Food Protection Trends - May/June 2024 - 157

the superheated steam unit. However, the RH at other
locations within these sites also increased by an average of
4%. Therefore, in spaces with a low ventilation rate, a small
percentage of change, but statistically insignificant increase
of indoor RH, was observed. Although the increase of indoor
RH was minimal, recurrent superheated steam exposure
during dry sanitation of the space could introduce water
vapor to the ambient environment and gradually increase
the overall indoor RH as it mixes with the surrounding air.
This increase in RH could affect the sanitation efficacy of
superheated steam and comfort of the personnel within
the facility (29, 38, 39, 50, 51). Specifically, the excessive
moisture could promote microbial growth and increase the
difficulty of removing organic food soil in the food processing
environment (9). Thus, dry food and produce handling
facilities with small spaces and low ventilation should
consider alternative dry sanitization methods or decrease the
indoor RH (e.g., venting out water vapor content or elevating
the ambient temperature) prior to superheated steam
sanitation treatments.
Similarly, visible saturated steam and surface condensation
formed at a distance of 0.3 m between the steam nozzle and
the surface (23.2°C) after several minutes (Fig. 2). Thus, the
formation of condensation may be attributed to the distance
between the steam nozzle and the surface. Previous studies
report that as the superheated steam moves away from the
steam source (nozzle), it interacts with the surrounding air
and rapidly cools down (38). Various factors contribute to
this: decrease in fluid flow velocity; formation of a boundary
layer; and temperature gradient between ambient air (or
surface) and the gaseous superheated steam (2, 10, 14, 24, 32,
34, 43, 44). Steam condensation could result as these factors
decrease the superheated steam temperature below 100°C
and condense the water vapor. As a result of condensation,
extended superheated steam exposure with a large distance
between the source and the surface could represent a worstcase
condition for accumulation of surface condensation.
Prior studies confirmed our qualitative observations,
suggesting that the temperature difference between the
superheated steam and the surface (or ambient air), as well
as the nozzle-surface distance, may be attributed to phase
change (condensation) and the transfer of latent heat to
surfaces (4, 8, 23, 53).
Depending on the facility, introduction of moisture
could be a drawback. For wet zones in low-moisture food
processing facilities, such as areas that are distant from the
final products, a limited amount of moisture would not
substantially affect dry food processing when moisture is
removed (e.g., wiping with dry rag) or air dried immediately.
However, dry food processing and produce handling
environments typically exclude wet sanitation and limit
the introduction of moisture because increases in RH and
surface condensation could facilitate microbial and fungal
growth within harborage sites or contamination of food
products and equipment (e.g., conveyor belts or harvester
equipment) (7, 15, 26, 33, 39-41, 47, 49, 52). Similarly,
significant increases in RH and temperature could decrease
the ease of removal of food and organic food soils, as well as
increase deterioration of cardboard, fiber-based products,
and other dry materials. Thus, dry food processing facilities
implementing superheated steam for surface sanitization
should examine the practical environmental considerations
within industrial food processing settings, including changes
of indoor RH and temperature, as well as manual operation
of handheld superheated steam units at close distances from
target surfaces.
Change in RH not time dependent through 5 h of
continuous superheated steam under test conditions
Twelve spaces with sizes ranging from approximately
11.7 m3 (413 ft3
) to 3,437.3 m3
(12,1387 ft3
) were analyzed
in this study to capture a range of diverse environmental
settings (Table 1). The following three types of ventilation
were included: natural, mechanical, and combination. The
ventilation rates ranged from 1.25 to >60 ACH. Within
commercial processing facilities, high ACH and positive
air pressure are commonly used as a good manufacturing
practice. Prior research has shown that low ACH and high
RH contribute to the spread of airborne infectious agents
(13, 16, 31, 54). Therefore, most manufacturing facilities
have already implemented 2 to 6 ACH as part of standard
operating procedures (46). In addition, the semi-enclosed,
naturally ventilated space also had very high ACH (>22
ACH). By contrast, the lowest ACH was observed in the
break room (1.25 ACH).
The two-way ANOVA test showed that exposure time
did not significantly affect ambient RH or temperature (P
> 0.05) among all spaces 1.5 m or greater from the source.
However, the average change in RH and temperature
fluctuated over time and were localized (Fig. 4). The largest
increase for RH and temperature was 47% and 9.3°C,
respectively, and occurred 0.3 m from the source, when
examining all facility spaces during a 5-h superheated steam
operation (Fig. 4). The greatest variation in temperature
change ranged from 2.85 to 9.3°C (large-sized facility with a
low ventilation rate, 0.3-m distance between the sensor and
the superheated steam source; Fig. 4B). The greatest variation
in RH change ranged from 8 to 47% (small-sized facility with
high ventilation rate, 0.3-m distance between the sensor and
superheated steam source).
The minimum change in RH and ambient temperature
may be attributed to two variables: the high efficiency of
both mechanical and natural air handling systems; and the
relatively low volume (1 liter) of water used to generate
superheated steam during the 5-h treatment. In practice,
a sanitation shift within a dry produce handling operation
may last for 5 to 8 h; the application of a sanitizer, such as
superheated steam, would likely constitute only a fraction
May/June Food Protection Trends 157
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