Optimization of the hop drying process with respect to alpha acid content

J. agric. Engng Res. (1979) 24, 233-248

Optimization of the Hop Drying Process with Respect to Alpha Acid Content

P. E. DOE; R. C. MENARY*

Variations in u-acid levels of hops dried in commercial kilns in Tasmania led to an investigation of the hop drying process. a-acid levels were found to increase at certain temperatures for a period during drying and then decrease. The u-acid-temperature-time characteristics of Pride of Ringwood hops were measured during three successive harvests. A mathematical model of a deep bed drier predicts a-acid values over a wide range of drying conditions. Optimum drying conditions are presented.

1. Introduction

The quality of dried hops is determined by colour, texture, aroma and u-acid content. In the past few years with the introduction of quantitative methods of assessing the brewing quality of hops there has been an increase in emphasis on u-acid content. ‘9 2 Breweries began buying hops on the basis of u-acid content in 1969. The traditional assessment methods based on colour, feel and smell led to kiln designs which met these requirements but did not necessarily minimize the loss of u-acid content. The current emphasis on u-acid yield has prompted a reappraisal of growing, drying and packaging techniques. Much research has been done in developing high a-acid cultivar? and in selecting the optimum harvest time.** 5

The aim of this study is to investigate the reasons for the changes in u-acid which occur when hops are dried and to find a method of operation of the deep bed hop drying kiln which will give the least loss of u-acid and maximize throughput.

1.1. Factors influencing a-asd At almost every stage of hop production, from the cultivation of the vines to the storage of the

dried hops prior to brewing, factors both natural and artificial may influence the formation and retention of u-acid. Hop variety and climate,6 the temperature and soil moisture during formation ‘9 * temperature and sunlight during hop formation,9 the time at which the vines are strung up, ;he age at harvest5 and the type and adjustment of the picking machine all play an important part in the u-acid formation.

During the drying of the hops the more obvious influences on u-acid retention are the physical conditions such as bed-depth, air speed, air temperature and humidity; however, the drying process cannot be considered in isolation. Other factors such as the amount of damage to the lupulin glands during picking and the physiological age and composition of the hops at harvest have an effect during drying which may be of prime importance. The drying will, in turn, have an effect on the stability of the hops in storage.

1.2. Commercial kilns Apart from one or two traditional oast houses, the hop drying kilns in Tasmania are of two

distinct types; the deep bed drier (Fig. I) and the falling bed drier (Fig. 2). The deep bed drier comprises a wire mesh floor over a plenum chamber. Air is blown by an axial fan past an oil burner into the plenum and passes up through the bed of hops. The bed may be up to 1 m deep.

* University of Tasmania, Box 252C, G.P.O., Hobart, Tasmania, Australia

Received 4 September 1978; accepted in revised form 2 January 1979

233 OOZl-8634/79/030233 + 16 $02.00/O Q 1979 The British ?aciety for Research in Agricultural Enpineering

234 OPTIMIZATION OF HOP DRYING PROCESS

Fig. 1. Deep bed drier

Conditioning conveyors

Fan and splay

Fig. 2. Falling bed drier

The current practice is to load the whole floor before starting the fan and burner. The hops dry progressively from the bottom of the bed to the top in around 8-12 hours. The dry hops are either pushed from the kiln or rolled out on a belt conveyor and left to “condition” for several hours before being pressed into bales or pelletized. The purpose of conditioning is to allow the relatively dry bracts to equilibrate with the strigs.

The falling bed drier contains up to five floors, four of which are perforated metal plates pivoted to enable the hops to be dropped periodically onto a lower floor. The bottom floor is part of an open mesh belt conveyor used to remove the dried hops. Bed depths of around O-3 m are used and drying time is about 6-8 hours. The hops are conditioned on a series of belt conveyors with air passing up through the conveyors.

