GB2217969A

GB2217969A - Travelling bread ovens

Info

Publication number: GB2217969A
Application number: GB8910356A
Authority: GB
Inventors: Christopher David James; Gordon Alexander Beech
Original assignee: Ranks Hovis McDougall Ltd
Current assignee: Ranks Hovis McDougall Ltd
Priority date: 1988-05-06
Filing date: 1989-05-05
Publication date: 1989-11-08
Anticipated expiration: 2009-05-05
Also published as: GB8810777D0; GB8910356D0; GB2217969B

Abstract

A method of baking bread in an oven comprising applying a relatively high initial heat flux e.g. 375 DEG C to the dough in a first or front section of the oven while the surface of the dough is moist and then reducing the heat flux e.g. below 200 DEG C to a level insufficient to burn the surface of the dough, but sufficient to maintain heat transfer to the dough, the heat flux after the first section should still be sufficient to maintain the rate of temperature increase at the centre of the loaf. It has been found that this rate can be maintained even though the heat flux applied to the loaf is substantially reduced.

Description

CONTINUOUS BREAD BAKING This invention relates to methods for continuous bread baking and apparatus for carrying out such methods.

Continuous bread baking involves passing pieces of dough, usually located in individual containers, into and through an elongate oven, the pieces of dough being located, normally within tins, on a conveyor which runs through the oven. Ovens for continuous bread baking are known as travelling bread ovens.

In a typical travelling bread oven, the oven tunnel through which the conveyor passes is divided into several longitudinally separated, heated sections in order to allow control of the temperature along the tunnel. Each section of the tunnel may be heated by a separate burner arrangement. Alternatively, the same heating gases may be used for the whole length of the oven, but additional burners are provided at spaced apart positions down the oven to maintain the desired temperature. Usually no one burner section (in either type of oven) is less than one eighth of the total oven length and oven temperatures are set within the range 1500 to 3500C.

In some ovens, the burners heat air which is then blown towards the support surface of the conveyor. Such ovens are known as forced convection ovens. In a direct fired forced convection oven it is possible to vary the air flow or turbulence within the oven. In other ovens, heated air is directed through pipes which run through the oven. One such oven is an indirectly fired oven, which has limited turbulence control.

Conventional travelling bread ovens usually expose the product to a constant or rising temperature profile. This tends to produce loaves which achieve a relatively low maximum loaf centre temperature (LCT) during the baking cycle for any given moisture content.

It is desirable to maximise the LCT whilst retaining optimum moisture content, crust colour and crust firmness.

The higher the LCT the better cooked is the middle of the loaf. For instance, by achieving an LCT of 960C, a loaf is produced in which the middle is particularly acceptable to eat, the bread "dissolving" readily in the mouth.

Achieving the desired LCT in the shortest possible time would reduce the bake time and thereby reduce the amount of heat input for producing the cooked loaves. An optimum bake time would be one which is less than 22 minutes.

However, in a typical current commercial process, the baking of a loaf with a weight loss as low as 40g in a time of only 22 minutes would result in an LCT of approximately 900C. This LCT is too low to produce loaves with satisfactory eating qualities in the centre.

It has now been appreciated that important advantages can be achieved by careful control of the amount of energy being transferred into the dough piece or loaf during its passage through the oven. In the above described ovens energy is transferred as heat flux and this term will be used hereinafter. However, it should be appreciated that the energy being transferred may be heat (conductive, convective and/or radiant), microwave energy or radio frequency energy or other form of energy, or a combination of different forms of energy. The above described conventional travelling bread ovens, with individually heated sections and provided with fans to force hot air towards the conveyor, are in practice controlled so that the dough piece is subjected to a constant or rising temperature/heat flux profile as the baking proceeds.It has been observed that the four sections of a direct fired forced convection oven have been operated with temperatures of 2400, 2650, 2850 and 2950C from the inlet to the outlet end, (or, for a constant temperature profile operation, wit'n each section at about 3000C) the temperatures being measured by thermocouples installed in the baking chamber.

In each section the turbulence dampers are set at least half open to produce moderately high levels of heat flux for any given temperature.

It has now been surprisingly discovered that improved results can be obtained by an application of high heat flux during the initial part of the passage of the dough piece through the oven, followed by a rapid decrease in heat flux and the maintenance of relatively low heat flux during the remainder of the passage of the dough piece through the oven.

The thermal conductivity of moist dough is relatively high.

Accordingly, relatively large quantities of heat can be taken up by the dough at the front of the oven while the dough is moist. When the dough surface dries out and begins to form a crust, the thermal conductivity of the crust rapidly decreases and if high levels of heat flux are still applied, overbaking of the crust results.

