Risk Assessment Studies
Report No. 7
SEPTEMBER 2001
Food and Environmental Hygiene Department
HKSAR
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This is a publication of the Food and Public Health Branch of the Food and Environmental Hygiene Department of HKSAR Government. Under no circumstances should the research data contained herein be reproduced, reviewed, or abstracted in part or in whole, or in conjunction with other publications or research work unless a written permission is obtained from the Department. Acknowledgement is required if other parts of this publication are used. |
Correspondence:
Risk Assessment Section
Food and Environmental Hygiene Department
43/F, Queensway Government Offices,
66 Queensway,
Email: enquiries@fehd.gov.hk
Table of Contents:
Potential Microbiological Hazards
Microbiological Quality of Ice-cream
Presence of Specific Pathogens
Figure 1: Flow Chart of Ice-cream Production
Figure 2: Trend Analysis of Hygienic Quality of Ice-cream
Table 1: Hazards and Typical Control in the Production of
Table 2: Microbiological Surveillance Results
of Ice-cream
Risk Assessment Studies
Report No. 7
An Evaluation of Microbiological
Surveillance
of Ice-cream
1998 – 2000
Ice-cream is a frozen
dairy product. It is sold in soft
or hard status. The production of
ice-cream includes totally eight steps.
Pasteurization, freezing and hardening are the main steps to eliminate
the microbiological hazards. In
Ice-cream represents a
congealed dairy product produced by freezing a pasteurized mixture of milk, cream,
milk solids other than fat, sugars, emulsifier and stabilizers. Products of dairy origin are the main
ingredients of ice-cream. These
include whole milk, skimmed milk, cream, frozen cream, condensed milk products
and milk solid. Other ingredients include
flavouring matters and water.
Fruits, nuts, candies and syrups are optionally added into ice-cream for
flavour enrichment. Two types of
ice-cream, soft and hard, are available on the market.
2. This paper describes the manufacturing
procedures, identifies the potential microbiological hazards and outlines the
local regulatory framework of ice-cream products. The food surveillance findings in the
period of 1998-2000 are reviewed and analyzed to evaluate the actual microbiological hazards in
ice-cream.
3.
The production of ice-cream
includes many steps classified under the following three main parts (Figure
1) –
1.
Ice-cream
mix making (mixing of ingredients, pasteurization and homogenization);
2.
Soft
ice-cream production (aging and freezing); and
3.
Hard
ice-cream production (packaging, hardening and storage).
4. The manufacturing of
ice-cream mix involves mixing of ingredients, pasteurization and homogenization. For soft ice-cream, pre-made ice-cream mix is supplied to
the retail outlets under refrigeration (< 7°C). Aging and freezing at around -5°C are performed in the
vending machines at the retail level.
Other ingredients, such as fruits and nuts, may be added to the soft
ice-cream at the time of sale for flavour enrichment. In frozen confection factories, the soft
ice-cream will be packed and hardened to produce the hard ice-cream. The following paragraphs describe the
technological details of each processing step.
5. The first step of
preparing ice-cream mix is to combine the liquid ingredients and heat them to
around 43°C. Then, sugar and other dry ingredients,
except nuts and fruits, are added to the lukewarm mix.
6.
The mixture is then pasteurized by a heating
process, either in batch or continuous modes, depending on the production
size. Pasteurization temperatures
for ice-cream mix, around 70°C for 10 – 30 minutes (1),
are higher than that for plain milk because high fat and sugar contents tend to protect bacteria
from heat treatment.
7.
The pasteurized mix is then
homogenized. High pressures of 4.1
x 106 Pa (600 psi) to 1.7 x 107 Pa (2500 psi) are used
to break down fat globules(2).
This pressure can clump fat globules and, together with
the added emulsifiers, can prevent churning of fat into butter granules (milk fat)
during freezing step. Homogenization
also improves the texture of ice-cream. In other words, the ice-cream becomes smoother.
After that, the homogenized mix is cooled down
to 4°C for further processing.
For production of soft ice-cream using vending machines, it is packed
and delivered to the retail outlets.
8. Aging is the first step
in soft ice-cream production. The
mix is held in sterilized vats from 3-24 hours at temperature of around 4°C or lower. It allows some physical changes, such as
fat crystallization, adsorption of protein onto fat globules, and incensement of the mix’s viscosity. These changes lead to quicker whipping
to
the desired overrun[1] in freezing process,
smoother ice-cream body and texture, and slower ice-cream melt-down.
9. During freezing, air is
incorporated in the mix and cooled down to around -5°C. Freezing must be performed as quickly as possible to prevent the
formation of large ice crystals.
The air cells in ice-cream should be small and evenly distributed in
order to maintain a stable frozen foam.
At this low temperature, -5°C, not all water
particles are crystallized and, therefore, ice-cream is only in
semisolid state. The semisolid
ice-cream emerging from the freezer is similar to the constituency of soft ice-cream
produced at the retail level. Other
ingredients, like fruits, nuts, or syrup, may then be added to enrich the
flavour of ice-cream.
10.
To produce hard ice-cream,
the semisolid ice-cream is packed into cartons or drums for hardening process
to form specific shape of ice-cream products and to have longer shelf-life (more
than one year).
11. The pre-packed semisolid
ice-cream is then placed in a hardening room where the temperature of about -34°C is maintained. This low temperature keeps the core
temperature of ice-cream at around -18°C. Hardening should be performed
quickly to prevent the formation of large ice crystal and to maintain better quality of
ice-cream.
12. After hardening, the hard
ice-cream will then be placed back in the cold store rooms with temperature of around -18°C. From this stage, the hard ice-cream must
be kept
at -18°C or below throughout
storage, transportation and display.
POTENTIAL MICROBIOLOGICAL HAZARDS
13.
Ice-cream, a milk-based
product, is a good media for microbial growth due to high nutrient value, almost
neutral pH value (pH ~6-7) and long storage duration of ice-cream
(3). However,
pasteurization, freezing and hardening steps in the production can eliminate most of the microbiological
hazards. According to the Frozen
Confections Regulation under Chapter 132, ice-cream must be heat-treated
during the production process. Pasteurization is most commonly applied
heat treatment in
the dairy industry. This can destroy almost all pathogenic bacteria in milk. The subsequent process that subjects the
mixtures to freezing temperature can also inhibit the growth of any remaining
flora. Hardening is also the important
control point that further reduces the hazards (4). Furthermore, as automatic machines are
commonly used for ice-cream making in dairy industry, the chance of contamination
through direct hand manipulation can be reduced.
14.
Nevertheless, there are
some steps
in the production of ice-cream that (Figure 1) can lead to the microbiological
hazards. Table 1
summarizes the possible hazards associated with different stages of ice-cream
production
(5). Heat
treatment by pasteurization can destroy most of the specific pathogens that
pose risk to public health. However,
the potential microbiological hazards found in the final products can still
be introduced after pasteurization through adding contaminated ingredients and improper
handling procedures (6).
This is especially important in the preparation of soft ice-cream as
its final stage of the production is carried out at point of sale. Some pathogens that can survive in food
even at low temperature include Salmonella spp., Listeria monocytogenes, Campylobacter spp. and Yersinia spp. (7).
15.
For ice-cream products,
L. monocytogenes is of significant food
safety concern worldwide (3).
In Hong Kong, regular surveillance to monitor L. monocytogenes in ice-cream has been in
place. In 1997, L. monocytogenes was found in 4 imported
ice-cream samples. Consequently,
the importer had voluntarily recalled all incriminated ice-cream products on sale. No report of any persons being affected
was received in that case.