Wafers
Traditional forms of wafers are baked by depositing batter on to one of a pair of plates, with the top plate closing on the lower to seal the batter between the two (Wade, 1995). The plates are heated rapidly, usually by direct-fired gas burners, and the rapid heat transfer quickly turns the water in the batter to steam. Considerable steam pressures are generated (up to 2.1 kgf/cm2has been reported by Pritchard and Stevens (1973)), and vents in the edges of the plates permit escape of the steam (and a very small quantity of batter). The number and dimensions of the steam vents both affect wafer sheet thickness (Pritchard and Stevens, 1973).
Wafer sheets are commonly baked for 1.5–3 min to a moisture content of less than 3%.
been standing empty at the appropriate baking temperature for some time before products are due to enter, the radiant heat component of the inner surfaces may build up. In such circumstances the first prod-ucts introduced into the oven may well be baked with excessive crust colour. Bakers often refer to this heat as ‘flash-heat’ (Cauvain and Young, 2001) as more product batches enter the oven the problem progressively disappears. A practical baker’s ‘trick’ to deal with this situation is to introduce a short burst of steam or water vapour into the oven be-fore loading the products to remove some of the radiant heat that has built up. This problem is more likely to be seen with deck-type oven rather than rack or travelling-type ovens, especially those with ceramic hearths. Flash-heat may be a potential problem with all types of bakery products.
Bread
In the manufacture of many bread and fermented products, the use of steam adds to the appearance through the formation of a gloss on the surface and influences the texture and eating qualities of the crust. In general, with bread and fermented products, the aim of using steam is to increase the ‘crustiness’ of the products, although it may also be used to achieve a more uniform expansion of the product in the early stages of baking through its impact on heat transfer rate.
The formation of a gloss on the surface of bread products derives directly from the introduction of steam and control of the quantity that is made available for a given mass of dough in a given baking chamber.
When dough pieces are first placed in the oven, their surface temperature is low – typically below 45◦C (113◦F) – and the introduction of steam at 100◦C (212◦F) results in some condensation on the surface of the dough piece. This excess of water combines with the starch and with the action of the enzymes present and encourages the formation of dextrins and sugars (mainly maltose) on the dough piece surface. As the temperature begins to rise, the surface begins to dry and the enzymic activity ceases but the dextrins and sugars remain and will colour as baking proceeds.
The dextrins are the components that are mainly responsible for the formation of the surface gloss. The whole process lasts a short time because the crust temperature climbs rapidly above the boiling point of water. The length of time over which steam will be introduced into the oven temperature is short, commonly less than 90 s and typically less than 60. Wiggins and Cauvain (2007) discussed the importance of having an excess of water vapour introduced into the oven. They considered that an excess is needed to encourage paste-type gelation, which leads to gloss formation, whereas insufficient steaming leads to crumb-type gelation and lack of crust gloss.
In addition to changing the appearance of the product, steaming will affect product crustiness: the thicker the gelatinised starch layer that is formed, the greater will be the cracking of the crust on the baked product. This cracking occurs when the product leaves the oven and begins to cool. The crumb of the product has a higher moisture content and is more flexible than the lower moisture content crust and so readily contracts as the temperature of the air within the cells falls. Being less flexible, the crust is not able to contract as fast or to the same degree as the crumb, and the strains placed on the crust by attached portions of crumb become such that splits begin to occur in the surface. A similar effect occurs when baked products are frozen, as discussed below.
Even when product crustiness is not a prime objective, the introduc-tion of steam can make a positive contribuintroduc-tion to the final quality of bread and fermented products. In particular, steam may be used to pro-vide a more uniform heat transfer rate to the fermented products so that the rate of crust formation is slowed and the rate of expansion at the centre of the dough is increased (Cauvain and Young, 2006). This delivers a more controlled oven break or oven spring in the product.
Part-baked breads
Part-baking is a process in which proved doughs are baked just suffi-ciently to inactivate the yeast and enzymes, and set the structure with a minimum of crust colouration and moisture loss. A second baking stage is used to re-freshen the crumb properties and develop a normal crust colour. Steam may be applied during one or both of the baking stages. In the first baking stage, baking temperatures are lower and times longer than used in scratch production, and steam may be used to help control the oven spring, or break. Collins and Ford (1985) studied the appli-cation of steam in the production of part-baked soft rolls and French bread, and found that limited steaming in the first bake improved final product quality. In the preparation of part-baked rolls in a rack oven, only a ‘few seconds’ were required and longer steaming led to severe wrinkling of the product. Steaming times of 18 s were recommended for part-baked French-type breads, and extending the steaming period led to product collapse and severe wrinkling. Collins and Ford (1985) also studied the application of steam at bake-off and found that it improved final product appearance.
Cakes
It is not common to introduce steam into ovens for cake baking, since the formation of a hard, glossy crust is not normally required. There is evidence, however, that the humidity level in the oven during cake
baking can affect aspects of product quality. To a large extent differences in humidity levels during cake baking come from the use of different types of ovens with their different heating methods. The effect of extrac-tion systems in most ovens is to lower chamber humidity, which leads to drier baking conditions.
Robb (1987) experimented with the introduction of humidity using water injected into a tunnel oven and examined the effect on the quality of sponge cakes. The introduction of steam had no direct effect, but did change the effects of baking times and temperatures; steam injection with combinations of lower temperatures and longer baking times re-duced sponge cake volume. The introduction of steam appeared to have no effect on cake weight losses during baking.
Choux pastry
The benefits of raising atmospheric humidity during the baking of choux pastry have been discussed above. In cases where the use of lids or coffins would be difficult, e.g. in a tunnel oven, similar improve-ments in product quality have been obtained through the introduction of steam into the oven soon after the point at which the product enters the chamber.
Rye breads
Gluten formation in rye bread doughs is restricted by the lack of any sig-nificant quantities of gluten-forming proteins, and starch gelatinisaton plays a much greater role in structure formation than in wheat breads.
Bread baked from 100% rye flour is much denser compared with wheat flour bread, and it is common for rye and wheat flours to be mixed to improve bread volume and palatability. Baking times for rye and mixed rye–wheat breads are much longer than 100% wheat (typically 45–55 min compared with 25–30 min for the latter). The length of time over which steam is applied is also greater, possibly as much as 20 min. This extended steaming period helps to improve bread volume and softness by allowing greater expansion of the dough before gelatinisation of the starch occurs.
Pumpernickel and wholemeal rye breads may be produced by the spe-cialised Horlbeck steam-baking process, which utilises a steam-baking chamber which might best be described as a steam pressure vessel.
The dough pieces are packed tightly into closed pans and steam baked for many hours. Baking conditions may vary from 16 to 24 h at 100◦C (212◦F), to 5 to 8 h at 200◦C (392◦F). The product crumb darkens consid-erably during the baking process but moisture retention is higher in the
product due to increased solubilisation of the carbohydrates which are present.
Chinese steamed breads
Chinese steam bread, or ‘man-t’ou’, has a long history in China, proba-bly stretching back for more than 3000 years. It is the staple food of the wheat-growing areas of northern China. The key features of Chinese steam bread are their semi-circular shape, white crust colour, smooth surface and shiny skin. The latter characteristic derives from the heat setting process carried out by baking the proved dough pieces in an at-mosphere of steam until the structure has become set. The high level of water that condenses on the surface of the heated dough pieces encour-ages starch gelatinisation but discourencour-ages the usual Maillard browning reactions.