Staling

The subject of bread staling is certainly not something new and studies on this particular topic go back many years. Various definitions have been given to the term "bread staling." Very broadly speaking, bread staling refers to all of the changes which occur in bread after baking and has been defined as "a term which indicates decreasing consumer acceptance of bakery products by changes in the crumb other than those resulting from action of spoilage organisms."

Some of the changes which occur in bread as a result of staling are:

Al though changes occur both in the crust and in the crumb of the bread, the majority of the studies involving bread staling have been concerned with changes in the crumb. Various methods and techniques have been used to measure staling in bread:

Crumb firming is one of the most obvious and important manifestations of bread staling and various instruments are available to measure this property. One of the problems encountered, however, is that the initial softness of the bread must be known since the firmness value by itself cannot be translated into a freshness rating.

Taste panels have been used to evaluate changes in bread as a result of staling. The panel must first be trained to recognize changes resulting from the staling process. If carefully controlled, results obtained using taste panels can be considered satisfactory.

As will be discussed later, changes in the starch are still considered to be the major factor responsible for firming and other changes in the bread crumb during storage. As a consequence, many researchers have attempted to use the changes in the starch to follow the bread staling process. As bread undergoes staling, there is a decrease in the amount of soluble starch that can be removed from the bread crumb. Also, the starch becomes less susceptible to enzyme attack. There is a change in the crystallinity of the starch and in the X-ray diffraction patterns.

Bread staling is an extremely complex phenomenon. Wheat flour, the primary constituent of bread, contains in addition to carbohydrates, proteins and lipids or fats, a whole series of transitional compounds. The various components present in wheat flour which eventually become the constituents of bread play an important role in bread staling.

Starch

One of the major components of wheat flour is starch (70 to 75 percent) and, as already indicated, numerous studies have been conducted which have shown that changes in starch play a major role in causing bread firmness.

Starch is the most abundant constituent and most important reserve polysacchqride of cereals. On a molecular level, its major constituents are the glucose polymers ammylose (25 - 28 %) and amylopectin (72 - 75 %)

amylose

amylopectin

Amylose consists of some 500 to 6000 a-(1,4)-linked D-glucopyranosyl units. Although a fraction of the amylose molecules is slightly branched by a-(1,6)-linkages, it is for all practical purposes considered to be a linear molecule. In contrast, amylopectin consists of linear chains of 10 to 100 a-(1,4)-linked D-glycopyranosyl units which are connected by a-(1,6)-linkages, forming a very large and highly branched polysaccharine of up to 3 million glucose units.

Starch occurs as intracullular, water-insoluble semicrystalline granules. In contrast to most plant starches, wheat, rye and barley starches have a bimodel size distribution. They consist of lare (10 - 35 m) lenticular and small (1 - 8 m) spherical granules. The partially crystlline nature of starch is predominantly attributed to structural elements of the amylopectin. Cereal starches have an A-type X-ray diffraction pattern and retrograded starch a B-type pattern.

The swelling or gelatinization properties of starch are another important parameter which should be considered. As indicated, starch itself exists in the form of distinct granules. When placed in water starch is insoluble, but when heated in water, the starch granules will begin to swell at a particular temperature. The temperature at which swelling begins is called the "gelatinization temperature." During the bread baking process, gelatinization of the starch takes place during the oven stage. However, the extent of gelatinization is limited due to the limited amount of water that is present.

The starch granule contains both amorphous and crystalline regions. The term "amorphous" indicates the lack of a definite or defined structure as opposed to "crystalline" which indicates a definite structure. When starch undergoes gelatinization, there is a change in the X-ray pattern or in the degree of crystallinity. On aging, the gel retrogrades or changes to another form which gives a different type of diffraction pattern. How then does this relate to the problem of bread staling?

It has been shown that the rate at which crystallinity develops in concentrated starch gels is similar to the rate of increase in bread firmness. If we assume, then, that the principal change in starch associated with bread staling is crystallization, which component of starch, amylose or amylopectin, is responsible for the crystallization or bread staling? There are differences of opinion with regard to this question.

The following table shows the effect of staling on the quantity and composition of starch extracted from bread crumb stored at 20C. In this particular experiment, the breads A and B were produced from flours having different protein contents. The flours used to produce breads A and B contained 11.0 and 13.9 respectively.

bread

dag

% soluble starch

% amylose % amylopectin

A

0

3.34

0.52

2.82

 

1

2.16

0.19

1.97

 

2

1.72

0.14

1.58

 

5

1.22

0.10

1.12

B

0

2.36

0.39

1.97

 

1

1.60

0.12

1.48

 

2

1.14

0.08

1.08

 

5

1.08

0.07

1.02

The composition of the soluble starch indicates that the soluble starch leached from the crumb of fresh bread is predominantly amylopectin. Although the amylose content of the soluble starch leached from fresh bread crumb is small, it decreased progressively during bread staling. These results indicate that amylose retrogradation or a change in the amylose fraction of the starch is important during the first day of storage. It is believed by many workers that the reason there is very little amylose present in the bread crumb is that it has retrograded or changed during the oven stage itself. These results also imply that the effect of the amylose fraction of the starch on bread staling diminishes as the flour protein content increases.