The falling bed drier produces a semi-continuous throughput with counter-flow conditions. The hops are exposed to progressively drier and hotter air as they traverse the kiln. For this reason, the hops are dried faster and more uniformly than in a deep bed drier where the hops at the top of the bed are in a saturated air stream for several hours before starting to dry.

1.3. Factors in&encing drying The factors which influence the drying can be separated into two categories; those which can be

directly controlled such as air temperature, air velocity and bed depth, and those which are the

P. E. DOE; R. C. MENARY 235

result of conditions external to the kiln and are not subject to control. Hop moisture content, air humidity and hop condition are in the latter category.

The factors influencing the loss of u-acid during drying are less easily identified. Burgess3 shows that u-acid content decreases with increasing temperature, but gives no indication of the rate at which this occurs. Zeisig,‘O in an investigation of hop drying at elevated temperatures, measured a-acid changes with air temperature and speed but did not explain or correlate the observations. Zeisig reported u-acid values of dried hops up to 1.06 times greater than the level before drying but offered no comment as to why this should occur.

2. Theory of deep bed hop drier operation Early in this investigation it was found that the variations of u-acid which occur during drying

are dependent on temperature and time and are not affected by moisture content or air humidity. This was demonstrated by holding hops at various temperatures in a saturated atmosphere; the hops showed the same variations of u-acid with time as hops dried at the same temperature. This discovery led the investigation along two paths; first to obtain measurements of u-acid-time- temperature behaviour and establish that the measurements were repeatable and secondly to develop a mathematical model of the deep bed hop drier which, with the measured u-acid-time temperature variations, can be used to predict the behaviour of a commercial kiln and so choose the optimum operating conditions.

2.1. Mathematical model The mathematical model is based on the method of Norden and Bainbridge.” The parameters

involved are air temperature, humidity and velocity; the depth of the bed of hops and the moisture content of the hops. Consideration of the heat and mass balances in the air stream and the hop bed leads to the following two partial differential equations relating the air and hop temperatures, the air humidity and the hop moisture content.

VaH aH PaM o -Jy +at+at= 7 . . . (1)

vaT, + aT, + ,waT, /GM ax

-&- at --i-=0 . . . (2)

where G = (C, + C,M)/(C, + C,H) . . . (3)

and 6 = (L + C,(T, - TA)/(C, 3 CS). . . . (4)

Two further equations express the rate of transfer of heat and mass to and from the surface of the hops. These equations are

?f$” + KJ,(H, - H) + K,(T, -.- TA) = 0 .

and

@.$.$ + K&H, - H) = 0 . . .

where KT and KY are lumped heat and mass transfer coefficients. These coefficients are impossible to measure directly, depending as they do on the detail of air flow over the hops and hence the shape and size of the hop cones. It is thus necessary to measure the behaviour of a real kiln and choose the heat and mass transfer coefficients which give a computer simulation which is closest to the measured kiln behaviour.


KEY TO SYMBOLS Symbol Significance Units C., C,, C,, C, specific heat of dry air, water, dry hops and water vapour respectively J/kg “C F1, F2, F3 functions relating a-acid, temperature, time and distance through bed

of hops H H,

K” K”*

KT

L M M* MU2

P T* T” t V

;

specific humidity of air specific humidity of air in equilibrium with hops according to measured sorption isotherm mass transfer coefficient mass transfer coefficient after end of constant rate drying period heat transfer coefficient latent heat of vaporisation of water moisture content of hops (dry basis) moisture content of hops at end of constant rate drying period moisture content of hops in equilibrium with surrounding air according to the measured sorption isotherm exponent in expression defining KY* temperature of air temperature of hops time velocity of air distance through bed of hops ratio of enthalpy of evaporation to specific enthalpy of air ratio of mass of dry hops to mass of dry air in a given volume of bed ratio of specific enthalpy of hops to air

kg/kg

kg/kg S-’

s-1

S-’

