Induction of conductance and capacitance changes by food-borne fungi

Food Microbiology,

1989, 6, 231-244

Induction of conductance food-borne fungi Irene A. Watson-Craik*, Department Strathclyde, Received

and capacitance

K. E. Aidoot

of Bioscience

changes

by

and J. G. Anderson

and Biotechnology, Glasgow Gl lXW, UK.

University

of

14 May 1989

A detailed examination was made ofthe conductance and capacitance changes caused by moulds and yeasts when cultured on a wide range of commercial and experimentally formulated media. Media which comprised only soya peptone and yeast extract (SPIYE) consistently elicited an increasing conductance curve of good quality. ‘Spiked’ fruit juice inocula resulted, however, in signal reversal. A comparable reversal was induced by medium supplementation with selected monosaccharides. Incorporation of a range of glucose and ammonium concentrations into SPIYE confirmed that reversal of the conductimetric responses could be induced by appropriate supplementation. Product interference may thus prove problematic. Capacitance signal responses, however, were both consistent and produced, with selected media, earlier detection times.

Introduction The use of impedimetric measurements for the detection and quantification of food-spoilage micro-organisms is now well recognised, particularly for quality assurance in food and related industries. Impedance changes associated with microbial metabolism have been used to determine bacterial numbers in a variety of foods such as fish (Ogden 1986), confectionery materials (Pugh et al. 19881, meat (Bulte and Reuter 19841, milk (Firstenberg-Eden and Tricarico

correspondence should be *To whom addressed at Department of Bioscience and Biotechnology, Division of Applied Microbiology, University of Strathclyde, Royal College, 204 George Street, Glasgow Gl lXW, UK t Present address: The Queen’s Glasgow, Glasgow G3 6LP, UK 0740-0020/89/060231

+ 14 $02.00/O

College

1983, Khayat and Richardson, 1986) and cheese (Khayat et al. 1988). Rapid detection of fungi is also of great importance in quality assurance systems. The traditional colony count method is both labour- and time-intensive, and the results subject to a wide range of influential factors such as sample size and preparation, nature of the diluent, culture medium, plating technique, dilution error and competitive inhibition (Jarvis et al. 1983). Moreover, Sharpe (1987) considered that the inability of counts either to relate to any instant other than the instant of sampling or to predict the ability of organisms to multiply in the food reduced their Impedance changes, credibility. however, are as much a function of microbial metabolic activity as of microbial number and may be used, therefore, not only to estimate bioburden but also to predict shelf lives. A fundamental requirement in the impedimetric monitoring of fungi in 0 1989 AcademicPressLimited

232

I. A. Watson-Craik

and J. G. Anderson

foodstuffs is the development of media which give consistent signal responses with the target range of organisms and are compatible with the wide variety of foods to be tested. It is clear that conventional fungal media, such as malt extract agar, which support the growth of a wide range of species, do not, in many cases, elicit impedimetric changes that are either significant or consistent (Firstenberg-Eden and Zindulis 1987, Schaertel et al. 1987). Diluents used to prepare pure culture inocula have included 0.1% peptone (Zindulis 1984, Schaertel et al. 19871, and a range of test media such as glucose/malt extract broth (Williams and Wood 198.6) or carbon baselammonium sulphate agar (Zindulis, 1984). Few studies have, however, assessed the effects on the impedimetric responses when spiked or naturally contaminated foods are used as the inocula, although Zindulis (1984) resuspended a yeast, isolated from concentrated orange juice, in dilute (1: 10) orange juice prior to inoculation. It was not clear, however, whether signal responses were significantly affected by the selected diluent. A research programme was therefore initiated to investigate the impedimetric changes in both capacitance and conductance induced by fungal growth. A wide range of both commercially available and specially formulated media was screened against 31 mould/yeast culture collection species and 127 fungal food isolates, and various media formulations were developed which elicited conductance and capacitance curves of good quality with a wide range of test species. Medium modifications were, however, necessitated as a result of conductimetric responses to fruit juice inocula ‘spiked’ with test species. The possible implications of interactive foodfmould responses are discussed, and the applicabilities of conductance and capacitance monitoring critically assessed.

