MONITORING AND CONTROLLING YEAST PROPAGATION

Abstract

A computer-implemented method for controlling and/or monitoring yeast propagation includes providing a mixture of yeast cells and wort, determining an original gravity of the wort or of the mixture and a temperature of the mixture at a start time, and calculating a theoretical end time for the yeast propagation on the basis of a logistic propagation model for the yeast propagation, on the basis of the original gravity and on the basis of the temperature at the start time.

Claims

1-15. (canceled)

16. A computer-implemented method for controlling and/or monitoring yeast propagation, comprising: providing a mixture of yeast cells and wort; determining an original gravity of the wort or of the mixture and a temperature of the mixture at a start time; and calculating a theoretical end time for the yeast propagation on the basis of a logistic propagation model for the yeast propagation, on the basis of the original gravity, and on the basis of the temperature at the start time.

17. The method according to claim 16, further comprising: determining a free amino nitrogen (FAN) content of the wort; wherein the FAN is taken into account for the calculation of the theoretical end time on the basis of the propagation model.

18. The method according to claim 16, wherein the temperature of the mixture is determined during the yeast propagation.

19. The method according o claim 16, further comprising: determining an actual value for an extract content of the mixture during the yeast propagation.

20. The method according to claim 16, further comprising: determining an alcohol content of the mixture during the yeast propagation.

21. The method according to claim 16, further comprising: determining a dissolved oxygen content of the mixture during the yeast propagation.

22. The method according to claim 16, wherein a model for an extract content in the mixture is used as the propagation model, wherein the extract content is determined on the basis of the original gravity at the start time, a target value for a residual extract content, a substrate uptake rate, a temperature rate, and a duration for a lag phase during the yeast propagation.

23. The method according to claim 22, further comprising: determining a reference value for the extract content on the basis of the propagation model.

24. The method according to claim 23, further comprising: comparing an actual value for the extract content with the reference value for the extract content; and when a deviation between the actual value and the reference value exceeds a predefinable limit value, varying as a function of the deviation at least one influencing quantity for the yeast propagation.

25. The method according to claim 24, wherein the at least one influencing quantity is the temperature, the end time of the yeast propagation, or a concentration of dissolved oxygen.

26. The method according to claim 24, further comprising: determining a dissolved oxygen content of the mixture during the yeast propagation; adjusting an aeration of the mixture during the yeast propagation on the basis of the deviation between the actual value and the reference value of the extract content and the dissolved oxygen content.

27. The method according to claim 16, further comprising: determining a pitching concentration of the yeast at the start time.

28. The method according to claim 27, further comprising: determining a biomass concentration of the yeast on the basis of the propagation model, the pitching concentration, and a target value for the yeast concentration that is related to the target value for the residual extract content.

29. The method according to claim 28, determining a viable yeast cell concentration during the propagation on the basis of the biomass concentration.

30. The method according to claim 16, further comprising: determining a pH value of the mixture.

Description

[0035] The invention and its advantageous embodiments are explained in further detail with reference to the following figures. In the figures:

[0036] FIG. 1 shows an example of a container for performing yeast propagation; and

[0037] FIG. 2 shows the residual extract and the biomass concentration, in each case in the form of measured curves and reference curves calculated by means of the propagation model. In the figures, identical elements are provided with the same reference signs.

[0038] FIG. 1 shows a schematic and exemplary illustration of a container 1 for yeast propagation in a brewery. The container 1 comprises an inlet 2 for pitching the hopped wort W with yeast H. Advantageously, the pitching concentration of the yeast H, the temperature T inside the container 1, and, optionally, the FAN content of the yeast H are determined at the beginning of the yeast propagation. Subsequently, the process of yeast propagation begins for the mixture G.

[0039] The container 1 further comprises an outlet 3 for removal and a device 4 for aerating the container 1.

[0040] For monitoring and/or control purposes, an oxygen sensor 5 and a thermometer 6 are further incorporated into the tank in the example shown here. A heating/cooling device [not illustrated] may further be provided for adjusting the temperature T in the container 1 during the yeast propagation.

[0041] The present invention provides a way to monitor and/or control yeast propagation. A growth process under controlled conditions and harvesting of the yeast at the right time, i.e., when the cell number and vitality of the yeast are as high as possible, are made possible.

[0042] Numerous descriptions of various influencing quantities of yeast propagation, as well as models for describing the process of yeast propagation, are already known from the prior art, such as from the dissertation, Mathematically Based Management of Saccharomyces sp. Batch Propagation and Fermentations, by T. Kurz (2002) at the Technical University of Munich, or in Modeling of the Bacterial Growth Curve by M. H. Zwietering et al. in Applied and Environmental Microbiology, pp.1875-1881, 1990. However, since yeast propagation is a complex, multivariant problem, the available models make use of a plurality of variables or different influencing quantities and are of limited suitability for practical control and/or monitoring of yeast propagation. Moreover, many of the influencing quantities are difficult to obtain directly, in particular continuously, during yeast propagation. The present invention thus relates to a simplification of the known models and descriptions, which allows targeted control and/or monitoring of yeast propagation. An essential finding underlying the invention relates to targeted selection of significant influencing quantities in the creation of the propagation model.