J/kg kg/kg kg/kg

kg/kg

“C “C

S

m/s m

P cr

In 1973, a full sized commercial drying kiln was instrumented by placing resistance tbermo- meters at fixed depths within the hop bed and recording the temperature variation with time during drying. The lumped heat and mass transfer coefficients KY and K, were adjusted so that the computed temperature profiles matched those measured. To obtain a reasonable match it was found necessary to provide for “falling rate” behaviour. This was done by specifying a moisture content in the hops, the “constant rate drying limit”, M*, below which the hops were assumed to dry under diffusion governing rather than convection governing conditions. “Falling rate” drying was simulated by adjusting the mass transfer coefficient according to the relationship

K,* = K,[(M - A&,)/(44* - M,)]P . . . (7)

where Kv* is the mass transfer coefficient replacing KY in Eqns (5) and (6) when the hop moisture content falls below M*. The values chosen in this manner were KY = 1.5 s-l, KT = 2.25 s-l, M* = 1.5 kg H,O/kg dry matter and P = 3.

The physical properties of air and water for use in the model are readily available.’ * Data for the hops were found experimentally. The specific heat of dry hops was measured by pressing dry hops into a pellet (40 mm x 25 mm dia.), heating the pellet and immersing in water in a calor- imeter. The specific heat of dry hops determined in this manner was l-4 kJ/kg.

The sorption isotherms required for the determination of the equilibrium relative humidity (I&) were found by drying hops to constant mass under controlled constant conditions of temperature and humidity.’ 3

2.2. Solution of the mathematical model The solution of Eqns (1) to (6) together with the appropriate initial and boundary conditions

to obtain the values of hop and air temperatures, the air humidity and hop moisture content at


Fig. 3.

1

Characteristic JI

A.x -. *

Distance tfx-ough bed x

Computinggridfor finite diflerence solution of mathematical model q/deep bed hop drier showing characteristics

various bed depths and times was achieved by the method of finite difference approximations to the partial derivatives. The computing is considerably reduced by using the method of characteristics.“’ The characteristic

dxldt = v . . . (8)

forms part of the computing network. Solving Eqns (1) to (6) along this characteristic corresponds to following the air through the bed of hops.

The process can be shortened further by following characteristics which are relatively widely spaced in time. This is possible as the hop temperature and moisture content vary much more slowly with time than the temperature and humidity of the air. Fig. 3 shows the characteristics in the time-distance plane.

The expression

vafqax + afqat

is written in terms of the complete derivative dH/dt as

vafrlax + afqat = (dH/dt), . . . (9)

where the subscript I indicates that the differentiation is carried out along the characteristic I shown in Fig. 3.

The other partial derivatives are similarly written in terms of complete derivatives as

val;lax + a7yat = (dT,/dt), . . . (10)

aT,l& = (dTH/dt),l . . . (11)

and

ahflat = (dM/dt),,. . . . (12)

On the substitution of Eqns (9), (lo), (11) and (12) into Eqns (l), (2), (5) and (6) the following equations in characteristic form result :

(dH/dt), = Fr . . . (13)

(dT/Adf)I = GI . . . (14)


(dWdt),, = --Fulp . . . (15)

and

(dTHldt)ll = CF&W - G&P 1.. (16)

where F,, Frr, GI and G1r are the values of

F = K&f, - H) . . . (17)

and

G = K,(TH - TA) ..* (18)

evaluated along the appropriate characteristics. Eqns (13) to (15) are solved by explicit “marching type” finite difference techniques. For

example, Eqn (13) appears as

(TV - T,)ldt = &@WH) - WY . . . (19)

which can be solved for T,. Eqn (15) is evaluated along characteristic II with the larger time step and becomes

(MU - M,)lndt = -Kv(K(TH) - H),/P * . . (20)

Typical values for the calculation mesh were 20 steps in the direction through the bed of hops and 150 time intervals (n = 150) between characteristics.

Once the air and hop temperatures, air humidity and hop moisture content have been determined, the u-acid levels are computed by using the measured u-acid-temperature-time relationship shown in Fig. 4.