Methods Inocula

preparation

Twenty-seven mould species (Aspergillus oryzae, A. niger, A. flavus, A. parasiticus, A. terreus, A. nidulans, A. versicolor, A. ochraceus, Botrytis cinerea, Byssochlamys fulva, Cladosporium herbarum, Geotrichum candidum, Monascus ruber, Moniliella acetobutens, Mucor hiemalis, Paecilomyces variotii, Penicillium chrysogenum, P. citrinum, P. digitatum, P. expansum, P. griseofulvum, Rhizopus stolonifer, R. oligosporus, Stachybotrys atra and Trichoderma viride) and four yeast species (Saccharomyces cerevisiae K2NI3, Schizosaccharomyces pombe, Hansenula anomalu and Rhodotorula sp.) were obtained from the Commonwealth Mycological Insitute, Kew. One hundred and twentyseven moulds and yeasts, isolated from foodstuffs such as yoghurt, orange juice, grapes, soft cheese and tomato paste, were identified to at least genus level. All cultures were grown on potato dextrose agar (PDA) slopes at 30°C for 7 days or until sporulation was evident. The spores or yeast cells were then washed off the slopes with 5 ml of a sterile solution of 0.05% (v/v) Tween 80 in distilled water. The suspensions were immediately diluted with either sterile distilled water or sterile fruit juice solutions to produce inocula containing from 103 to 105 cfu ml-i, as determined by direct microscopical counts. Instrumentation Capacitance (0 and conductance (G) changes were simultaneously monitored by use of a Bactometer Ml20 Microbial Monitoring System. The wells (16) in each disposable module were filled with 0.9 ml of the appropriate sterile medium. Media containing agar were allowed to set prior to inoculation with a 0.1 ml volume of cell suspension. The modules were then inserted into the instrument, in which the incubators were maintained at 25”C, and C and G were monitored for a period (2448 h) which depended on the species under examination.

Media Commercially available media. Wart, soytone and tryptic soya agar media, all from Difco; eugon agar and broth, from BLL; malt extract (ME), Czapek-Dox (CD), wort, potato dextrose and Sabouraud dextrose broths and

1. Media

formulated Medium

and tested

Note:

Medium

Components

supplied

by a BDH

Carbon Base/ammonium tartrate (CBAT) Carbon Base/ammonium sulphate (CBAS) Malt extract/yeast extractfglucoselpeptone Roes & Luckner (1984) medium Glucose/peptone/yeast extract Glucose/yeast extract/tryptone/phosphate Glucose/yeast extract/ammonium sulphate Yeast extract/phosphate/starch Carbon Base/tryptone/yeast extract Starch/ammonium sulphate Starch/potassium nitrate Starchkryptone Starchlsoya peptone Williams & Wood (1986) medium Tryptonekoya peptone Starch/yeast extract Soya peptone Glucose/yeast extract Tryptonelyeast extract Tryptonefsoya peptoneiyeast extract Soya peptonelyeast extract (0.5%) Soya peptone/yeast extract (0.25%)

Table

b Difco

d Sigma

capacitance

e Oxoid

for both

10

5

20

20

10

50

15

15 15 15 15

15 15

22 22

5 5 5 2.5

.lO

4

3 5 5 4 5

3

and conductance

5

5

15 15

15

5

5

5

5 5 2-5

20

5

15

Medium

changes.

20

3

2

0.7

3 3.7

components

3

1

5

0.1

(g l-1)

4

o-3

l-5 0.05

234

I. A. Watson-Craik

and J. G. Anderson

ME, CD, tryptone soya and neutralised soya peptone agar media, all manufactured by Oxoid, were screened. Formulated media. These media, detailed in Table 1, were solidified with agar 11.5% (w/v)] (Oxoid, No. 3). Medium development. By use of conductance and capacitance monitoring, those media were recorded which producedsignificant and consistent signal changes, when inoculated with a cell suspensionin distilled water. These media were then tested with inocula in which the cell suspensions,washed from the agar slopes,were suspendedin UHT concentrated fruit (orange, grapefuit, apple) juices (‘Jucee’trademark). The resultant signal changeswere compared with those produced when the test inocula were suspended in distilled water and usedto inoculate media which had beensupplementedwith 10%(w/v) glucose,fructose, galactoseand sorbose. The effects of glucose [O-3% (w/v)] and nitrogen [O-0.05% (w/v) NH4-Nl supplementation, both individually and in combination, were then investigated. The patterns of signal responseswere recorded in terms of both strength and direction, and used to select those media which generated signal changes unlikely to be affected by the addition of food inocula containing significant concentrations of either labile carbohydrates or nitrogenous substrates.