Storage temperature is an important factor to be considered in any discussion of bread staling. Staling becomes more rapid as the temperature of storage is reduced from room temperature to 35F. Below 35F., staling becomes slower as temperature is lowered, until at 0F. it is very slow, and bread products will keep for months without apparent staling. Studies on the effect of bread storage temperatures on the role of starch in staling have been conducted. It has been shown that changes in the starch contributes about 93, 50 and 20 percent of the total crumb firmness at 20C, 30C and 36C, respectively, during five days of storage. The results imply that changes in the starch in the crumb are about one-half and one-fourth as fast at 30C and 36C., respectively, than at 20C. The results suggest that at elevated temperatures, some factor (changes in protein or moisture redistribution or both) in addition to the starch plays an important role in the firming process undergone by bread.

What effect does protein content of the flour have on bread staling? It is generally believed that flours with higher protein levels produce breads that stale at a slower rate. The effect appears to be due primarily to a dilution of the starch rather than a direct effect of protein.

The following table shows the effect of the protein content of flour on the "time constant." The term "time constant" is a value derived from an equation known as the "Avrami equation." It has been shown that the rate of firming of bread can be described quantitatively by this particular equation which was developed to describe the rate of crystallization of super-cooled melts. The time constant values are the reciprocal of the rate constants obtained from the Avrami equation. Therefore, the higher the time constant, the slower the rate of staling. The time constants shown for the breads produced from flours of different strengths (stability values of 12.5 and 5.5 minutes as obtained with a laboratory recording dough mixer) but essentially the same protein content are identical, suggesting that the staling rate of bread is independent of protein quality.

f lour
% protein

flour
stability (min.)

bread
time constant

10.6

12.5

3.75

11.0

5.5

3.74

13.9

16.0

5.44

21.6

21.0

11.25

It is known that bread does not necessarily lose moisture during stating and that stale, firm bread often contains as much moisture as does fresh, soft bread. It is generally agreed that increasing absorption in bread dough enhances softness and retards the firming process. More starch will be gelatinized at the oven stage as more water is added.

Probably more important than moisture in relation to bread staling, is the moisture redistribution which occurs between components during bread storage. This has been a controversial subject among various investigators. Reports exist in the literature which indicates that moisture transfer occurs from the starch to the gluten in the crumb during stating while other studies have shown a moisture transfer from gluten to the starch phase.

A minor component of wheat flour is a carbohydrate material referred to as pentosans. Pentosans are present in wheat flour at a level of two to three percent. However, they have been examined extensively in terms of their functionality in baking. Whereas we have seen that starch is made up of one sugar unit, glucose, the pentosans are made up of two sugar units called xylose and arabinose.

One important property of the pentosans is their ability to increase the water-binding capacity of a flour and thus the baking absorption. The next table shows the effect of the addition of pentosans on the stating rate of bread at 20C. Pentosans increased the overall time constant for the bread, that is, they decreased the rate constant or the rate of bread firming with the effect exerted by the water-insoluble pentosans being more pronounced than that of the water-soluble pentosans.

bread

overall time
constant (days)

control

5,44

with 1 % soluble pentosans

6,53

with 1 % insoluble pentosans

8,54

The use of higher protein flour, higher absorption and the importance of storage temperature have been discussed. What else can the baker do to retard firming in bread? By far the most important ingredients presently used to retard stating are the surface-active agents. When a surface-active agent is used in bread for softness, the entire mechanism of what occurs is unclear. It is believed that a complex is formed between the surface-active agent and the amylose or the amylopectin components of the starch. The crystallization of starch is thus reduced by the incorporation of a surfactant, but the effect of temperature on this reaction is unchanged. In other words, breads with softeners still firm faster in a refrigerator than at room temperature. The primary means, therefore, to retard firming in bread is to retard the changes occurring in the starch or what is referred to as starch retrogradation.

The effect of adding two different commercial emulsifiers on the amylose content in the water solubles extracted from the bread crumbs is presented in the following table. The amylose content was found to decrease as the bread aged, with the greatest decrease occurring during the first 24 hours of storage. The lower levels of amylose present after incorporation of emulsifier play an important role. Also of interest here is that the two emulsifiers differ in their ability to lower the amount of amylose. The type or composition of emulsifier that one uses to retard firming is, therefore, of utmost importance.

storage
hours

control
% amylose

surfactant A
% amylose

surfactant B
% amylose

10 minutes

10.45

3.29

7.95

1

8.45

3.05

6.37

2

5.21

3.01

4.74

5

4.99

1.99

3.79

12

4.95

1.93

3.26

24

3.91

1.87

3.16

48

3.27

1.87

2.79

72

3.22

1.84

2.63

96

2.82

1.65

2.65

In addition to emulsifiers, other ingredients used in the formula will help to improve freshness retention. In particular, the use of shortening, sugar, and amylase enzymes, particularly of the bacterial type, will retard stating. Proper mixing, fermentation and baking conditions are processing factors which will retard the stating rate to a degree.

In conclusion, I would like to indicate that the problem of bread stating is one which has been studied by different investigators for many years. Changes in the starch, what is referred to by the cereal chemist as starch retrogradation, are considered to be the most important single factor in bread stating. Although by the incorporation of surface-active agents (emulsifiers) or changes in formulation and processing, the stating process can be retarded to a limited degree, many additional studies will have to be conducted to better understand the mechanism of stating and how the shelf life, of baked products can be extended.



e-mail
Nol Haegens

Home