Chapter 5
Effects of Water on Product Textural Properties
Table 5.1 Average moisture contents for UK breads (based on Chamberlain and Knight, 1987).
Bread type Moisture content (%)
800 g, tin, white, wrapped 40.4
800 g, tin, white, unwrapped 37.6
800 g, free-standing (crusty), unwrapped 35.6
800 g, tin, wholemeal, unwrapped 38.6
400 g, tin, white, wrapped 37.3
400 g, tin, white, unwrapped 35.4
400 g, tin, wholemeal, unwrapped 36.8
400 g, French bread (baguette) 29.2
60 g, crusty roll 26.4
not expected to have the crisp and hard eating character of the latter. On the other hand, the crust of baguette has a lower moisture content than the baked crumb and is therefore expected to have a hard, crisp eating character compared with the softer, chewier character of the crumb.
Bread and fermented goods
Bread and other fermented goods fall into the intermediate moisture range of foods and have the highest water levels of virtually all baked products. They are characterised by a relatively high moisture content in the baked crumb of the product and a lower moisture content in the crust. This moisture differential is essential in those fermented products that are expected to have a hard or crisp eating crust which contrasts with the softer, chewier crumb, such as baguette. The moisture content of fermented products varies according to the required crust character for the product and is mainly a consequence of the heat input during baking.
The longer the baking time, the thicker the crust region will be for bread (Wiggins and Cauvain, 2007) and therefore the perception of crustiness.
The data for the average moisture content of UK breads given in Table 5.1 are taken from Chamberlain and Knight (1987) who derived their figures from a nationwide survey of the nutritional composition of bread undertaken by the UK Ministry of Agriculture, Fisheries and Food (Wenlock et al., 1983). The data show that ‘crusty’ bread products have a lower average moisture content and also show the effect of wrapping in retaining moisture in the product during storage. The data given in Table 5.1 are averaged for the combination of crust and crumb so that breads with a higher ratio of crust to crumb, e.g. French bread, will have a lower average moisture content but possibly the same crumb mois-ture content, as breads with thinner crusts, e.g. sandwich-tin breads.
While the data used in Table 5.1 are now over 20 years old, they remain appropriate for UK products.
It is well known that the moisture content of a bread crumb is a ma-jor contributor to the perception of product freshness and that, within limits, the higher the moisture content, the fresher the bread will be perceived by the consumer. Some of the influence of a higher crumb moisture content is seen when the bread crumb is compressed with the fingers: the higher the moisture content, the easier it will be to deform the crumb and the softer (fresher) it feels. Too much water and the crumb may be easily deformed but it may not recover to the shape that it was before compression. This combination of easy compression with a good recovery is commonly assessed by the ‘squeeze test’ carried out by con-sumers at the point of purchase, especially when the bread is cold on the store shelf and the direct link between product warmth and freshness has been lost. Increasing the thickness of the crust on such products will often result in a loaf that is firm or hard to touch and may be rejected as stale by the consumer.
Because of the importance of bread softness to the consumer, there have been various attempts over the years to imitate the consumer squeeze test. One of the earliest reported in detail was by Hlynka and Van Eschen (1965) who modified an earlier Hill–Darby system (Matz, 1962). The modified device which comprised a lever system connected by low-friction bearings arranged in such a way that a weight placed on the top cross-bar caused jaws to exert pressure on the sides of a loaf of bread placed between them. In the ‘pre-digital age’, there was a link-age which connected the cross-bar to a Helecoid gauge movement and which translated changes of distance between the jaws into readings on a pointer on a graduated scale.
More recently introduced devices include the Bread V-Squeeze as shown in Fig. 5.1 (www.stablemicrosystems.com). This rig comprises a pair of ‘V-shaped’ rounded fingers which are lowered onto the surface of a loaf, and the force required to compress the product is measured.
The force so measured can be related to product freshness. One advan-tage of this testing rig is that it delivers the ability to measure softness on a whole loaf, but one possible disadvantage is that the data so-obtained will be strongly influenced by the thickness of the crust on the loaf (see below). The test can be carried out on wrapped or unwrapped and sliced or unsliced bread. Correlation of such objective data with sensory evaluation should be carried out to determine the relevance of such information in a commercial context.
