mixing is also satisfactory for frozen dough.
  3. The addition of shortening at four to six percent of flour weight or less, with the inclusion of a surfactant such as diacetyl tartaric acid ester or sodium stearoyl-2-lactylate, is recommended.
  4. Flour having a protein content of 11 to 13 percent, with a laboratory recording dough mixer development time of four to eight minutes is suitable. Starch damage should not be high.

Considerable effort has been made to establish quality parameters affecting frozen dough. It is known that the overall quality of bread dough deteriorates gradually during storage at -18°C. This can be seen because there is an increase in proofing time, a decrease in loaf volume and poor bread characteristics. The two factors playing a major role in this deterioration are:

  1. a decrease in gassing powder of the yeast due to a decline in its viability and activity
  2. the gradual loss of dough strength

Yeast is a living micro-organism and during fermentation it produces CO2 which is the main reason why the dough expands during proofing. Yeast is able to produce CO2 over a wide range of temperature. It has it's optimum temperature around 38°C and yeast cells are killed at around 55°C. As the temperature goes down, the CO2 production slows down and basically stops around -1°C. But apart from the fact that the yeast stops producing carbon dioxide, other important changes are taking place, one of them being the damage of the membrane of the yeast cell. Disrupted yeast cells release proteolytic enzymes and reducing agents, in particular glutathione, which have a negative effect on the capacity of the dough to retain the CO2 produced by the yeast.

There are also fierce debates about the freezing rate: what is the best system? Slow freezing or rapid? As a matter of fact both have positive and negative effects. In the case of slow freezing, the salts and other ingredients in the dough will get more and more concentrated in the non-frozen water.

Both yeast cell viability and fermentative capacity are affected by dough freezing. The volume of CO2 produced has been utilised as a measure of yeast activity. All the research done concluded negative effect of dough freezing and frozen storage on gassing power and on the viability of the yeast. In this respect temperature fluctuations are more damaging then the sub-zero temperatures as such. So freeze damage is promoted by frequent fluctuations in the storage temperature especially at storage temperatures close to the freezing point of water. Freeze-thaw cycles have a negative influence on the yeast. And this doesn't mean that the temperature has to become positive. A temperature fluctuation from say -18°C to -12°C is also to be considered as a freeze-thaw cycle. It should be kept in mind that a considerable amount of water is not frozen even at a dough temperature of -18°C. This phenomenon is called crio-concentration i.e. as the water freezes the concentration of the salts, sugars etc in the remainder of the water becomes so high, that the solution doesn't freeze any more.

The following table shows how much water has changed from the liquid state to the solid state at various temperatures:

Quantity of frozen water as a function of the temperature

product
moisture content
- 5C
- 10C
- 15C
- 20C
- 30C
moisture content
bread
40 %
15 %
45 %
53 %
54 %
54 %
40 %
yeast
72 %
68 %
80 %
85 %
88 %
89 %
72 %
eggs
74 %
85 %
89 %
91 %
92 %
93 %
74 %
egg yolk
50 %
80 %
85 %
86 %
87 %
87 %
50 %
cod
81 %
77 %
84 %
87 %
89 %
91 %
81 %

This table shows that for bread (and for dough it is pretty much the same) that at -5°C, 15 % of the moisture in the loaf is frozen, at -10°C 45 % of the moisture is frozen and so on.

Yeast damage induced by freezing can be explained by the events that occur during freezing. The manner in which the cells equilibrate their content during freezing depends on the rate of cooling. If cells are cooled slowly, water can leave the cell due to the vapour pressure differential with external ice. If they are cooled rapidly, water does not leave the cell, resulting in the formation of inter-cellular ice. Cell dehydration concentrates internal solutes, which in turn can damage cell membranes. In addition intracellular ice formation and expansion could exert sufficient force to rupture the membranes. Consequently, an optimum freezing rate is supposed to exist that is slow enough to prevent intracellular ice formation but rapid enough to minimise the cell exposure to high solute concentrations. Different studies showed that a slow freezing rate is preferable for preserving yeast activity.

For this reason one can introduce a two stage freezing system: once the interior of the dough piece has been reduced in temperature to approximately 0C to -5°C, and the outer shell is sufficiently firm, further reduction of the interior temperature can proceed in a holding room at -20C. By adopting this two-stage freezing operation, and thus reducing the time of holding each piece of dough in the main freezing chamber, both the throughput of the plant is increased and the product quality is improved.

In attempts to improve the poor baking performance of baker's yeast strains, different studies have been carried out to obtain strains with improved tolerance to freezing. Accumulation of the disaccharide trehalose in yeast is widely believed to be a critical determinant of dough tolerance to freezing. A strong correlation between trehalose content and stress tolerance has been demonstrated.

In any calculation of refrigeration requirements for dough freezing, specific heat values of 0,65 prior to solidification and 0,45 after solidification may be applied. The water content of the dough will generally be from 43 to 45 %. The total quantity of heat to be removed in freezing and lowering the whole mass of dough to -20C is approximately 270 kilojoule per kilogram of dough.



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Nol Haegens

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