Results Screening of commercially available and formulated media Conductance and capacitance changes generated in commercially available media were both medium- and speciesspecific. Thus, although growth on malt extract and Czapek-Dox agar of all species tested was moderate to heavy, only the growth of A. niger on malt extract agar elicited an increasing conductance signal (Fig. 1). Again, although the growth of Paecilomyces variotii resulted in a decreasing conductance signal in both media, capacitance responses differed (Fig. 1). With wort agar (Fig. 21, although significant capacitance increases were observed with all species tested, curve quality was poor. Significant conductance increases were evident

only with A. niger. No medium tested produced good quality-increasing conductance or capacitance curves with all species tested, although curve qualities were better on agar-supplemented than on liquid media. Promising conductance responses were, nevertheless, recorded with neutralized soya peptone agar, on which no species tested generated decreasing conductance signals. Individual species curve qualities, however, varied (Fig. 3). It was clear that signal responses could be manipulated by selection of an appropriate culture medium, and a range of carbon and nitrogen sources (Table 1) was therefore screened. Although consistent capacitance increases were recorded with several media such as the Williams and Wood (1986) medium, CBAS and CBAT, baseline drifts were excessive and accelerations poor. One medium in particular, soya peptone (0.5% w/vYyeast extract (OZ?YLJ w/v) (SP/YE) elicited good quality increasing conductance curves with all species screened (Fig. 4). Curve qualities were comparable when component concentrations were reduced to 0.25%. Signal responses to fruit juice inocula SPNE medium generated good quality increasing conductance curves when the inoculum was suspended in distilled water. Signal responses to ‘spiked’ fruit juice (orange, apple, grapefruit) inocula were then assessed. With all species tested, the presence of fruit juice induced a signal reversal (Fig. 5), such that a decreasing conductance curve was recorded. When the inoculum was suspended in diluted (1:2) fruit juice, the signal reverse was temporary, whereas inoculum suspension in diluted (1: 10) juice elicited only a minor and transient signal reversal. It was clear that the direction and strength of the response was dependent on the identity

Conductance

and capacitance

. ,60

-

Molt

Extract

,

agor

in fungi

235

,

,

-

I60

, 140

120

I00

80

60

40

20

5 BASE STEP

IO

I5

20

= 3673 = 18.28

160

25 Time

Czopek-Dox

30

35

40

45

(h )

BASE STEP

= 892 q 2.84

160

ogor

140

I20

60

40

20

I 5 BASE STEP

= 2603 = 5.38

I

I

I

I

I

I

I

I

10

I5

20

25

30

35

40

45

Time

( h )

BASE STEP

= 960 = I.56

Fig. 1. Capacitance (C) and conductance (G) changes recorded in malt extract and Czapek-Dox (-), agar each inoculated with three of Cladosporium herbarum Paecilomyces uariotii (------I, A. niger (-----I, Geotrichum candidurn (- -----).

236

I. A. Watson-Craik

160 I-

wart

and J. G. Anderson

ogor

,

-

160

-

100 : s t 2 5 ”

-so

5 BASE STEP

= 2479 = 0.02

IO

15

25

20 Time

30 (h)

35

40

-

60

-

40

-

20

45 BASE STEP

= 1170 =5.41

Fig. 2. Capacitance (C) and conductance (G) changes in wart agar inoculated with A. niger ), Gee. candidum c---------Jand P. viridicatum (- - - - - -). (of the diluent. To determine the influential factor, inocula suspended in distilled water were dispensed into media BP/ YE) supplemented (10% w/v) with organic acids (lactic, citric) or monosaccharides (glucose, fructose, galactose, sorbose). Signal reversals could not be induced by the addition of the organic acids. When, however, the medium was supplemented with simple sugars, conductance decreases were recorded on the growth of all species tested. Medium reformulation It was apparent, therefore, that SP/YE was not buffered against the effects of foodstuffs containing significant concentrations of simple carbohydrates. It was possible that foodstuffs, such as eggs, which contain significant concentrations

of nitrogenous substrates, would have comparable repercussions. The effects of supplementation of SPM3 based media, at decreasing concentrations of both yeast extract and soya peptone, with varying concentrations of glucose and/or ammonium sulphate or chloride were thus assessed,with the aim of developing media formulations to which the addition of either labile carbohydrates or a nitrogen source would have no effect on the quality of the impedimetric response. The quality of the response was described in terms of both signal strength and direction. The qualities [good (G), fair (F), poor (P)] of acceleration, baseline and maximum values (FirstenbergEden and Eden 1984) were used (Table 2) to determine the overall signal strength of each curve. This was assigned on the

Conductance

and capacitance

in fungi

237

I

140

Neutrolised

soyapeptone

/’

ogar

I’ I’ I’

120

/

I’ I’ I’

100

-

/ / I’

2 6 t -2 6 0

,! . -;/

EO-

. 60

-

,I

.