The measured a-acid-temperature-time relationship can be represented mathematically as

indicating that a is a function Fl of hop temperature, TH and time t.

a = Fl(t, TH)

Time after start of drying (h I Fig. 4. Measured a-acid-temperature-time characteristics, 1976 harvest (used in mathematical model)

.*. (21)

P. E. DOE: R. C. MENARY 239

The computed hop temperature-(distance through bed)-time relationship can be represented as

T, = F2(x, t). . . . (22)

What is required is the function relating a-acid to distance through the bed and time, namely

a = F3(x, t). . . . (23)

Eqn (21) can be differentiated with respect to the two variables t and TH to give

. . . Likewise from Eqn (21)

dT, = (g$dx + (g),dr- . . . (25)

Putting Eqn (25) in Eqn (24) gives

(24)

da = [(T)T, + (g)t. (T)X ]dt + [(I%); (z)jdx ... (26)

but from Eqn (23)

da = ($+t + ($$lx. . . . (27)

Comparing Eqns (26) and (27) gives

(f-qx = (2) T, + (gf . (fg); ... (28)

The partial derivatives on the right-hand side of Eqn (28) were calculated from the previously found relationships, Eqns (21) and (22), at 10 equally spaced depths through the bed of hops using a double linear interpolation procedure to determine intermediate values.

The required function, Eqn (23), was then found by multiplying the partial derivative on the left-hand side of Eqn (28) by the appropriate time interval and accumulating the increments. By this process the variation of u-acid with bed depth and time is computed. Mean u-acid values were obtained by averaging the values obtained at each bed depth to give the variation of u-acid with time for the whole bed.

Computer programmes were written in the FOCAL and ALGOL languages and run on Digital PDPS/E and Burroughs B6700 computers.

3. Materials and methods

3.1. Hops Throughout this investigation the hops used were “Pride of Ringwood” variety grown in hop

gardens at Bushy Park and Ranelagh in southern Tasmania. Except where indicated the hops were machine picked and dried or otherwise tested within 3 hours of cutting the vines.

3.2.1. Sample preparation 3.2. Analysis

Solvent extraction of u-acids was carried out in a stomacher (Colworth Stomacher 400) using 15 g of green hops and 50 ml of toluene. Extraction time was 10 minutes.


3.2.2. Moisture content determination The moisture content of green and dried hops was determined by oven drying about 10 g of

hops at 107°C for one hour.

3.2.3. Determination of a-acid Several methods were used for the determination of a-acids. Details are as follows: (a) conductometric titration of a-acids with methanolic lead acetate,’ 5 (b) spectrophotometer determination using petroleum ether as solvent,’ 6 (c) nuclear magnetic resonance spectroscopic determination of a-acid composition.’ ’

3.3. Drying behaviour of hops Hops were dried in around 0.3 kg lots on glass fibre gauze trays in a recirculating air conditioned

wind tunnel. The height of hops on the trays was not more than 30 mm so that all the cones were equally exposed to the air stream. Air flow was horizontal over and under the hops. Details of the wind tunnel are given in Doe, James and Young. la At intervals during drying, hop cones were removed at random from the racks, placed in polyethylene bags and stored at -20°C before analysis. The maximum period between drying and analysis was 14 days. Samples were analysed in duplicate for a-acid levels and triplicate for moisture content.

During the 1974 harvest over 150 samples were dried and analysed by the polarimetric method. These samples were stored at -20°C and reanalysed in 1975 by the conductometric method. In the 1975 harvest the measurements of 1974 were checked, the analysis being by the conductometric method.

The measurements were repeated during the 1976 harvest with one difference; the resistance thermometer forming part of the temperature controller of the recirculating air conditioned wind tunnel was placed on a tray amongst the hops. The air temperature was thus controlled so as to keep the hops at constant temperature. The air temperature was initially higher than the hops but fell as the hops dried. In this way the measurements of drying behaviour were made at constant hop temperature. Some 114 samples were analysed conductometrically.

