2012 IAFP SYMPOSIUM:
Salmonella in low moisture foods: a continued challenge
INACTIVATION OF SALMONELLA ON
RAW NUTS USING LOW-ENERGY
X-RAY
Sanghyup Jeong, Ph.D.
2012 IAFP ANNUAL MEETING
July 23, 2012
Providence, Rhode Island
GOAL and OBJECTIVES
Pasteurizing almonds/walnuts with X-ray while
maintaining wholesomeness
1. To quantify the relationship between aw and the
D10-value for low-energy X-ray inactivation of
Salmonella on almonds and walnuts.
2. To determine the impact of X-ray irradiation on
the sensory quality.
3. To quantify post-irradiation survival of
Salmonella on nuts during storage.
HARVESTING ALMONDS
• High risk of natural
contamination
–
Danyluk, M.D., Brandl, M.T., Harris, L.J., (2008).
Migration of Salmonella enteritidis phage type 30
through almond hulls and shells. Journal of Food
Protection 71(2), 397-401.
• Catastrophic re-hydration
–
Shaker
RAIN
Uesugi, A.R., Harris, L.J., (2006). Growth of
Salmonella Enteritidis Phage Type 30 in almond hull
and shell slurries and survival in drying almond hulls.
Journal of Food Protection 69, 712.
Sweeper
SPEAR vs. SHIELD
• OUTBREAKS
– California almonds were implicated in two outbreaks
of salmonellosis in 2000 and 2003 that were traced to
Salmonella Enteritidis PT30, prompting the recall of
nearly 6 million kg of raw almonds.
• REGULATIONS (2007)
– “Almonds grown in California outgoing quality control
requirements” (7CFR Part 981.442)
– Mandate minimum 4-log reduction of Salmonella on
all California almonds.
PASTEURIZING ALMONDS
•
•
•
•
•
•
•
Fumigation-propylene oxide
Steam
Dry Heat
Moist-Air Heat
Acidic Spray
Hydrostatic pressure
IRRADIATION
IONIZING IRRADIATION
•
•
•
•
Gamma (1 MeV)
E-beam (5 MeV)
X-ray
Low-Energy X-ray (70 keV)
1. Low penetration power
2. Small footprint
3. Less shielding requirement
FACTORS  RADIATION SENSITIVITY
• Temperature
• Dose rate
• Physiological state of the
microorganism
• Compositions
• Favorable/unfavorable
microbial byproducts
• Availability of water
THE DRIER, THE HARDER TO KILL…
• Thermal resistance
increases as water activity
decreases.
• Non-monotonic
relationship with water
activity (Laroche et al.,
2005)
– Thermal resistance in
food powders
• Solute types (O’DonovanVaughan, Upton, 1999)
Microbial
Growth
Microbial
Resistance
INOCULATING NUTS
SE PT30
(Outbreak
Strain)
Almonds
(Nonpareil)
Salmonella
Tennessee
(3 strains)
Walnuts
(Juglans
regia)
CONDITIONING aw
• CH3COOK
• NaNO2
• K2CO3
0.23
0.45
0.64
0.84
• KCl
X-RAY IRRADIATION
•
•
•
•
•
70 keV, 4 kW irradiator
5 dose levels (0.3-5.5 kGy)
Dose rate: ~20 Gy/s
Double side treatment
Radiochromic film dosimeter
(GAF3001DS, GEX Corp)+
Spectrophotometric method
MICROBIAL ASSAY
• 45 ml of 0.1% peptone  5 g nuts
• Homogenization (Stomacher)
• Serial dilution on TSAYE (ferric
ammonium citrate (0.05%) +
sodium thiosulfate (0.03%)
– Characteristic black precipitate
• Incubation (35 ± 2ºC for 48 h)
• Counting survivors  Log
Reduction
SENSORY TEST
• Dept. Food Sci. (MSU)
• Almonds (1.13 kGy)/Walnuts
(2.37 kGy)
• Triangle Test (Almond/walnut)
• Acceptance Test (walnut only)
– Appearance, aroma, flavor,
texture, and overall acceptability
– 9 point hedonic scale
POST-IRRADIATION SURVIVAL
•
•
•
•
•
•
Almonds & Walnuts
0.2 & 0.7 aw
SE PT30 & S. Tennessee
1.13-4.32 kGy (~5 log reduction)
6 case studies
1, 30, 60, and 120 days at 4°C/70%RH
QUESTIONS WERE…
1. How effective? (i.e., D10-value)
2. What factors matter?
– D10-value = f(Water Activity)
– D10-value = f(Bacterial Serotype)
– D10-value = f(Nut Type)
3. Still tastes good?
4. Any survivals after storage?
D10-value = f(aw, Serovar, Variety)??
SENSORY??
• 67 panelists
• Triangle Test:
– Almonds - no
significant difference
(α=0.05)
– Walnuts – “fishy
taste” (oxidative
rancidity)
ACCEPTANCE TEST
Attribute
Control
Walnuts
Irradiated
Walnuts
Appearance
7.07a
7.03a
Aroma
5.81a
5.79a
Flavor
7.28a
5.89b
Texture
7.31a
7.03b
Overall
7.27a
6.09b
POST-IRRADITATION SURVIVORS
CONCLUSION-Highlights
1. Low-energy X-ray irradiation was an effective intervention
for Salmonella on almonds and walnuts.
2. Radiation resistance (D10-value) did not change
monotonically with water activity.
3. Sensory changes of almonds at the 5 log reduction X-ray
dose were not significant.
4. Post-irradiation bacterial counts did not change over 120
days of storage.
Jeong, S., Marks, B.P., Ryser, E.T., Harte, J.B., (2012). The effect of X-ray
irradiation on Salmonella inactivation and sensory quality of almonds and
walnuts as a function of water activity. International Journal of Food
Microbiology 153(3), 365-371. (OPEN ACCESS).
ACKNOWLEDGMENT
This work was supported (in part) by the Technical Committee on Food
Microbiology of the North American Branch of the International Life Sciences
Institute (ILSI). ILSI North America is a public, non-profit foundation that provides
a forum to advance understanding of scientific issues related to the nutritional
quality and safety of the food supply by sponsoring research programs,
educational seminars and workshops, and publications. ILSI North America
receives support primarily from its industry membership. The opinions expressed
herein are those of the authors and do not necessarily represent the views of the
funding organization.
QUESTIONS