[0043] The starting point for modeling yeast propagation is a logistic propagation model as described in (Speers, et al., 2003), which describes the extract decrease S as a function of time t, a substrate uptake rate ?.sub.s, an extract difference ?S=S.sub.1?S.sub.2 between the original gravity S.sub.1 and a target value for the residual extract content S.sub.2, and a duration for the lag phase ?:

[00001]$S\left(t\right)=S_2-\frac{?S}{1+e^{\left({?}_s\left(?-t\right)\right)}}$

[0044] The determination of the substrate uptake rate ?.sub.x is based upon the application of Monod kinetics to account for a dependence of the growth rate R upon a limiting substrate concentration during the growth process due to nutrient exhaustion, accumulation of toxic metabolic products, and the ion balance. As described in the article, Growth of Saccharomyces cerevisiae is controlled by its limited respiratory capacity: formulation and verification of a hypothesis, by B. Sonnenleitnert and O. K?ppeli, published in Biotechnology and Bioengineering, Vol. XXVIII, pp. 927-937, John Wiley & Sons, Inc., 1986, the substrate uptake rate ?.sub.x is given by:

[00002]${?}_s={?}_{s\max }.Math.\min \left(\frac{Z}{Z+K_Z},\frac{N}{N+K_n}\right).Math.\frac{K_E}{K_E+E}$

[0045] Here, Z describes the sugar concentration, N the nitrogen concentration, E the ethanol content, ?.sub.s,max the maximum specific growth rate, and K.sub.s the half-maximum concentration of the substrates.

[0046] The dependence upon temperature T in the growth of microorganisms as the dominant factor influencing the growth process is based upon an extended form of the B?lehr?dek model for the temperature rate r, as described in Model for Bacterial Culture Growth Rate Throughout the Entire Biokinetic Temperature Range by D. A. Ratowsky et al., published in Journal of Bacteriology, pp. 1222-1226, 1983:

?{square root over (r)}=?*(T?T.sub.max)*{1?e.sup.[b*(T?T.sup.max.sup.)]}.

[0047] Here, T.sub.min and T.sub.max describe a minimum and maximum growth temperature, respectively, and a, b empirical parameters. Based upon this temperature rate, the duration of the lag phase ? is given as a function of the temperature T:

?=(?(T?T.sub.min).Math.{1?e.sup.[b.Math.(T?T.sup.max.sup.)]}).sup.?2,

[0048] as proposed in Modeling of Bacterial Growth with Shifts in Temperature by M. H. Zwietering et al., Applied and Environmental Microbiology, pp. 204-213, 1994.

[0049] The extract content S can thus be determined as a function of time t on the basis of the following propagation model S(t):

[00003]$S\left(t\right)=S_2-\frac{\left(?S\right)}{1+e^{-.Math.{?}_{s\max }.Math.\min \left(\frac{S}{S+K_S},\frac{N}{N+K_n}\right).Math.{\left(a.Math.\left(T-T_{\min }\right).Math.\left\{1-e^{\left[b.Math.\left(T-T_{\max }\right)\right]}\right\}\right)}^2.Math.\left(t-{\left(a.Math.\left(T-T_{\min }\right).Math.\left\{1-e^{\left[b.Math.\left(T-T_{\max }\right)\right]}\right\}\right)}^{-2}+c\right)}}$

[0050] Here, c represents a further free parameter.

[0051] On the basis of the propagation model S(t) according to the invention, it is possible to determine a theoretical end time t.sub.end for the yeast propagation on the basis of the original gravity E.sub.1 and the temperature T at a start time t.sub.start. dS(t)

[0052] The extract decrease dS(t)/dt is related to the biomass increase dX(t)/dt, i.e., to the biomass concentration X(t) as a function of time t. The extract decrease dS(t)/dt corresponds to the nutrient uptake of the yeast, and therefore the biomass concentration X increases proportionally with the decrease in 25 extract S. The proportionality factor Y is referred to as biomass yield and represents the amount of biomass per substrate. Thus, the rate of biomass increase dX(t)/dt or the biomass growth rate ?.sub.x is obtained on the basis of the substrate uptake rate ?.sub.s as:

?.sub.x=Y.Math.?.sub.s

[0053] It is thus possible to calculate the increase in biomass ?X from the extract difference ?E. The biomass concentration X or the yeast concentration can be determined from the decrease in the extract S without the need to perform another direct measurement. Cell concentration monitoring is thus also possible in this way.

[0054] In FIG. 2, the residual extract S and the biomass concentration X are each shown as a function of time tin the form of measured values (S.sub.m(t) and X.sub.m(t), respectively) determined in a test measurement, and of reference curves (dashed and solid line) calculated by means of the propagation model S(t) and X(t), respectively. In this context, for the propagation model S(t), an extract difference ?S=4%, while the measured extract difference ?S was slightly larger. The model reflects the process of yeast propagation in a satisfactory manner.

REFERENCE SIGNS

[0055] 1 Container [0056] 2 Inlet [0057] 3 Outlet [0058] 4 Aeration device [0059] 5 Oxygen sensor [0060] 6 Thermometer [0061] W Wort [0062] H Yeast [0063] T Temperature [0064] FAN FAN content [0065] G Mixture of wort and yeast [0066] S Extract decrease [0067] t Time [0068] ?S Extract difference [0069] S.sub.1 Original gravity [0070] S.sub.2 Target value for residual extract content [0071] ? Duration of lag phase [0072] ?.sub.x Substrate uptake rate [0073] R Growth rate [0074] Z Sugar concentration [0075] N Nitrogen concentration [0076] E Ethanol content [0077] ?.sub.s,max Maximum specific growth rate [0078] K.sub.s Half-maximum concentration of substrates [0079] T.sub.min Minimum growth temperature [0080] T.sub.max Maximum growth temperature [0081] a,b Empirical parameters [0082] S(t) Propagation model [0083] t.sub.startStart time [0084] t.sub.end End time [0085] X(t) Biomass concentration [0086] Y Proportionality factor, biomass yield [0087] ?.sub.x Biomass growth rate [0088] ?X Increase in biomass [0089] S.sub.m(t) Measured values for extract content [0090] X.sub.m(t) Measured values for biomass concentration