I

I’ If

,

/-

/ /’

,I

I /

,

-

/’

,I

I

.‘-

/

II

,

40

_-- -

/’

,I /’

/

20 /

-we-t-----+-----J 7 EASE STEP

,J’ 14

21

= 1640 = 9.05

,

I 35

28 Time

I 42

I. 49

(h)

Fig. 3. Conductance increases in neutralised soya peptone agar after inoculation A. niger (---------), Geo. candidum (- - - - - -), Paec. uariotii (-------I herbarum (-), P. roquefortii (--- -- -).

basis of the lowest quality parameter. Thus, for example, a curve whose acceleration, baseline and maximum value were assessed as good, good and fair, respectively, would have an overall fair (F) signal strength. From Table 3 it can be seen that growth on media which contained yeast extract and/or soya peptone only, at concentrations which ranged from 0.25 to 2.0% (w/v), generated increasing conductance signals of good strength. Capacitance signal responses were, however, poor. Media supplementation with glucose only resulted in conductance decreases. Capacitance responses were dependent on the concentrations of yeast extract and/or soya peptone. Good quality

with

C. and

increasing and poor quality capacitance signals were recorded with concentrations of al.0 and <0.5%, respectively. Supplementation with only ammonium chloride or sulphate elicited signals of variable quality. In general, better quality increasing conductance signals were recorded with increasing YE and/or SP concentrations. Capacitance responses were uniformly poor. Addition 10.05% (w/v) NHb-Nl of ammonium chloride or sulphate to media supplemented with glucose yielded interesting data. The responses were related to the concentrations present of both glucose and YE and/or SP; with low (~0.25%) yeast extract concentrations, supplementation with glucose (l+O3.0%) generated good quality increasing

238

I. A. Watson-Craik

and J. G. Anderson

, 160

-

140

-

120

-

iti 100

-

6 t g 6 ”

BO-

60

-

40

-

---___a-*

5 BASE STEP

= 403 = I.55

IO

15

20

25 Time

30

35

40

45

50

( h)

Fig. 4. Conductance changes in SPNE agar after inoculation with Botrytis cinerea (-), candidum (------), Syncephalastrum racemosum terreus (- --- -) and P. funiculosum t------J.

conductance curves. At higher (31.0%) yeast extract concentrations the responses were dependent on the glucose added and were more variable. Thus, although the addition of 3.0% glucose always resulted in a decreasing conductance signal, signal strengths of D or iD, and D or DI were recorded with glucose supplementations of 1-O and O-5,%, respectively. The data presented in Table 3 suggested that the impedimetric responses of glucose/ammonium-based media may also be vulnerable to yeast extract supplementation. The use of such media has been documented. Carbon base/ammonium tartrate (CBAT) medium, for example, has been reported (Schaertel et al. 1987, Connolly et al. 1988) to gener-

(- - - -), Fusarium

culmorum

(- ----),

Go. A.

ate significant conductance decreases with a wide range of yeasts. The conductance changes generated by the growth of moulds on CBAT supplemented with a range [O-2*0% (w/v)1 of yeast extract concentrations were therefore recorded. From Fig. 6 it can be seen that the direction of the conductimetric response could be reversed by appropriate yeast extract supplementation. Capacitance responses were, however, more consistent, and with no media formulation were decreasing capacitance signals recorded. An increasing signal was elicited by the addition of glucose, and although supplementation of media with only ammonium chloride or sulphate did not generate an increasing signal, incorporation of nitrogen into

Conductance

BASE STEP

= 339 = I.54

and capacitance

in fungi

239

Time(h)

Fig. 5. Temporal conductance changes on inoculation of SPNE medium with an inoculum (A. famigutus) suspended in distilled water (--) or orange (---------I, apple C-------j or grapefruit (--------) juice.

media already supplemented with glucose enhanced the quality of the signal, particularly at yeast extract concentrations <0.25%. On all the SP/YE based media the pattern of response of all species tested was similar, although the strength of the signals and the detection times varied. As can be seen from Fig. 7, it was not possible to differentiate yeasts from moulds by either conductance or capacitance monitoring with the range of media tested. Although some of the differences in the detection times could be attributed to differences in inoculum size, detection times were, at least in species-specific. Thus, with pa& medium 4.9, a standardized inoculum (102 spores ml-i) of A. niger, Geotrichum

candidum

and P. viridicatum resulted in capacitance detection times of 19.5, 14.4 and 47.6 h, respectively. It was also clear that detection times could be affected by the choice both of medium and the signal to be monitored (Table 4).

