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.

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 High-Temperature 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 z-value 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 low-temperature 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 z-value

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 z-value can be calculated using equation 1 (Mullan, 2016; Mullan, 2020).

FT = FR x 10^TR-T   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 z-value 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^TR-T    Equation 2
FR             z

Taking logs to base 10 and simplifying gives equation 3.

Log (FT) =   TR-T x log(10)
        FR           z

logFT-logFR = TR-T x 1
                             z

1 = logFT-logFR  Equation 3
z           TR-T    

The z-value 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 z-values for the HTST processes approved in the US and NZ are shown in Table 2.

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).

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/resource-gallery/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/10157-dpc-3-animal-products-dairy-approved-criteria-for-the-manufacturing-of-dairy-material-and-prod. Accessed 1 November, 2021.

Mullan, W.M.A. (2016). How to derive an equivalent heat process at a higher (or lower) temperature. [On-line]. Available from: https://www.dairyscience.info/index.php/thermal-processing/302-equivalent-process.html . Accessed: 15 November, 2020. Updated August 2018.

Mullan, W.M.A. (2020). How do you calculate the z-value given two equivalent thermal processes?. [On-line]. Available from: https://www.dairyscience.info/index.php/thermal-processing/386-z-value-calculation.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.