1. Elham A. Milania, Richard Gardnerb, Filipa V.M. Silvaa*
The University of Auckland, Auckland, New Zealand
aChemical and Materials and Engineering Department
bSchool of Biological Sciences
*Email: filipa.silva@auckland.ac.nz
Introduction
• Beer is one of the most consumed beverages worldwide.
• The oldest evidences of beer production dates from 6000 years ago in the Sumerian region of
Mesopotamia.
• The industrial production of beer ends with a process of pasteurization which aims to increase
beer shelf life.
• Saccharomyces cerevisiae and Saccharomyces pastorianus, the yeasts used to produce top and
bottom fermenting beers, respectively, are often used as the criteria to design appropriate thermal
pasteurization processed for beer.
Thermal resistance of Saccharomyces yeast ascospores in beers
Objectives
To determine the thermal resistance (D- and z-
values) of ascospores of four different strains of
Saccharomyces in Gold Export lager beer (4%
alc/vol.).
To investigate the effect of different alcohol content
(0%, 4% and 7% alc/vol.) on the thermal resistance
of S. cerevisiae DSMZ 1848 ascospores.
Acknowledgments
 Keith Richard and Miguel Roncoroni from the School of Biological Sciences at the
University of Auckland.
 DB Breweries Morton Coutts grant, Auckland.
 Centre for Microbial Innovation, University of Auckland.
References
 Bigelow, W.D., and Esty, J.R. (1920). Thermal death point in relation to time of typical thermophilic organisms. J. Infectious Diseases 27: 602-617.
 Silva, F.V.M., and Gibbs, P.A. (2009). Principles of thermal processing: Pasteurisation. In Engineering Aspects of Thermal Food Processing, edited by R.
Simpson. Contemporary Food Engineering Series. Boca Raton, Florida, USA: CRC Press.
 Splittstoesser D.F., Leasor S.B., and Swanson K.M.J. (1986). Effect of Food Composition on the Heat Resistance of Yeast Ascospores. Journal of food
science (USA) v. 51(5) p. 1265-1267.
Methods
Beers and strains
• The thermal resistance of Saccharomyces cerevisiae strains DSMZ 1848, DSMZ 70487 and
Ethanol Red®, and Saccharomyces pastorianus ATCC 9080 was first determined in Gold
Export lager beer (4% alc/vol) (DB breweries, Auckland).
• Then, the thermal resistance of Saccharomyces cerevisiae DSMZ 1848 was determined in
0% alc/vol. (Bitburger Privatbrauerei, Germany) and 7% alc/vol (Taniwha beer, MATA
breweries, New Zealand).
Production and enumeration of spores
Suspension of yeast (DSMZ 1848) in the pre
sporulation liquid medium (0.8% yeast extract, 0.3%
peptone, 10% glucose, zinc sulphate 25 mg/L) and
overnight incubation at 28˚C.
Suspension of yeast in the sporulation liquid
medium (potassium acetate, bacto yeast
extract, glucose, and zinc sulphate) and > 7
days incubation at 18˚C.
Thermal inactivation experiments
Autoclaved heat resistant pouches were used to pack the beer.
For each strain, 5 ml of previously filter-sterilized beer was inoculated with yeast ascospore to
yield a final concentration of ca. 106 cfu/mL.
Thermal inactivation experiments were carried out at temperatures between 50°C and 65°C.
Spore numbers before and after heat treatment were obtained by plating the spores on agar
plates containing YEPD, and counting colonies after incubation for 2 days at 28°C.
• First-order/Bigelow model pattern (Bigelow and Esty,
1920; Silva and Gibbs, 2009) was observed:
N - number of microbial spores
N0 - initial number of microbial spores
DT - decimal reduction time at temperature T (min)
t - time (min)
• For each strain/temperature, the ascospore survival
curve (Log N vs. time) was plotted.
• After linear regression, the D-value was calculated
from the inverse of the slope.
• Then, z- value was determined from the inverse of the
slope obtained from the linear regression of
Log(D-value) vs. temperature data.
DTref- decimal reduction time at a reference temperature
z - number of degrees Celsius required to reduce D by a
factor of 10.
TD
t
oN
N 
10
10
ref
ref
T T
zT
T
D
D
 
 
 

D- and z-values of Saccharomyces ascospores
in Gold Export DB beer (4% alc/vol.)
Yeast
Temperature
(°C)
D-value
(min)
z-value
(°C)
Saccharomyces
cerevisiae
Strain DSMZ 1848
50 70
10
55 22
60 7.2
65 2.3
Saccharomyces
cerevisiae
Strain DSMZ 70487
50 49
11
55 18
60 6.4
65 2.3
Saccharomyces
pastorianus
Strain ATCC 9080
50 30
13
55 13
60 5.3
65 2.2
Saccharomyces
cerevisiae
Strain Ethanol Red
50 26
13
55 11
60 4.5
65 1.9
• The strain DSMZ 1848 presented higher D-values
followed by strains DSMZ 70487, ATCC 9080, and
lastly Ethanol Red.
• Similar z-values (10°C to 13°C) were obtained for all
strains.
• Splittstoesser et al. (1986) got a D55°C -value of 57 s for
S. cerevisiae (API20C) ascospores in Chenin Blanc
wine , while 106 min was registered in apple juice.
Thermal resistance of
S. cerevisiae DSMZ 1848 ascospores in
different alcohol content beers
Fig. 1. Thermal resistance of Saccharomyces cerevisiae DSMZ 1848
in different alcohol content beers.
• The D55°C-values for 0% alc/vol. was 35 min,
followed by 22 min for 4% alc/vol and lastly 8 min
for 7% alc/vol.
• With respect to 50°C, the alcohol effect was
noticeable at 7%, reducing the D-value from 70 min
for 4% alc/vol to 7.9 min for 7% alc/vol.
• The higher the alcohol content the more effective
the inactivation of the yeast ascospores.
• The results obtained will enable the design of
appropriate thermal pasteurization conditions to
preserve the beer.
Results
Bigelow model (first order kinetics)
0
10
20
30
40
50
60
70
80
90
0% alc/vol. 4% alc/vol. 7% alc/vol.
D-value(min)
50°C
55°C