Discussion The results obtained from the screening of commercially available media confirmed that impedimetric monitoring of yeasts and moulds requires media developed for that specific purpose. Indeed, media such as PDA and ME have previously proved unsatisfactory for the impedimetric monitoring of yeasts (Zindulis, 1984). The poor quality capacitance curves recorded on wort agar (Fig. 2)

240

I. A. Watson-Craik

and J. G. Anderson

Table 2. Assessment of quality of conductance acceleration, baseline and maximum value.

and capacitance

Conductance

curve

components,

Capacitance

Response Quality

Acceleration

Baseline

Maximum value

Acceleration

Baseline

Maximum value

Good (G) Fair(F) Poor(P)

al.3 30.7, <1.3 n.a.

co.12 >0.12, ~2.0 n.a.

213.0 36.5, <13.0 n.a.

21.3 20.7, <1.3 <0.7

so.2 >0.2, ~0.6 >0.6

216.5 210.0, <16.5 c10.0

Note: n.a. denotes not applicable. Table 3. The effects of medium composition conductance and capacitance responses. Medium Medium 1.1 1.2 1.3 1.4 1.5 1.6 2.1 2.2 2.3 2.5 2.6 2.7 2.8 3.1 3.3 3.4 4.1 4.2 4.4 4.5 4.6 4.7 4.8 4.9 4.11 4.13 4.14 5.2 5.3 5.4 5.5 5.6

YE

SP

2.0 2.0 2.0 2.0 2.0 1.0 1.0 1.0 1.0 1.0 1.0 1.0 0.5 0.5 0.25 O-25 0.25 0.25 0.25 0.25 O-25 0.25 0.25

2.0 2.0 1.0 1.0 1.0 0.5 0.5 0.25

0<5 0.1 0.1 0.1 0.1 0.1

0>5 -

O-25 0.25 0.25 -

composition

1.0 3.0 3.0 3.0 ‘1.0 1.0

0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 0.05 O-05 -

the

strength

Conductance Signal

(% w/v)

Ammonium-N, as Glucose NH&l (NH&SO4 1.0 1.0 3.0 3.0 1.0 0.5 3.0 3.0 3.0 3.0 1.0 0.5 3.0 1.0 3.0 1.0

on

0.05 0.05 0.05 0.05 0.05 O-05

Strength

and

direction

of

Capacitance Signal

Direction I I I D iD I I I D D D DI I I D D I D D D Di I I ii D N D dI I I aI

Strength P P : G P P E F F F P nm P P P P P P P F F F E P nm : F F

Direction N N N I I N N N I I I I N run N N N N ii N I I I I I N nm I I I I

Note: I denotes signal increase; D, signal decrease; iD, small signal increase prior to decrease; DI, signal decrease then increase; Di, signal decrease prior to small increase; d1, small decrease prior to signal increase; N, no significant signal change; nm, signal not monitored.

Conductance

and capacitance

in fungi

241

160

80 60

k

BASE = 1002 STEP = 6.12

t7

6 5

BASE = 1089 STEP = 5.07

I-

CJ 160

:

80

,

60

I

7 I4 BASE = 1063 STEP = 5.18

21 28 Time(h)

35

42

L

I

1

14 7 BASE = 2227 STEP = 5.35

,

? I\

_I , ,’

21 28 Time (h)

I

I

35

42

I

49

Fig. 6. Conductance decreases recorded on growth of A. niger (-), Geo. candidum (---------I, and P. roqwfortii (----I on CBAT agar supplemented with a range [(a) 0; (b) 0.1; (c) 0.25; (d) 2-O%] of yeast extract concentrations.

were of particular interest since this medium was reported (Schaertel et al. 1987) to result in good quality curves and consistent detection times. Since the instrumentation (Bactometer Ml201 and medium (Difco) were comparable in the

two studies, and since adjustment of incubation temperature to that (28°C) used by Schaertel et al. (1987) did not resolve the discrepancy, it is diffkult to reconcile these results. They do, however, emphasise the importance of

242

I. A. Watson-Craik

and J. G. Anderson

_,-/ -. ,‘/ , ‘.\

_----

- ,,’ .-

4 BASE STEP

8

i2

16

I

I

I

I

1

I

I

20

24

28

32

36

40

44

= 3881 = 8.21

.