Cakes
The baked moisture content of cakes is somewhat lower than that of breads, but cake products still fall within the intermediate moisture range of foods. The crust formed on cakes during baking is usually
Figure 5.1 Bread V-Squeeze. (Reproduced with permission of Stable Micro Systems.)
considerably thinner than that of breads; the formation of a hard crust on cakes is not usually seen as desirable by the consumer. Nevertheless, the moisture content of the cake crumb has a major impact on the perception of freshness in cake products. Guy et al. (1983) used a sensory panel to relate cake moisture content and freshness ratings. They showed that as the moisture content of the cake increased, a sensory panel perceived that the cake was fresher (see Fig. 5.2). Raising the moisture content of the crumb may have other less desirable effects on cake quality, such as increasing the product ERH and decreasing its mould-free shelf-life (see Chapter 6).
Cauvain and Screen (1990) examined the effects of some ingredients on the properties of the cake crumb and found that the level of water used to make up the batter and the baked crumb moisture content had a significant effect on the textural properties of cakes, as assessed with texture profile analysis. In part, the effects were the result of chang-ing water levels on cake crumb density; however, when the data were corrected for variations in crumb density, increasing the cake moisture content still affected the cake texture profile making the crumb more cohesive, chewy and gummy.
Figure 5.2 Relationship between panel score and moisture content of the cake crumb. (Based on Guy et al., 1983.)
Pastries and laminated products
The moisture content of pastries and laminated products is usually much lower than that of cakes, ranging from around 5% for puff pastry up to around 17% for short and Danish pastries. This means that the eating qualities of such products are characterised largely by their dryness or crispness. If the pastries have higher moisture contents, e.g. as a re-sult of moisture migration from creams or fillings (see Chapter 7), such products are generally considered to be unacceptable in terms of eat-ing quality and may be classified as stale. There are no precise limits to the level of moisture acceptable in pastry products because they vary according to product type and consumer preference.
Butcher and Hodge (1984), in a study of the causes of softening of pork pie pastry during storage, used a five-point panel rating for pastry crispness, where 0 rated soft, and 4 rated crisp, with a neutral crispness point (i.e. neither soft nor crisp) at 2. They correlated their neutral crisp-ness value with pastry hardcrisp-ness measured using a puncture test with an Instron and established a force value that corresponded with their neutral crispness point (see Fig. 5.3). In the same study, they correlated crispness values with pastry moisture contents; by extrapolating their data, they found that the neutral crispness value for side-wall pastry corresponded to a moisture content of around 11%. This would mean that moisture contents above 11% in side-wall pie pastry were likely to be considered unacceptable to their panel. Robb (1991) studied mois-ture migration in apple pies and found that his panel rating of neutral
Base pastry
Side pastry 75
50
25
0
0 1 2 3 4
Neutral crispness
Panel score
Soft Crisp
Instron value (force)
Figure 5.3 Relationship between sensory rating and pastry crispness. (Based on Butcher and Hodge, 1984.)
crispness for the sweetened pastry corresponded to a moisture content of about 17%. The difference between the two neutral crispness mois-ture contents may arise from the use of two different panels, but is more likely to be associated with the level of moisture content considered to be acceptable for the two different product groups.
Biscuits and cookies
The moisture contents of freshly baked biscuits and cookies are usually below 5%, with a water activity of around 0.2. Prolonged exposure of the product to many ambient storage conditions can lead to the absorption of water from the atmosphere into the biscuit matrix, making the biscuit soft. It is therefore common to seal biscuits in a moisture-impermeable film to prevent this moisture uptake. As biscuits absorb moisture from the atmosphere they lose their crispness, and become soggy (Manley, 2000) and less acceptable to most consumers. Even biscuits that may eventually be ‘dunked’ (deliberately immersed in a suitable hot liquid such as tea) benefit from having a low initial moisture content; otherwise they are prone to falling to pieces in the liquid during dunking.
The absorption of moisture by dry products such as biscuits and cook-ies can lead to a change in product dimensions, even though the structure has been set during baking. The usual form of change is an increase in size, commonly diameter or length, although an increase in thickness is possible. Such dimensional changes can lead to quality problems where
Table 5.2 Product characters that change during storage.
Crust or product crispness Crumb and crust moisture Crumb firmness
Crumbliness Taste Aroma
the biscuits have been coated, for example with chocolate. Barron (1977) studied the effect of moisture absorption by wafers on the cracking that occurred in the chocolate coating. He observed a gain in length of be-tween 0.33 and 0.42% of the original dimension for each 1% increase in wafer sheet moisture content (storage relative humidities ranged from 11.7 to 75.5%). The time taken for cracks to occur in the coating depended on the initial moisture content of the wafer sheet and the relative hu-midity of the atmosphere, as well as the thickness and completeness of the chocolate coating.