3.4. Alpha-acid determinations on green hops An important aspect of the work was establishing the validity of the increase in a-acid during the

early stages of drying. It is generally recognized that there are problems associated with the reliability of a-acid determinations on green hops. In order to investigate the effectiveness of the extraction process, two methods of mechanical disruption were used, namely homogenizer (Sorvall Omnimixer) and stomacher. The stomacher level gave a significantly higher LCV (lead conductance value) value from green hops than did the homogenizer; the means and standard errors of 20 determinations by each method being 11.9 + 0.25 and 9.88 + 0.58 respectively. The stomacher gave smaller variations between determinations. In both methods the addition of free moisture caused a slight reduction in the LCV measured; the both cases an emulsion was found which settled to the bottom of the filtrate. The emulsion had a higher LCV than the supernatent. When the emulsion was titrated separately a correction could be made and this accounted for the observed decrease in LCV due to the presence of free moisture. The emulsion can be eliminated by the addition of 10 g anhydrous sodium sulphate before extraction. The two methods of mechanical disruption were equally effective for dried hops.

During a drying run at 48”C, samples were taken at 30 minute intervals and analysed for a-acid by both LCV and spectrophotometric methods. There was no significant difference between the a-acid levels determined by the two methods. When lead salts were washed with methanol, dried and converted to free acids, the measured a-acid on a spectrophotometric basis corresponded to the LCV. Furthermore, when equal masses of lead salts from each of the drying run samples were converted to free acids and subjected to NMR the level of acids was the same for all samples as were the proportions of humulone, cohumulone and adhumulone (63 : 27 : 10). This evidence


suggests that substances other than u-acids are not contributing to the increases in LCV’s obtained during drying.

3.5. Experimental kiln During 1976 it became apparent that there may be advantages in drying at higher air speeds and

temperatures than were available in existing kilns. lo It was thought unwise to extrapolate the one set of data obtained in 1973 at a temperature of 51°C to the higher temperatures contemplated. For this reason an experimental kiln was built (Fig. 5). The experimental kiln has a vertical working section (A) 0.61 m square and 1 m long. Temperature can be controlled up to 140°C. Fan (B) has sufficient capacity to provide air speeds in the working section in excess of 1 m/s. Heat input is from a 30 kW electric finned tube heater bank (C) and humidity can be adjusted by use of a flap valve (D) to obtain partial recirculation of the exhaust air. Up to 38 kg of fresh hops are contained in glass-fibre gauze basket (E) which may be removed from the kiln for weighing. Air temperatures at different depths in the bed of hops can be recorded with a multipoint temperature recorder.

During the drying runs, samples are taken from different levels in the bed and together with a sample of fresh hops are analysed for moisture content and n-acid level.

Fig. 5. Detail of experimental kiln

4. Results

4.1. Alpha-acid-time-temperature relationship The object of determining the u-acid-time-temperature relationship was to provide data for use

in the mathematical model. The first measurements (1974) reported in Doe and Menary,13 although showing a rather confused pattern of variation of u-acid with temperature and time, did indicate that levels of u-acid increased in the early stages of drying. The re-analysis of the 1974 samples by the conductometric method confirmed this increase and showed that the increase was relatively rapid and could easily be missed if insufficient samples were taken over the period of the increase. A typical LCV time curve is given in Fig. 6. At an air temperature of 48”C, u-acid content reached a maximum level after 6 hours of drying. The effect was temperature and time dependent and was not related to the moisture content of the hops; hops heated in a saturated atmosphere showed the same characteristics as hops dried at the same temperature.

The measurements of 1975 (Fig. 7) confirmed the 1974 measurements.


Fig. 6.