160

-+

- \\ *..

----------

.

\

(b) ‘\

80

60

‘.

‘.

. ‘. ‘*..

BASE STEP

1

I

I

I

I

4

8

I2

I6

20

= 1013 = 1.1 I

.

I 28

1 i24 Time

I 32

I 36

\

. \

--..__ 40

-.

. *. 44

(h)

Fig. 7. Capacitance (a) and conductance (b) changes on inoculation of medium 4.12 with Rh. arrhizus (), A. flavus c---------j, Saccharomyces cerevisiae c-----j and Schizosaccharomyces pombe (-------).

Conductance Table media

4. Comparison 4-12 and 5.3.

of conductance

and capacitance

(G) and capacitance

(C) detection

in fungi

243

times

(h) in

Medium 4.12

Species P. chrysogenum Geo. candidum A. flaws Sacch. cerevisiae Note:

n.d. denotes

5.3

C

G

18.3 7.2 21.7 18.4

18.6 8.6 26.8 18.5

C

G

n.d.

n.d.

9.5 21.8 n.d.

17.2 34.8 n.d.

not determined.

developing media, for impedimetric monitoring, that are not vulnerable to perturbation or to manufacturing variations. These may not visually affect mycelial growth but may have a significant effect on electrochemical changes in the medium. The effects of medium composition were exemplified by the varied responses to the formulated media (Table 1). At this stage, interest was focused primarily on the conductance signal since both impedimetric instruments currently available (Malthus and Bactometer) can monitor this signal. It was found that media containing only soya peptone and yeast extract @P/YE media) gave good increasing conductance curves with all 127 fungal food isolates tested. This medium was, therefore, selected for further screening against ‘spiked’ food products. The resultant signal reversal, with orange, grapefruit and apple juice, was attributed to the presence of simple carbohydrates in the juices, since a comparable effect was induced by supplementing SP/YE with monosaccharides (10% w/v) and since the reversal was diminished by dilution (X 10) of the juices. It seems likely that the conductance decreases documented with carbon base-supplemented media (CBAS and CBAT) also derive from the carbohydrate content. Decreasing conductance

signals on CBAS and CBAT, comparable to those recorded in the present study with filamentous moulds, have previously been documented with yeasts (Schaertel et al. 1987, Connolly et al. 1988). Indeed, it was apparent that the impedimetric responses of yeasts and moulds were not significantly different on any of the media tested (Fig. 7). The variable nature of possibly contaminated foods make it imperative the media developed for impedimetric monitoring are not excessively sensitive to product interference. As already indicated, the increasing conductance signal induced by moulds in SP/YE medium proved particularly vulnerable to glucose supplementation. Further examples of this problem, shown in Table 3, occur with media containing low (<0.25%) yeast extract concentrations, in which conductance increases, reversed by glucose supplementation, could be restored by glucose/ammonium incorporation. In general, the results of this study suggest that media formulations which elicit a decreasing conductance signal may be more appropriate fcr the conductimetric assay of moulds and yeasts. With yeast media such as CBAS and CBAT similar reports were previously recorded (Connolly et al. 1988). It was, however, clear (Fig. 6) that the conductimetric responses of glucose/ammo-

244

I. A. Watson-Craik

and J. G. Anderson

nium-based media such as these may be vulnerable to interference from nitrogenous food products. No media formulations were encountered during this study which resulted in a capacitance decrease. Glucose supplementation of SP/YE or YE media elicited increasing capacitance curves whose quality was enhanced by the further incorporation of ammonium. The unidirectional change in the capacitance signal exhibited by such media formulations minimises the problem of product interference. With an appropriate medium, the faster response of the capacitance signal (Table 4) would also enable more rapid detection of contaminated products. Monitoring of the capacitance

signal was also recommended for the detection of yeasts by Zindulis (1984) and for moulds and yeasts by Schaertel et al. (1987). The results of this study demonstrated that with the appropriate medium both moulds and yeasts could be detected by either conductance or capacitance monitoring. Nevertheless, the operational advantages of capacitance monitoring suggested that further study of this was warranted.

Acknowledgements This research was supported by the Science and Engineering Research Council (I.A.W.-C.) and by Bactomatic Ltd (K.E.A.).

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