Accordingly, it is preferred that there is a rapid decrease of heat flux as soon as the crust starts to form.

Preferably the heat flux should be sufficiently low so as to quickly reduce crust temperature and prevent overbaking of the outside of the loaf. However, heat transferred to the dough should be maintained even after the initial high heat flux section.

Accordingly, the present invention can be regarded, in a preferred form, as a method of baking bread in an oven, comprising applying a relatively high initial heat flux to the dough while the surface of the dough is moist and then reducing the heat flux to a level insufficient to burn the surface of the dough, but sufficient to maintain an adequate rate of heat transfer to the dough. In particular the heat flux after the first section should still be sufficient to maintain the rate of temperature increase at the centre of the loaf. Surprisingly, it has been found that this rate can be maintained even though- the heat flux applied to the loaf is substantially reduced.

Preferably, the first or front oven section is maintained at a temperature of 3750C with high air turbulence, for example, with an air flow of from 30 to 40 m/s from jets located above and below the conveyor surface.

Typically, the jets may be provided with 5mm slots and arranged at 200 mm intervals and at a distance of 80 mm from the top and bottom surfaces of the bread tin.

In a forced convection oven this leads to an estimated rate of convective energy delivery (an estimate of heat flux based on an assumed average tin temperature of 1000C) of from 3.0 to 3.5 watts per cm 2 It is preferred that the front oven section represents from 10 to 25% of the total oven length.

Preferably the temperature in the oven after the front oven section is reduced to less than 2000C, preferably to between 950C and 2000C. More preferably, the oven is divided into at least two sections after the front section (the first section). In the second section it is preferred that the air flow is reduced and, in the third section, it is preferred that the air flow is increased again, thereby increasing the heat flux. Increasing the heat flux in the third section has the effect of imparting the required crust colour and firmness to the finished product.

Typical settings of the second and third sections are 950C to 2000C at 10m/s (nozzle velocity) and 1250C to 2000C at 25m/s (nozzle velocity).

In another preferred method in accordance with the present invention, the oven includes four heating sections. The first section is preferably very short, between one quarter and one sixteenth of the total length of the oven.

The second section preferably has a length between two and five times that of the first section. It is operated at a low turbulence and with the temperature reduced from the high temperature of the first section down to perhaps 950 to 2000c.

In the third section, preferably having a length between 1.5 and 2 times that of the second section, more preferably a length twice that of the second section, the air velocity is maintained at a low level, the temperature perhaps being in the range 950C to 2000C.

In the fourth section there is high turbulence to provide firmness in the crust and give the appropriate finish to the bread. The length of the fourth section may be about one quarter of the total length of the oven. The temperature perhaps being in the range 1250C to 2000C.

A preferred four section oven has a front oven section whose length is one eighth of the total length of the oven, a second section whose length is one eighth of the total length of the oven, a third section whose length is half the total length of the oven and a fourth section whose length is one quarter of the total length of the oven. It is also possible to use a travelling oven with a number of sections of equal length, by the provision of suitable rates of heat transfer for each section. Alternatively, continuous ovens may be used in which the conveyor is arranged in a vertical spiral form, suitably sectioned to provide areas of temperature difference.

The result of operating in a three or four section oven as described above is to achieve a relatively high heat flux in the first section and a relatively low heat flux in the remaining sections.

An embodiment to the present invention will now be described, by way of example only.

A travelling oven is provided with three oven sections each with an independent control of temperature and air flow and connected together in series to form a large composite baking chamber. A conveyor is used to carry product from the feed (or front) end to the delivery (or rear) end of the oven.

Each oven section has an individual burner to maintain the desired baking temperature. A circulation fan is used to force the hot air from the burner, through a series of ducts with slot-like nozzles, directly onto the top and bottom of the product. This air stream is continuously heated and recirculated and is controlled by a series of dampers which vary the volume of air to the nozzles and thereby control the amount of heat flux to the product.

The front oven section is less than 10% of the total oven length and the temperature of this section is 3750C and the air flow at the nozzles is 25 m/s. In the second section the temperature is 1700C and the air flow 10m/s.

This is equivalent to a calculated rate of convective energy delivery (an estimate of heat flux based on an assumed average tin temperature of 1250C) of 0.2 watts per cm2. In the third section the temperature is 1600C and the air flow 25m/s . This was equivalent to a calculated rate of convective energy delivery ( an estimate of heat flux based on an assumed average tin temperature of 1200C) of 0.4 watts per cm2.

The heat flux profile produced by the above settings in the various oven sections is found to provide improved efficiency of the baking process and results in higher LCT's whilst retaining optimum moisture content, crust colour and crust firmness. Loaves are produced with an oven weight loss of 40g and an LCT of 940C in a baking time of 19.5 minutes.