7 I I I I I I I I I I 0 2 4 6 8 IO 12 14 16 18 91 ’ 20

Time after start of drying (h)

Typical lead conductance value-time curve, Pride of Ringwood hops at 48°C

Time after start of drylng (h 1 Fig. 7. a-acid-temperature-time characteristics, 1975 harvest

The 1976 results (Fig. 4) obtained at constant hop temperature rather than constant air temperature were used as the u-acid-time-temperature function Fl [Eqn (21)] in the mathematical model of the deep bed drier. The results are expressed as a ratio of LCV at particular times during the drying run to LCV before kilning and represent 114 samples analysed conductometrically. The times after the start of drying at which the maximum a-acid values occur are rather less in Fig. 4 than in Figs 6 and 7. The reasons for this have not been fully investigated but there is some evidence to suggest that the physiological age of the hops at harvest can influence the u-acid changes which occur during drying.

4.2. Experimental kiln results During the 1977 harvest, a total of 12 drying runs was carried out in the experimental kiln. Air

speeds were in the range 0.22 to 0.81 m/s at temperatures up to 89°C and with various bed depths. Table I gives the complete results. It can be seen that several drying runs produced increases in the u-acid content while others resulted in no change or some loss. Two of the runs are suspect- one marked * had an unusually high value of u-acid in the fresh sample and the other marked * * took place after the harvest and used hops held for several days at 4°C in a cool store.


Run

1 14 March 1977 2 22 March 1977 3 24 March 1977 4 28 March 1977 5* 29 March 1977 6 30 March 1977 7 31 March 1977 8 5 April 1977 9 6 April 1977

10 7 April 1977 11 15 April 1977 12** 18 April 1977

TABLE I

Pride of Ringwood hops dried in experimental drier during 1977 harvest -

I

-

Air ‘emperature,

“C

71 81 84 89 64 49 82

80-+63 7 86 88 89 49

7--

-

Bed depth, m

-___ 044 0.46 0.43 0.43 0.51 0.53 0.84 0.50 0.59 0.55 0.52 0.34

-

._

-

Air velocity,

mls -___

0.59 0.81 0.63 0.56 0.54 0.23 0.22 0.44 0.23 0.33 0.40 0.61

-

-

a-acid a-acid before after drying, drying,

% %

8.2 11.1 10.1 9.8 10.4 12.8 12.6 12.7 16.1 14.8 14.5 16.2 14.1 12.5 9.0 10.5 8.5 9.9

11.2 10.4 ? 10.5

10.4 9.8

-

_-

-

Moisture content

(w.6.) after drying

-- 14.3 10.1 3.9 7.3

13.8 16.8 3.8

16.2 4.2 3.7 2.6 8.9

-

_-

-

Drying time,

h

3.0 2.1 2.1 1.5 3.3 7.5 3.5 2.2 2.3 2.1 2.5

10.3

*, ** See section 4.2 in text t Air temperature falling linearly with time during drying

4.3. Mathematical model result.\ Fig. 8 shows measured air temperatures for one of the 12 drying runs in the experimental kiln

together with the computed air temperature variations for the data shown. Figs 9,10 and 11 show the computed air humidity, hop moisture content and hop temperature for the same run. It can be seen that the hops dry progressively through the bed-hops at the bottom of the bed (corres- ponding to x/d = 0 in the figures) dry first and reach a temperature close to that of the air while hops at the top of the bed stay for some time at low temperatures in near saturated air before starting to dry.

Fig. 22 shows the computed variation in u-acid level with bed depth and time; also shown (dotted) is the variation in total u-acid with time, obtained by averaging over the bed depth.

201 I I 0 I 2

Time after start of drying (h)

Fig. 8. Measured and computed air temperatures, Run 1, 1977 (refer to Table I for details of air speed, temperature, etc.). Numbers on curves are dimensionless distance through bed: disrance from bottom/bed depth


Time after start of dry& (h)

Fig. 9. Computed air humidities, Run I, 1977

1 I I 0 I 2

Time after start of drying(h)

Fig. 10. Computed hop moisture content, Run 1, 1977

4.4. Correlation between measured and computed results Measured and computed a-acid values, averaged over the whole bed, are plotted in Fig. 13.

