Effect of Evaporative Cooling, Fat Content and Food Type on Pathogen Survival during Microwave Heating

Effect of Evaporative Cooling, Fat Content and Food Type on
Pathogen Survival during Microwave Heating
by
April Hix

Thesis submitted to the Faculty of the Virginia Polytechnic Institute and State
University in partial fulfillment of the requirements for the degree of
Master of Science In Food Science and Technology
Defense date: 28 July 2000

Approved:

_____________________________ _____________________________
Susan S. Sumner, Co-Chair
Cameron R. Hackney, Co-Chair


___________________________
_____________________________
Joseph D. Eifert
Kumar Mallikarjunan





Key Words: Food borne pathogens, microwave

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Abstract

Due to the rapid nature of microwave heating, the microbiological safety of
foods prepared in the microwave has been in question for several years. Because
foods are heated from the inside out and are strictly governed by their own
internal properties such as ionic content, moisture level and specific heat, work
must be done to further master control of such properties so microwave cooking
can be more predictable, controlled and ensure control pathogens.
This study concentrated on the effect of fat content, evaporative cooling
and food type on the rate of food borne pathogen survival rates in microwave
heated foods. Foods investigated in this study included fresh, raw broccoli
spears; a regular, whole muscle breaded chicken patty and a fat free, breaded,
formed chicken patty; and raw ground beef patties at three differing fat
percentages. All foods were tested in triplicate. A Sharp® 1000W Light-Duty
Commercial Microwave Oven was used to treat inoculated samples according to
their recommended cooking times. Two sets of samples were treated, one
wrapped with Saran? Wrap and the other without wrap.
F- values were determined for each product. Raw ground beef patties at
fat contents of 30%, 15% and 7%, heated for the same time had F-values ranging
from 0.03 to 126.20. The lower the fat content, the lower the lethality. Regular and
fat free chicken tenders had similar patterns. F-values for fresh broccoli indicted
that vegetative pathogens survived the recommended microwave process.
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Covering in Saran? Wrap had some preventive effect on evaporative cooling
depending on the food tested and significantly (p < 0.05) increased most F-values.
Inoculated pack studies were performed in triplicate on each food with
Listeria monocytogenes, Salmonella and Escherichia coli O157:H7. Survival was
determined by presence or absence of growth of each pathogen after enrichment.
Listeria monocytogenes survived in all samples except for the 30% fat ground beef
patties. The Salmonella species had a lower survival rate; however, it was still
present in uncovered 15% fat ground beef, covered 7% fat ground beef,
uncovered chicken patties (both types) and in all broccoli samples tested. E. coli
O157:H7 survived in all samples except the 30% fat ground beef samples.
Results indicate that higher fat contents seem to ensure lower rates of
pathogen survival. This was especially true for the raw ground beef, which had
received no prior processing other than the grinding of the whole muscle. There
were fewer survival differences in the preprocessed, frozen chicken patties. Both
were shown to support no pathogen survival in covered samples, except the fat
free chicken patties. Listeria monocytogenes was shown to consistently survive the
suggested cooking time in these samples. This is consistent with expectations
that fat free food samples would display more survival than regular fat samples.
Overall, covering samples with Saran? had little effect on pathogen
survival rates. There were survival differences in some covered and uncovered
samples consistent with expectations that covered samples would show less
survival than uncovered, but further work including more samples would be
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necessary to ensure that the covered or uncovered variable made the true
difference in pathogen survival. Finally, broccoli demonstrated consistent
pathogen survival in all categories of testing. This indicates microwave oven
prepared vegetables could be a prime source of pathogen transmission to
consumers. Further work needs to concentrate on determining the correct
processing times and parameters that need to be met to ensure safe food.
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Acknowledgements
First, I thank God for the strength, health and the opportunity to come to
Virginia Tech and for His mercy, love and lessons He has taught me while here.
Second must come Dr. Susan Sumner for her guidance, patience and friendship
through this effort. Also, my thanks to Dr. Cameron Hackney whose insight and
scope richly added to this research. Thank you to the rest of my graduate
committee as well, Dr. Kumar Mallikarjunan and Dr. Joseph Eifert, whose
patience and time are also appreciated. Special recognition goes to
USDA/CSREES/NRICGP Grant # 1999-02764, which made this work possible.

