The invention relates to a toaster and a method of toasting bread

When making toast, the temperature of the toaster is not generally taken into account, or is only briefly taken into consideration which means that second or subsequent slices of toast either become too dark or too light compared to the first slice. This inconsistency of browning is annoying for the customer.

To address this problem in electronic toasters, the toaster control software is programmed with one or two time reductions for a particular toasting temperature setting. Toasters are provided with control circuits, which are generally only capable of measuring a fixed set point, typically a temperature of 40C

Typically, if the control circuit measures between a temperature above the set point then the toaster will reduce the timing cycle by 20 seconds for the second toasting cycle. Due to the toaster running for the same amount of time whether at 40°C or 80°C, the resulting toast will be inconsistent, being too dark for the higher parts of the range and too light for the lower parts of the range. Examples of such toasters can be found in the prior art documents GB2318502, GB2420966, US6198078, US6006656 and US2003/0196556.

The present invention therefore seeks to provide a toaster and a method of toasting bread that provides for a more uniform toast between toasting cycles.

According to a first aspect of the invention there is provided a toaster comprising the features of Claim 1.

According to a second aspect of the invention there is provided a method of toasting bread in accordance with Claim 7.

Preferably, the adjustment to the base length of the toasting cycle is made in accordance with the formula: where At is change in cycle length and Tl the measured temperature

Preferably, a is in the range of 15-25 C, more preferably 20-22 C. Preferably b is in the range of -1 to -1.8, more preferably -1.2 to -1.5. Preferably, PCB cavity temperature is measured.

A particular advantage of the invention occurs if the ambient temperature is above 30 or much lower than ambient temperature as the toaster timing can be decreased or increased, respectively, for the first toasting cycle as well as subsequent cycles. A further advantage is that the use of a formula takes up less memory on the control chips than introducing a plurality of set points, which enables lower cost processors to be used with the standard thermistors.

An exemplary embodiment of the invention will now be described in greater detail with reference to the example and drawings in which:

Fig. 1 shows a toaster

Fig. 2 shows a selection of toast made with a prior art toaster

Fig. 3 shows a graph of toaster temperature v time alteration

Fig. 4 shows a selection of toast made with a toaster of the invention

Figure 1 shows a toaster Figure 1 shows a schematic plan view of a toaster 1 comprising a main housing 2 having first and second bread or toasting cavities 3,4. Heating elements 5,6 are arranged in each bread cavity 3,4 and in use are adapted to toast a piece of bread located on a respective bread support 7,8. Each bread support 7,8 comprises a plurality of alternating metal supports and extends through a respective slot 9,10. The bread supports are also supported at the end of the bread cavities 3,4. The two bread supports 7,8 are attached to a carriage 11, which is adapted to slide on two vertical rods 12,13, the lower ends of which are mounted on the base of the housing 2 and the upper ends of which are attached to a flange 16 extending from the upper surface of the toaster. The rod 13 is located substantially equidistantly from the two bread cavities 3,4. A spring 15 is attached to both the flange 16 and the carriage 11, so that the carriage is urged into a raised position in which the bread or toast is raised above the upper surface of the housing.

The switch 17 for actuating the timer, pop-up and heating elements comprises two pairs of upstanding elements having opposing contacts at their distal ends. The downward projection 14 which is integral with the carriage 11, in use, will force the respective contacts together when it is inserted between the two pairs of upstanding contact arms. As the upstanding contact arms are resilient, the downward projection is urged away from the lowest position by the resilience of the contact arms. The carriage 11 is held in the lower position against the force generated by the spring by means of an electromagnet, which is actuated by the closing of the contacts. The toaster is provided with an NTC thermistor, which is adapted to measure the temperature in the PCB cavity, ideally adjacent to the toasting cavity and supply this data to a control unit, which determines the appropriate length of the toasting cycle as described in more detail below. Once the predetermined toasting time has elapsed, the timer circuit breaks the contact so that the carriage is released from the lowest position.

In known toasters, a common control algorithm is to provide each toaster setting with a duration:

For darker toast, a higher setting is selected and the base timing is increased as the toaster element will operate at a constant setting as to toast the bread, the sugars need to caramelise. . If the NTC detects the temperature is above the setpoint temperature of typically 40C the toast cycle is reduced by 20 s from the base timing. If the temperature is below the setpoint, the base timing is used.

Figure 2 shows a selection of toast made with a prior art toaster in which the operating time is simply reduced by 20 s for the second toasting cycle. This shows that the first piece of toast is toasted to an acceptable level but that the subsequent pieces are too light or under toasted for most consumers' tastes. The reverse problem where subsequent slices are burnt also occurs.

In an exemplary embodiment of the invention, when the toasting cycle is actuated, the temperature of the NTC thermistor is determined and this temperature is used to calculate the time of the toasting cycle. At the end of the calculated time, the toasting cycle is ended and if appropriate the toaster pop up is released.

Figure 3 shows a graph of NTC thermistor temperature at actuation of the toasting cycle against calculated time reduction. The graph has five points on it corresponding to the following example designed for setting 5 with a base time of 150s in the known toasters:

1 152 18

30 sec gap until next cycle

2 124 54

3 mins gap until next cycle 3 133 42

15 mins gap until next cycle

4 148 23

20 sec until next cycle

5 119 60

The points follow a substantially linear relationship that can be described with the following equation:

Tl - a

b where is the change in toasting cycle time, Tl is the measured NTC thermistor temperature and a and b are constants depending on toaster configuration. In the toaster used in this example (a Dualit® Lite toaster) a= 20.8 and b = -1.27.

Thus the toaster of the invention can also adjust for ambient conditions as in the first example, the NTC is below room temperature at 18 C and so the toasting cycle time is increased. In the second example above, the NTC temperature is 54 C and so the time is decreased by 26 s, which is more than the standard toasters. Unlike in the known toasters, the length of the toasting cycle is increased from the base time when the ambient temperature is below standard room temperature of 21 C.

Figure 4 shows examples of the toast produced according to the previous example, which shows that the toast has a much more uniform colour than in the known toasters.