Thermal processing
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- Written by Michael Mullan
Technologists must be able to calculate the cumulate lethality of a heat process normally referred to as F. This is done by defining a reference temperature, e.g. 121.1 °C for a F0 calculation using a low acid food or e.g. 93.3 °C for an acid product, at which the equivalent lethal effects experienced during heating and cooling at lower temperatures are calculated.
The area under the lethality curve is normally calculated using numerical integration. The most commonly used method is the trapezium or trapezoid method. An alternative, more accurate, but slightly more complicated method, is to use Simpson's rule or to be more correct Simpson's rules.
Site users can choose to download:
1) an Excel spreadsheet based-lethal rate calculator programmed using the trapezoid rule;
2) a more accurate and easier to use Excel workbook using both of Simpson's rules and also offering the option of using the trapezoid rule. This download also contains a spreadsheet to convert Z-values in Fahrenheit or Celsius to the desired measurement system and a PDF document listing D, F, Z values and reference temperatures for the major spoilage organisms of significance to processing low acid and acid foods. A link to a free On Line resource that details thermal resistance data for many spoilage and pathogenic organisms significant to the food and pharmaceutical industries is also provided;
3) all the spreadsheets above and the author's Ebook "Thermal processing of acid fruit and vegetable products. Significant microorganisms, recommended processing time / temperatures, and public health significance of spoilage."
Options 1 to 3 are available as immediate downloads.
- Details
- Written by Michael Mullan
Food technologists producing acidic foods such as pickles and sauces often find it difficult to get information on the processing conditions required to obtain commercial sterility. Following experience of working with processors experiencing technical issues including spoilage problems and difficulties in exporting products I have produced a focussed and concise report (Thermal processing of acid fruit and vegetable products. Significant microorganisms, recommended processing time / temperatures, and public health significance of spoilage.) that:
1) Identifies the potential spoilage organisms of acidic foods
2) Discusses the decimal reduction times and Z-values of the major spoilage organisms of acid foods
3) Lists F or P values and reference temperatures for ensuring the production of commercially sterile acid foods
4) Explains how to calculate F values and the number of log reductions of spoilage organisms following processing
5) Explains how to calculate equivalent processes e.g. at higher temperatures using published data
5) Explains the importance of measuring pH over the shelf life of acidified products
6) Provides a summary of the major causes of spoilage of acidic foods and their control
7) Lists literature, including a free On Line database containing around 6000 D-values, concerning the manufacture and control of acid foods.
The booklet can also be used on an IPhone or Android device that can view PDF files.
This report contains 17 pages, 5 tables and 19 references (the table of contents is shown in figure 1). Worked examples are provided and the author is prepared (within reason) to help users having problems providing they are posted on the forum.
The report was updated to version 1.3 on the 30th June 2012 and can be downloaded using the original download URL. The next significant update will be released around December 2012 and will contain more information on the equations underpinning thermal process calculations, more worked examples and Excel spread sheets configured to undertake the calculations. There will also be additional sections including the manufacture of acidic milk products (not covered in this report), validation and quality assurance of heat processes. Donors will be advised of updates which will be available at no cost.
Figure 1. Table of contents of the thermal processing report.
The report is available as an immediate download following a donation of £19.99 (about $32 or 28 Euros) to the Diary Science and Food Technology website. Only make this donation if you understand that you are receiving an educational aid to help you understand the scientific and technological factors influencing the production of commercially sterile acid and acidified foods.
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£19.99 (about $32 or 28 Euros)
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In this section we are going to use simple mathematical techniques, associated with thermobacteriology, to investigate the survival of Mycobacterium avium subsp. paratuberculosis during high temperature short time, HTST, pasteurisation.
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Calculator for determining the F value of a thermal process
The calculator converts temperature readings to lethal rates, plots the lethal rates against time, and determines F values for a heat process. The area under the curve is determined using the trapezoid rule. Accurate F determinations for most thermal processes can be obtained. In general the more values, the more accurate the value for F will be.