Exceptional thanks goes to my sisters, Melani (my twin) and Lauren, my
brother, Jonathan, and my father, Alan, and his wife - my friend , Vivian. Their
support and love kept me focused on my work and out of complete
discouragement many days. Laura S. Huffman and Judy Scott deserve special
credit as well for their time, support and advice these past two years.
Last and most importantly, I would like to dedicate this thesis to my
mother, Rita York Hix. Her support, friendship and love through the first 20
years of my life have helped make me who I am today.
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Introduction
Pathogen survival during microwave reheating and cooking has emerged
as a growing concern for consumers, the government and the food industry. Prior
research at the Virginia Tech Department of Food Science and Technology has
indicated that evaporative cooling, a phenomenon where the surface of a
microwave treated food is actually cooled by a developing jacket of air around
the product, may be a factor in the survival of food borne pathogens on
microwave treated food. Another investigation at the University of Nebraska
Department of Food Science identified a food’s fat content as a potential source of
pathogen destruction citing higher fat contents might result in lower pathogen
survival rates.
As utilization of the microwave grows in popularity, it is increasingly
important to understand the nature of microwave heating and how to ensure safe
food cooked under such conditions. There are few homes without a microwave
in America. In fact, it is projected that there are roughly 150 million microwaves
in US homes resulting in an approximately 95% saturation level (Schiffman,
1997). People are conditioned to having this convenience and will take advantage
of it even when microwave preparation may not be a top choice for cooking a
certain food. Some people do not read the directions provided on the box of a
microwave entrée (Schiffman, 1995). That person prepares all microwave foods
the same: cooked on the highest setting for the shortest amount of time, even if a
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hold time is recommended for temperature equilibration (Schiffman, 1995). Few
people are ever aware of it because they did not even take the time read that
section of the box (Schiffman, 1995). A food may be undercooked in some areas,
overcooked in other areas and the consumer eats it anyway. Undercooked areas
present a possible health risk that health officials are worried about. Those are
the areas in which food borne pathogens can survive and even grow if not stored
properly after heating (Fields et al., 1986).
Research is necessary to determine how to design microwave foods and
microwave processes so that the food will be free of pathogens. It must be
recognized that no matter how many warnings are given in the case of
microwave foods, people will still have the “it could never happen to me”
mentality. Safety needs to be designed into microwave foods, microwave ovens
and microwave cookbooks. This research targets the problem of designing safer
foods and processes for the microwave. It helps answer more of the standing
questions pertaining to the survival of food borne pathogens – whether fat
content and moisture loss play a role in this problem. Also, little vegetable tissue
research has been conducted regarding pathogen survival during microwave
cooking. Inoculated, fresh broccoli spears were used in this project to determine
differences between vegetable and muscle food pathogen survival.
Prior research indicates that major factors affecting pathogen survival
include hot and cold spots, nonuniform heating, the dielectric properties of a food
(fat, salt, protein, moisture content), wattage and power output of the microwave
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being used, and the size, shape and orientation of the food once in the microwave
cavity (Carter, 1994; Flores, 1994; Schiffman, 1993). The diversity of microwave
ovens has also been implicated in the lack of heating uniformity of microwave
foods (Fakhouri and Ramaswamy, 1993a). There are no standards for consumers
to follow to correct this difference in microwave ovens across the United States
and only one set of heating directions provided with most products; therefore
many microwave foods are not completely cooked, leaving them vulnerable to
any surviving pathogens (Fakhouri and Ramaswamy, 1993a; Schiffman, 1997).

Because of the threat that food borne pathogen survival poses to the
young, old and immunocompromised population, research to correct these
problems is necessary. The overall objective of this research was to further
answer food product development questions on how to make microwave
products safer.
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Table of Contents

Abstract ..............................................................................................................................ii
Acknowledgements..........................................................................................................v
Introduction......................................................................................................................vi
Table of Contents.............................................................................................................ix
List of Figures . …………………………………………………………………………xi
List of Tables……………………………………………………………………………xiii
Chapter One: Review of Literature .............................................................................. 1
I.
Microwave Cooking vs. Conventional Methods .............................................1
A. Heating
Pattern.........................................................................................3
B. Heating
Rate..............................................................................................5
C. Heating
Uniformity..................................................................................8
II.
Food Safety Concerns of Microwavable Foods................................................9
A.
Proper Package Heating Instruction...................................................10
B.
Post Process Contamination .................................................................12
C.
Cooking and Reheating .........................................................................13
D. Storage
Abuse .........................................................................................16
III.
Microwave Mechanism of Microbial Destruction ........................................17
A. Electromagnetic
Radiation ....................................................................18
B. Dipolar
Rotation .....................................................................................19
C. Ionic
Conduction ....................................................................................20
IV.
Relevant Food Characteristics Involved in Microwave Heating.................20
A.
Dielectric Properties of Food ................................................................21
B. Specific
Heat............................................................................................23
C. Shape
and
Size ........................................................................................24
D. Orientation...............................................................................................26
E.
Molecular State of Water .......................................................................27
F.
Presence of Bone .....................................................................................28
G.
Presence of Fat ........................................................................................29
V.
Microorganisms of Concern..............................................................................30
A.
Listeria monocytogenes ............................................................................. 30

B. Salmonella
species....................................................................................33

C. Escherichia
coli
O157:H7..........................................................................36
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VI. Objectives.............................................................................................................40
VII. References............................................................................................................41

Chapter Two: The Effect of Evaporative Cooling, Fat Content and Food Type on
Pathogen Survival during Microwave Heating.......................................................50

I. Abstract ................................................................................................................51
II. Introduction.........................................................................................................53
III.
Materials and Methods......................................................................................61
A.
Source, Type and Classification of Products ......................................61
B.
Heating
Curves .......................................................................................61
C.
Inoculated Pack Studies.........................................................................62
1. Inoculum Preparation......................................................................62
a. Listeria monocytogenes ................................................................. 62
b. Salmonella spp..............................................................................63
c. Escherichia coli O157:H7..............................................................63
2. Study Design .....................................................................................64


3. Inoculation of Product .....................................................................64



4. Pathogen Recovery...........................................................................65
a. Listeria monocytogenes ................................................................. 65
b. Salmonella spp..............................................................................66
c. Escherichia coli O157:H7....................................................................66
D.
Pathogen Survival Curve Determination............................................67
IV.
Results and Discussion ......................................................................................67
A.
Survival
of
Listeria monocytogenes......................................................... 67
B.
Survival
of
Salmonella species ...............................................................70
C.
Survival
of
Escherichia coli O157:H7 .....................................................73

D.
Comparison of High Fat/Low Fat Survival Rates.............................73

E.
Comparison of Covered/Uncovered Survival Rates........................74
F.
Implications Against Evaporative Cooling.........................................75
V. Conclusions .........................................................................................................77
Acknowledgements............................................................................................79
VI. References............................................................................................................80
Figures..................................................................................................................84
Appendices..........................................................................................................92
Vita……………………………………………………………………………...102
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