The measured a-acid values are those obtained from the drying runs in the experimental kiln and reduced to a relative basis by dividing by the a-acid of the hops when fresh. The computed values are obtained from the mathematical model and are for drying at the same conditions of bed depth, air temperature and velocity used in the experimental kiln and for the same drying time.


With the two suspect runs, marked * and **, discarded for the reasons given in section 4.2 above, the correlation coefficient, r, for the remaining 9 points is r = 0.884. This corresponds to a 98.8 % confidence level for the 7 degree-of-freedom system. In other words, the probability that the agreement between the measured and calculated u-acid values is fortuitous is less than 1.2 %.

0 e 60- : 2 i 50-

E 8 40-

I

Tme after stcrt of drying (h 1 Fig. Il. Computed hop temperature, Run ?, 1977

093l 0

I I J I 2 --+-

Time after start of dryrig (h 1 Fig. 12. Computed a-acid variations, Run 1 1977

5. Optimum drying conditions The factors that may be controlled to maximize the u-acid yield during hop drying are air

temperature, air velocity and bed depth. Each can be varied during a drying run so that there is an infinite number of possibilities for kiln operation. In practice, the heater and fan capacities of a kiln will place an upper limit on air temperature and velocity while the maximum bed depth is limited by convenience of loading to about 1 m. (At air velocities above about 0.5 m/s a net or similar device must be placed above the hops to prevent them becoming airborne.) Fric and Makovec19 commented on the poor brewing qualities of hops dried at temperatures above 60°C at air speeds from 0.3 to 6.5 m/s.


51

Computed relative o-acid

Fig. 13. Computed and measured a-acid values, all runs, 1977

__-- _-- -~ __--- __---

o-acid mtio (o-acid after/o-acid before drvim)

Bed depth (ml

Fig. 14. Equal a-acid contours and drying times for hops dried to 10% moisture content at 0.5 m/s air speed

An examination of the LCV-temperature-time curves (Fig. 4) suggests that the best method of drying is one which results in the hops being dry enough to bale or pelletize after 3-4 hours of drying. In practice this would be difficult to achieve in a bed of a reasonable depth as the hops in the bottom layers would be dry in a much shorter time than those at the top. A series of runs of the mathematical model was used to produce the equal u-acid contours shown in Fig. 14. It can be seen that with constant temperature, air velocity and bed depth during drying, the optimum drying conditions for an air velocity of 05 m/s are 69°C with a bed depth of 0.4 m.

Also shown in Fig. 14 are the computed contours of the drying time required to dry to 10% m-c. at various temperatures and bed depths at an air speed of 0.5 m/s. Fig. 14 enables the kiln operator to select operating conditions which maximize throughput.

The possibility of varying one or all of the three factors with time during drying complicates the identification of optimum drying conditions.


Variation of air temperature with time as in Run 8 of Table I or the variation of air speed with time may be worthwhile. Another consideration becoming increasingly important to the industry is the cost of fuel oil for the kilns. It may be that some trade-off of hop quality for improved drying economy will occur.

6. Conclusion This project has established that there is a consistent variation of u-acid content in “Pride of

Ringwood” hops during drying. The variation was found to be dependent on hop temperature and time. The behaviour of hops measured at constant temperature has been used to predict the behaviour of a commercial hop drying kiln with the use of a mathematical model. This model takes into account the fact that temperatures, humidities and moisture contents vary both with time and position within a deep bed drier. The efficacy of the mathematical model was established by comparing computed a-acid variations with those measured in an experimental deep bed drier over a range of drying conditions. At present a survey is being conducted by Tasmanian hop growers of the characteristics (heater capacity, air speed, etc.) of existing kilns. This work will enable the optimum operating conditions to be selected within the limitations of the kilns. Further work is proposed on the operation of falling bed driers, with particular reference to economic operation. The physiological and biochemical reasons for the observed a-acid variations are under active investigation.

