Pasteurization is a milder form of heat treatment than commercial canning. It is
generally set at the minimum F value to destroy all potential vegetative pathogens
considered a hazard in a particular food product. Pasteurization will destroy some
spoilage organisms, however many will survive. Therefore a second food
preservation mechanism must be used such as refrigeration or low water activity.
There are three examples we will examine: pasteurization of liquids such as milk;
pasteurization of moist solids such as deli roast beef; and pasteurization of a dry
food such as almonds.
3
Let’s start with liquids pasteurization. Raw milk may contain: [read slide].
4
In the early 1900’s pasteurization of milk was targeted to the milk pathogen
Mycobacterium tuberculosis. This bacteria was the cause of a common disease
called consumption. In 1911, the National Milk Standards Committee in the United
States recommended a time-temperature combination of 62.8°C (145°F) for 30
minutes for the pasteurization of milk. This was slightly above what many
considered to be adequate for the destruction of M. tuberculosis, at 60°C for 20
minutes. However, in 1924 pasteurization of milk was standardized at 142°F for 30
minutes.
In 1937, the rickettsia Coxiella burnetii, responsible for Q-fever was first described.
It could be found in raw milk. It was found that this organism was more heat
resistant than M. tuberculosis, and that it could be isolated from pasteurized milk
that had been processed according to the then recommended standards 142°F for
30 minutes. BY the 1950’s the US standard was raised to 145°F for 30 minutes.
Today we know that the most heat resistant potential pathogenic bacteria in milk is
Coxiella burnetti. Therefore, milk pasteurization is based on its D and z values.
The reference temperature for milk pasteurization is 63°C. After many, many
years of research the accepted milk pasteurization process is D63 = 2.5 minutes and
a Z of 4.1°C. Since it is desired to destroy 12D, the resulting F63 = 30 minutes.
This is a current federal USDA grade A pasteurized milk requirement.
5
Just like in commercial canning, the thermal death time for the destruction of
Coxiella burnetti in milk can be calculated at different temperatures. The chart on
the slide provides the several options that can be used, including 161°F for 15
seconds and 191°F for 1 second. The challenge is how to get maximum lethality
with the least damage to the quality of the milk. The historic method of milk
pasteurization is the vat method. However, it is physically impossible to heat up
milk to 161°F in a vat for 15 seconds and cool it. So, in order to achieve these short
thermal death times, an in line pasteurizer is used.
6
Here is a very simplified view of a vat pasteurizer. In most cases it is heated by
steam or hot water. Direct fire would result in localized scorching. Milk is pumped
in. The steam heats the milk until it reaches 63°C or (145°F). The clock is started
and after 30 minutes the milk is pumped through a cooling system like a plate heat
exchanger. Note that much of the milk will receive more than 30 minutes of heating
as it sits in the vat waiting to enter the heat exchanger.
7
An in-line pasteurizer was created to rapidly heat and cool milk to preserve milk
quality.
8
Most liquid pasteurization has moved to an in-line continuous system.
Unpasteurized milk enters from the bottom right. It first enters a heat exchanger
where it is warmed. The flow rate is precisely controlled by a timing pump. The
flow rate controls the amount of time the milk will remain at the pasteurization
temperature in the heating section seen on the far left. The pasteurized milk then
flows through the first cooling heat exchanger. Here heat is transferred to the cold
raw milk used in the preheating step. Finally, the second heat exchanger cools the
milk to refrigeration temperature using a coolant.
9
USDA inspected establishments producing ready-to-eat roast beef, cooked beef
and corned beef products and certain ready-to-eat poultry products are required by
FSIS to meet the lethality performance standards for a 6.5 log reduction of the most
heat resistant potential pathogen, Salmonella. Unlike milk and some other liquids,
these foods are not that susceptible to heat damage. Therefore, it is common to
see these foods heated to their target temperature and held for the thermal death
time (TDT). Come-up and cool-down heat is not factored in. Poultry must meet a 7
log lethality. Both beef and poultry
must be cooked under high moisture conditions,
since it is know that dry heat is not nearly as lethal as wet (moist) heat.
10
Here is simple piece of equipment used to pasteurize and cook deli meat loaf.
Loaves of deli meats like roast beef or turkey are loaded into the combi cooker
cooler. Steam is pumped through coils inside the tank to heat the water to cooking
temperature. Once the time at temperature has been reached, the steam is
removed and a cooling water or coolant is pumped through the coils. Once cool deli
loaves are placed in normal refrigeration units and are ready to eat.
11
This is another method to cook and pasteurize meats packaged in bags.
Essentially bags are conveyed into a tunnel pasteurizer using either hot water,
steam or circulating hot water. The conveyor belt is used to modify the time that the
meats are in the pasteurizer.
12
Water is an efficient carrier of heat. Air is not. Due to that simple fact, moist heat
will be ten to a hundred fold more efficient at killing microorganisms than dry heat.
That is why canned foods will always have moisture in them and the USDA requires
high moisture levels in cooking-pasteurization processes.
13
In 2004 an international outbreak occurred of Salmonella enterica serotype
Enteritidis that sickened at least 30 people in both the USA and Canada. Raw
almonds distributed throughout the United States and internationally were
implicated as the source. This relatively small outbreak led to a recall of over 13
million pounds of raw almonds. As a result the Almond industry decided to adopt
pasteurization standards to prevent another outbreak. But the process was not that
simple. Almonds are a dry food.
14
Specifically, almond handlers must subject their almonds to a process that achieves
a minimum 4‐log reduction in Salmonella bacteria prior to shipment. There are
several heat pasteurization methods researched including: oil roasting, dry roasting,
and steam processing. These treatments are surface treatments only. The
nutritional and sensory characteristics of the almonds remain unchanged after
pasteurization. Here is a nut roaster that uses steam heat early in the process to
affect a 4 log destruction of Salmonella. Following the steam injection, the normal
roasting heat finishes the roast without and moisture damage to the almond.
15
This process is called the controlled condensation steam pasteurizer. Raw almonds
are hoppered in via item number 1. A ribbon mixer constantly mixes the almonds
while steam is injected from below. The process requires 2‐4 minutes. Water and
steam are drained. A vacuum (item 7) is pulled to remove as much moisture as
quickly as possible. That is the key to preventing deterioration of the almonds.
16
1.pdf17-High-Temp-2012-Pt2-1-up.pdf
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