EFFECTS OF AND RESISTANCE TO

WEAK ACIDS IN SPOILAGE YEASTS

Some weak acids:

E 200 sorbic acid 200 - 2000 mg Kg-1

E 210 benzoic acid 150 - 500 mg Kg-1

E 220 SO2 15 - 2000 mg Kg-1

E 260 acetic acid “quantum satis”

E 280 propionic acid 1000 - 3000 mg Kg-1

AH A- + H+

AH A- + H+

neutral cytoplasm

acidic environment

EFFECTS OF WEAK ACIDS

- Intracellular acidification

- Inhibition of glycolytic enzymes

- Decrease of fermentation activity

- ATP depletion

- Growth inhibition

- Enhancement of thermal death

- Intracellular acidification, but...

0 50 100 150

5.0

5.5

6.0

6.5

7.0

Intr

acel

lula

r p

H

[total acetic acid] (mM)

Intracellular acidificationS. cerevisiae

adaptationenergy

no adaptationenergy

no adaptationno energy

- Inhibition of glycolytic enzymes

- Decrease of fermentation activity

... but...

Specific glucose consumption rateS. cerevisiae

0 50 100 150

14

16

18

20

22

Qg

luc

(mm

ol g

-1 h

-1)

[acetic acid] (mM)

Inhibition of growth

ATP is diverted for maintenance purposes

...but how much ATP?

S. cerevisiaeacetic acid

0 2 4 6 8 10 12

8

16

24

32

QA

TP

(mm

ol A

TP

g-1h-1

)

v (mmol acetic ac. g-1 h

-1)

AH A- + H+

AH A- + H+ ATP

H+

A-

neutral cytoplasm

acidic environment

?

glucose

Extrusion of benzoic acid

S. cerevisiae

Adapted cells

Non adapted

0 40 80 120 1600

40

80

120

160

Lev

el o

f p

rese

rvat

ive

Time (min)

- Saccharomyces cerevisiae

- Zygosaccharomyces bailii

- Pichia sp.

0 2 4 6 8 100

5

10

15

20

up

take

rat

e

[benzoic acid] (mM)

Diffusion of benzoic acid

Pichia sp.

S. cerevisiae

Z. bailii

non adaptedadapted

AH A- + H+

AH A- + H+ ATP

H+

A-

neutral cytoplasm

acidic environment

conversion

1

2

3

4

Mechanisms of yeast tolerance to preservatives:1) Reduced permeability; 2) Proton extrusion; 3) Preservative conversion; 4) Preservative extrusion