The oven temperature profile was applied on a commercial travelling oven which had modified turbulence. The front oven section provided unusually high rates of heat transfer over approximately 10% of the oven length.

Figure 1 of the accompanying drawings shows the variation of temperature in a conventional directly fired forced convection oven operated at a current commercial bakery.

It can be seen that the temperature of the air between the tins rises to a peak of over 2000C at about 20 minutes.

The temperatures at the side and end of the tins follow similar profiles, but reaching somewhat lower peak temperatures. The loaf centre temperature begins to rise rapidly only after 20 minutes and achieves a peak at about 25 minutes.

Figure 2 of the accompanying drawings shows the effect of the process of the present invention on LCT. In the three section oven described above, the LCT begins to rise rapidly after about 12 or 13 minutes and reaches its peak at around 20 minutes.

The present invention also provides an oven for carrying out the process of the invention. A preferred oven in accordance with the present invention is provided with means for operating a first feed section of the oven at high heat flux and for operating the remainder of the oven at a relatively low heat flux.

Claims

1. A method of baking bread in an oven, comprising applying a relatively high initial heat flux to the dough in a first or front oven section while the surface of the dough is moist and then reducing the heat flux to a level insufficient to burn the surface of the dough, but sufficient to maintain an adequate rate of heat transfer to the dough, the heat flux after the said first or front section still being sufficient to maintain the rate of temperature increase at the centre of the loaf.

2. A method according to Claim 1, characterised in that the first or front oven section is maintained at a temperature of about 3750C with high air turbulence.

3. A method according to Claim 1 or Claim 2 characterised in that the temperature in the oven after the said first or front oven section is reduced to less than 2000C.

4. A method according to Claim 3, characterised in that the said temperature is reduced to between 95 and 2O00C.

5. A method according to any of the preceding claims, characterised in that the heat flux is applied to the dough by means of an oven comprising at least one section after the said first or front section.

6. A method according to Claim 5, characterised in that the said oven comprises two sections after the first or front section, in the second of which the air flow is reduced and in the third of which the air flow is increased again in order to increase the heat flux.

7. A method according to Claim 6, characterised in that the said second section is at a temperature of between 95 and 2000C with an air flow rate of 10 metres per second (nozzle velocity).

8. A method according to Claim 6 or Claim 7, characterised in that the said third section is at a temperature of between 125 and 2000C with an air flow rate of 25 metres per second (nozzle velocity)

9. A method according to Claim 5, characterised in that the oven comprises four heating sections.

10. A method according to any of Claims 5 to 9, characterised in that the first section is operated at high turbulence.

11. A method according to Claim 10, characterised in that the air flow in the first section is between 30 and 40 metres per second.

12. A method according to Claim 11, characterised in that the air flow in the said first section is produced from jets located above and below a conveyor surface by means of which the dough is conveyed through the oven.

13. A method according to Claim 11, characterised in that the jets are provided with 5 mm slots and arranged at 200mm intervals at a distance of 80mm from the top and bottom surfaces of a bread tin in which the dough is to be conveyed through the oven.

14. A method according to any of Claims 5 to 13, characterised in that the second section of the oven has a length between two and five times that of the first section.

15. A method according to any of Claims 5 to 14, characterised in that the second section of the oven is operated at a low turbulence.

16. A method according to any of Claims 5 to 15, characterised in that the temperature in the second section of the oven is reduced from the high temperature of the first section down to between 950C and 2000C.

17. A method according to any of Claims 9 to 15, characterised in that the third section of the oven has a length between 1.5 and 2 times that of the second section.

18. A method according to Claim 17, characterised in that the third section has a length twice that of the second section.

19. A method according to any of Claims 9 to 18, characterised in that the temperature in the third section of the oven is between 950C and 2000C.

20. A method according to any of Claims 9 to 19, characterised in that the fourth section of the oven is about one quarter of the total length of the oven.

21. A method according to any of Claims 9 to 20, characterised in that the temperature in the fourth section of the oven is between 1250C and 2000C.

22. A method according to any of Claims 9 to 21, characterised in that the oven has a first section of length one eighth of the total length of the oven, a second section whose length is one eighth of the total length, a third section whose length is half the total length and a fourth section whose length is one quarter of the total length of the oven.

23. A method of baking bread substantially as herein described with reference to the accompanying drawings.

24. An oven for baking bread according to the method of any of the preceding claims.

25. An oven according to Claim 24, characterised in that it comprises means for operating a first feed section at high heat flux and for operating the remainder of the oven at a relatively low heat flux.