Acknowledgements

The authors wish to acknowledge the support of the Reserve Bank Rural Credits Development Fund, Australian Hop Marketers Pty. Ltd., Mrs E. Williams and Mrs A. McEldowney for technical assistance.

REFERENCES

’ Clarke, B. J.; Hildebrand, R. P.; White, A. W. The application oj salt formation in the chemistry and technology of hop resins. J. Inst. Brew., 1976 82 212

’ Aitken, R. A.; Bruce, A.; Harris, J. 0.; Seaton, J. C. Quantitative analyses of beer bittering substances by thin-layer chromatography. J. Inst. Brew., 1967 73 528

3 Burgess, A. H. Hops: Botany, Cultivation and Utilisation. London, Leonard Hill, 1964 52 ’ Skber, R. N.; Kavanagk, T. E.; H&brand, R. P.; Clarke, B. J. Hop maturation and resin formation:

field trials. J. Sci. Fd. Agric., 1974 25 1121 s Skinner, R. N.; Kavanagh, T. E.; Hughes, G. A. ; Hi&hand, R. P.; Clarke, B. J. Hop maturation

and resin formation. Effects of extended harvestings on subsequent hop crops. J. Sci. Fd. Agric., 1975 26 1037

b Ha&e, P.; Petrlcek, D. The influence of environmental conditions on the proportions of cohumulone, adhumulone andhumulone in hop a-acids. J. Inst. Brew., 1967 73 51

7 Hautke, P.; Petricek, D. Vztahy mezi klimutem, v4;nosy a chemickl;m slozenim chmele (Interrelations between climate, yieldandchemicalcomposition of hops). Chmelarstvi, 1972 45 (1) 8

’ Hautke, P. ; Petricek, D. Correlation between chemical composition of hop cones, environment and yield. Brauwissenschaft, 1973 26 193

’ Smith, L. P. The influence of temperature and sunshine on the alpha-acid content of hops. Agric. Meteorol., 1974 13 (3) 375

lo Zeizig, H. D. Drying of hops in the small farms and co-operatives qf Western Europe. Proc. Int. Hop Growers Convention, Brussels, 1970

” Norden, P.; Bainbridge, N. W. An analysis of the process of through drying of wet textile materials. Applied Polymer Symposium, 1971 (18) 1111

Ia Zimmerman, 0. T.; Lavine, I. Psychrometric Tables and Charts, 2nd ed. Dover: New Hampshire Industrial Research Services Inc., 1964

‘3 Doe, P. E.; Menary, R. C. Optimisation of the hop drying process., a preliminary investigation. Inst. Engrs Aust. Proc. Annual Conference, 1975


u Smith, G. D. Numerical Solution of Partial Differential Equations. London: Oxford University Press, 1965 100

l5 Morgan, K.; Rees, T. C. Determination of hop a-acids by semi-automatic conductometric titration. J. Inst. Brew., 1968 74 383

I6 Alderton, G.; Bailey, G. F.; Lewis, J. C.; Stitt, F. Spectrophotometric determination of humulone complex and lupulone in hops. Analyt. Chem., 1954 26 (6) 983

‘7 Molyneux, R. J.; Wong, Y. Nuclear magnetic resonance spectroscopic determination of a and /I acid homolog composition in hops. J. Agric. Fd Chem., 1975 23 (6) 1201

I8 Doe, P. E.; James, D. G.; Young, F. Studies on theprocessing of abalone. V. Dried abalone: techniques and characteristics of drying. Fd. Technol. Aust., 1973 25 (3) 189

‘9 Fric, V.; Makovec, K. (The e&ct of different temperatures andflow rates of drying air on hop quality). Vliv rozdilntho teplotniho rezimu a rychlosti proudeni susichio vzauchu na kualitu chmele. Roslinnit Vsoba, 1976 22 (3) 303

Optimization of the hop drying process with respect to alpha acid content

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