What is the meaning of the term syneresis when applied to bread

Syneresis is the name given to a particular physical or colloidal change that takes place in starch and other gels as they age. It is caused by crystallisation or aggregation of polymers, causing loss of water from the surface of components. It is common with some starch gels, particularly those subjected to freezing and thawing. The released water may evaporate to be absorbed by other components by diffusion or vapour phase transfer, or may be lost from the product/component, causing it to dry out and shrink.

It is the change in starch crystallinity that brings about the staling of bread that is a day or two old, causing a sensory change equivalent to convey the impression that the bread contains less moisture and therefore has lost its freshness. Bread may lose actual water during the process of staling but there are many other changes occurring at the same time which will account for the dry-eating qualities of the bread (Pateras, 1998). This change in the condition of the starch is sometimes described as the 'process of syneresis' and is affected by the temperature and humidity under which the bread is kept.

If a loaf is kept for several days there is bound to be a loss of water by evaporation, but this water will not be lost regularly from the entire loaf. The loss is greatest at the part nearest to the crust. It has been shown that the loss of moisture from the centre of a loaf is exceedingly small and at the end of two weeks the moisture in the centre of a loaf is almost the same as the beginning. It would mean that if the outside portion of the loaf was cut off the interior would be almost as moist eating as a loaf a day old. However, this is not the case in sensory terms. A loaf several days old will be dry eating and stale equally throughout its whole structure. In fact there is a change in the method by which moisture is held in the loaf.

To illustrate this, prepare a stiff starch jelly and allow it to stand for a day or two. The water will partially separate out and will be seen on the surface while the gelatinised starch will seem to have become more solid. We can presume that something similar will occur in bread during storage. The starch will separate slightly from the water it was holding at the outset and the particles of bread will become dense and more insoluble, though apparently it does not mean that the small amount of water that has separated out will be evaporated. The bread particles, being more dense, will be harder to mix with saliva in the mouth so that a sensation of dryness and a difficulty in masticating the bread are experienced and make us think that the bread is dry.

Another point affecting the condition of the starch is the temperature at which the bread is stored. Bread stored at 4 °C stales more rapidly than that stored at room temperature or under frozen conditions. At a low temperature of —5 °C staling does not occur, though the act of freezing and thawing bread is the equivalent of 24 hours storage at ambient (Pence and Standridge, 1955).

References

PATERAS, I. (1998) Bread storage and staling, in Technology of Breadmaking (eds S.P. Cauvain and L.S. Young), Blackie Academic & Professional, London, UK, pp. 240-261.

PENCE, J.W. and STANDRIDGE, N.N. (1955) Effect of storage temperature and freezing on the firming of a commercial bread. Cereal Chemistry, 32, 519-526.

13.4 In some technical literature there is reference to batter specific gravity or relative density. What is this? How is it measured? What is its relevance to cake- and spongemaking? Why is the volume of the baked product referred to in terms of specific volume?

It would seem logical to use the same unit of measurement for expressing the mass concentration of unbaked batters and baked cake products. However, when considering how this property is measured, it becomes clearer why a large part of the industry continues to use the different measures.

The density of a substance is its mass (weight) divided by its volume. The amount of air occluded in a batter is monitored by measuring its cup weight, that is, the weight of batter required to fill a cup of known volume. As the same cup size or volume is used for the comparison, the cup weight relates directly to specific gravity (now more commonly known as relative density) and this figure is used in the bakery for process control purposes without having to make any calculations. The lighter the cup and its contents (i.e. the larger the volume of air in the batter), the lower the relative density.

Thus batter relative density and batter specific gravity essentially measure the same property, the degree of aeration of the batter. Batter relative density in cakemaking is most commonly related to product volume; usually the lower the batter relative density, the larger the cake volume in a given set of circumstances will be. In contrast, baked products are monitored by measuring their volume. Since this is an important physical characteristic related directly to specific volume, that is the volume of a known mass, it is convenient to use this term.

Thus, practical considerations lead to the continuing use of different units in these circumstances for measuring mass concentration.

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  • ermias
    What is syneresis in cooking?
    3 years ago
  • Gringamor
    How common is Syneresis in bread?
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    What syneresis mean in dessert?
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  • Chantal
    What do you understand by the term syneresis in relation to bread?
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  • Walton Johnson
    What is syneresis as used in baking?
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  • XANDER
    What is meant by syneresis as used in bakery?
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