Ice cream processors are required to meet or exceed the pasteurisation time temperatures specified in their jurisdiction. Details of minimum requirements are given in Table 1.
Table 1. Minimum temperaturetime criteria for pasteurising ice cream mixes in selected countries 

Country 
Heat treatment* 
Regulation/Statutory Instrument and reference 
Canada

69°C for 30 minutes 80°C for 25 seconds 
Code of Good Practice for HighTemperature ShortTime Pasteurizers (HTSTs). (Anon, 1991) 
New Zealand 
**69°C for 20 minutes **74°C for 10 minutes **79.5°C for 15 seconds **85.5°C for 10 seconds 
DPC 3: Animal Products (Dairy): Approved Criteria for the Manufacturing of Dairy Material and Product. (DPC 3, 2010) 
UK 
65.6°C for 30 minutes 
The Dairy Products (Hygiene) Regulations 1995. (Anon, 1995) 
US 
69°C for 30 minutes 80°C for 25 seconds 83°C for 15 seconds 
Grade A Pasteurized Milk Ordinance 2019 Revision. (PMO, 2019) 
Notes
*Holding times and temperatures are minimum values. Holding time refers to the minimum residence time of the fastest traveling particle. This depends on whether the liquid flow is laminar or turbulent.
** Applies to ice cream mixes containing particles of diameter less than or equal to 1000 µm.
Many larger manufacturers use HighTemperature Short Time (HTST) pasteurisation and use different HTST values from those given in Table 1. This is permitted providing the heat treatment is at least equivalent in its antimicrobial effect to that specified in the particular jurisdiction.
How can manufacturers calculate equivalent HTST processes based on local regulations?
This can be done accurately providing the zvalue of the process is known. Z is a thermal constant that expresses the increase in temperature necessary to obtain the same lethal effect in 1/10 of the time.
If z is not known it can be calculated if two equivalent HTST heat processes are specified. Ideally, the temperature range should be small to avoid linearity issues. While it can also be done if one lowtemperature process e.g 69°C for 20 minutes and one HTST process is specified the temperature range is normally too large for accurate predictions and additional caution is required.
The 2019 milk pasteurisation ordnance in the US (PMO 2019) provides two equivalent HTST time/temperatures for pasteurising ice cream mix (Table 1). This is similar to the situation in New Zealand (DPC 3, 2010).
Regretfully, the temperature differences in the values provided for the UK and Canada (Table 1) create uncertainty regarding the efficacy of calculating equivalent processes. However, the z values can still be calculated and additional safety achieved by increasing the holding time or temperature.
Calculation of the zvalue
This will be illustrated for HTST processing of ice cream mixes in the US. The minimum HTST processing conditions required are shown in Table 1.
Since both HTST heat treatments have been designated as equivalent, the zvalue can be calculated using equation 1 (Mullan, 2016; Mullan, 2020).
FT = FR x 10^TRT Equation 1
z
Where FT is the F value at the required temperature. This is the time at the reference temperature required to achieve the desired lethality and can take into account the lethal effects during heating, holding, and cooling at the target processing temperature. Only holding time at the reference temperature is considered for HTST pasteurisation. FR is the F value at the reference temperature, in this case, 80°C. TR is the reference temperature, 80°C and T is the Temperature for which we require the F value. z is the thermal constant. The higher the z value of the organism, the greater its heat resistance. Once the zvalue has been calculated this can be used to calculate other equivalent F values. To do this we need to solve Equation 1 for z.
Solving for z
Isolating the exponential function gives equation 2.
FT = 10^TRT Equation 2
FR z
Taking logs to base 10 and simplifying gives equation 3.
Log (FT) = TRT x log(10)
FR z
logFTlogFR = TRT x 1
z
1 = logFTlogFR Equation 3
z TRT
The zvalue of the equivalent processes over the temperature range 80°C to 83°C has been calculated using Equation 3. This gives a value of 13.5227 °C for z.
Using this value equivalent FT values can be obtained using Equation 1. Free calculators are available on this site to do these calculations.
The zvalues for the HTST processes approved in the US and NZ are shown in Table 2.
Table 2. Calculated zvalues for equivalent F values knowing reference F value and zvalues from minimum timetemperature data specified in the US and NZ. 

Equivalent F values

z° C 
80 °C,25 s and 83 °C,15 s (PMO, 2019) 
13.5227 
79.5°C, 15 s and 85.5°C, 10 seconds (DPC3, 2010) 
34.0732 
Using these values potential equivalent time/temperatures to those specified in Table 1 to meet the US, PMO (2019) ordnance requirements for ice cream mix and, the NZ requirements (DPC 3, 2010) were calculated using equation 1 (Table 3).
Table 3. Equivalent minimum timetemperature treatments (seconds) for pasteurizing ice cream mix between 80°C and 90°C that meet those cited in PMO (2019) and DPC3 (2010) 

Temperature, °C 
PMO (2019) United States 
DPC3 (2010) New Zealand 
79.5 
NC 
15* 
80* 
25 
14.5 
81 
21.1 
13.6 
82 
17.8 
12.7 
83* 
15 
11.8 
84 
12.7 
11.1 
85 
10.7 
10.3 
85.5 
9.8 
10* 
86 
9 
9.7 
87 
7.6 
9 
88 
7 
8.4 
89 
5.4 
7.9 
90 
4.6 
7.4 
* Specified in regulations. Note holding times will have to be multiplied by 2 if the pasteuriser is operating under laminar flow conditions. 
References
Anon (1991) Code of Good Practice for High Temperature Short Time Pasteurizers (HTSTs) Critical Design, Operational & Testing Criteria. Available from: http://www.bccdc.ca/resourcegallery/Documents/Educational%20Materials/EH/FPS/Food/DairyProcessingHTST.pdf Accessed 25th October, 2021.
Anon (1995) The Dairy Products (Hygiene) Regulations 1995. UK Statutory Instruments 1995 No. 1086. Available from: https://www.legislation.gov.uk/uksi/1995/1086/made Accessed 1 November, 2021.
DPC3 (2010). DPC 3: Animal Products (Dairy): Approved Criteria for the Manufacturing of Dairy Material and Product. New Zealand Food Safety Authority. Available from: https://mpi.govt.nz/dmsdocument/10157dpc3animalproductsdairyapprovedcriteriaforthemanufacturingofdairymaterialandprod. Accessed 1 November, 2021.
Mullan, W.M.A. (2016). How to derive an equivalent heat process at a higher (or lower) temperature. [Online]. Available from: https://www.dairyscience.info/index.php/thermalprocessing/302equivalentprocess.html . Accessed: 15 November, 2020. Updated August 2018.
Mullan, W.M.A. (2020). How do you calculate the zvalue given two equivalent thermal processes?. [Online]. Available from: https://www.dairyscience.info/index.php/thermalprocessing/386zvaluecalculation.html . Accessed: 15 November, 2020.
PMO. (2011). Grade A Pasteurized Milk Ordinance. Food and Drug Administration, College Park, MD. Available from: Regulation/GuidanceDocumentsRegulatoryInformation/Milk/ucm2007966.htm. Accessed 21st December 2015.
PMO. (2019). Grade A Pasteurized Milk Ordinance. Food and Drug Administration, College Park, MD. Available from https://agriculture.ny.gov/system/files/documents/2019/07/2017_PasteurizedMilkOrdinance.pdf . Accessed 10th October 2020.