Analysis of Thermal Death Kinetics of Microorganisms in Apple Pulp

This report investigates the thermal death kinetics of Listeria innocua in apple pulp, a common food processing scenario. The experiment involved heat-treating apple pulp spiked with Listeria innocua at varying temperatures (56, 58, 60, and 62°C) and measuring the decline in viable microorganisms over time. Key parameters such as D-values (time to reduce the microbial population by 90%) and TDT (Thermal Death Time) were calculated to assess the efficacy of heat treatment. The results demonstrated that higher temperatures led to a more significant reduction in microbial survival. The report discusses the impact of temperature and exposure time on microbial inactivation, comparing findings with existing literature and highlighting the implications for food safety and shelf-life stability. The analysis includes detailed data and discussion of the results at 58°C, along with a comprehensive abstract summarizing the overall experimental design and conclusions. The study underscores the importance of understanding thermal death kinetics in ensuring the safety of processed food products.

Abstract
The current experiment aimed to study the thermal death kinetics of microorganisms
commonly found in food. For the purpose of experiment, apple pulp spiked with Listeria
inocua was used. The sample was heat treated at four temperatures- 56, 58, 60 and 62oC and
log cfu/mL was determined for samples withdrawn at 0, 2.5, 5, 7.5, 10, 15, 20 and 30 minutes
intervals. Although heat is lethal to most of the microorganisms, not all microorganisms are
equally sensitive to heat. Further, thermal lethality is related to the number of microorganism
present, temperature to which the microorganism is exposed and time of exposure to heat.
From the study it was evident that the number of survivors declined at higher temperatures
with time and highest decline was observed for 62 oC. The D value refers to the time (in
minutes) required to destroy 1 log cycle (90%) of the target microorganism at a given
temperature (National Canners Association, 1968). Similarly, the z represents the difference
in temperature required to alter the D value by a factor of 10 (Van Doornmalen and Kopinga,
2009). The smaller z-value implies a greater sensitivity of the organism to heat.
Discussion
L. innocua recorded different responses during different heat treatments. Microorganisms
have been reported to exhibit higher tolerance to dry heat than to wet heat. Resistance to dry
heat is relevant for disinfection of devices and materials; while wet heat resistance is
significant in food microbiology. It is proposed that microorganisms are inactivated by
oxidation during dry heat treatment while during wet heating, protein denaturation and
membrane damage are reposible for the killing of microorganisms (Setlow and Setlow, 1998,
Coleman et al., 2007; Zhang et al., 2010). The straight line portion of the survival curves are
used for the calculation of D values by plotting the log of survival counts versus their
corresponding heating times (Juneja et al., 2009). At 56oC, D-value of 11.41 mins was
recorded along with a thermal death time (TDT) of 94.19 mins. This suggests that a ten-fold
reduction in the population of L. innocua at 56 oC would require 11.41 mins. The D value is a
convenient parameter for determining the length of thermal exposure for achieving
inactivation of microorganisms in a process (Perni, 2013). As evident from the curve, the
number of survivors at 56 oC did not exhibit a significant response till 5 mins of exposure to
this temperature. However, a steep decline was recorded after 10 mins of heat treatment. The
F value was 92.39 mins and 12 D value was 136.99 mins. This indicates that, at 56 oC,
exposure to heat for 136.99 mins would be required to reduce the number of survivors by 12

logs or 12- D values where 1 D represents 1 log value or a factor of 10. At 58 oC, a similar
response was observed and a decline in the population of L. innocua was recoded from 7.5
mins – 10 mins. The D value of 13.49 mins was obtained along with a TDT of 120.94 mins.
The F-value was 113.45 mins, thus implying that an exposure to 58 oC for 113.45 mins was
required for completely killing the bacterium. At 60 oC, the D value (17.76 min) as well as
the TDT (151.16 min) recorded and increased. Also, significant decrease in the population of
survivors was observed after 5 mins of exposure to 60 oC. The observations suggest that
exposure to higher temperatures triggered a protective response in L. innocua which resulted
in longer time period to achieve the inactivation of the organism. Also, the F- value was
increased further to 146.69 mins, hence indicating that longer exposure to 60 oC was required
to kill L. innocua. However, with further increase in temperature to 62 oC, the D-value, F
value and TDT were significantly reduced and recorded lowest values in comparison to other
treatment temperatures. A 12 D value of 115.72 mins at 62 oC implied that exposure of L.
innocua to 62 oC for 115.72 mins would reduce the population of survivors by 12 log values.
Also, at this temperature, an exposure for 80.71 mins was sufficient to kill the bacteria in the
sample.
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