AFDC 27 (2572) DTZS                                    
 
 
© TBS 202 4                                                                                        First Edition 202 4  
 DRAFT TANZANIA STANDARD  
 
Code of practice for the reduction of contamin ation of food 
with polycyclic aromatic  hydrocarbons (PAH s) from   smoking 
and direct drying processes   
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
TANZANIA BUREAU OF STANDARDS  
 
 
 
                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  Copyr ight notice  
This document is copyright -protected by Tanzania Bureau of Standards. Neither this document nor any extract 
from it may be reproduced, stored or transmitted in any form for any other purpose without prior written 
permission from TBS.  
Requests f or permission to reproduce this document for the purpose of use should be addressed as shown 
below:  
Tanzania Bureau of Standards  
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16103 Dar es Salaam  
Sam Nujoma Road/Morogoro Road,  
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Fax +255 22 2450298  
Hotline: 0800110827  
E-mail: info@tbs.go.tz  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  Code of practice for the reduction of contamination of food with polycyclic   aromatic 
hydrocarbons (PAH) from   smoking and direct drying processes  
 
Foreword  
This Tanzania Standard was app roved under the authority of the Board of Director of Tanzania Bureau of Standards.  
Tanzania Bureau of Standards (TBS) is the statutory national standards body for Tanzania established by the 
Standards Act Cap. 130 . 
Tanzania Standards are developed through  Technical Committees that are representative of key stakeholders 
including government, academia, consumer groups, private sector and other interested parties. The Technical 
Committees work under the supervision of Divisional (sectoral) Committees. The Sta ndards are developed in 
accordance with the Guide and Procedure for Development of Tanzania Standards and TZS 0, Guide for 
presentation of Tanzania Standards .  
Tanzania Standards are subject to review, to keep pace with  science and  technological advances. Users of the 
Tanzania Standards are therefore expected to ensure that they always have the latest versions of the standards they 
are implementing.  
This Tanzania Standard was developed under the supervision of the Agriculture and Food  Divisional Standards 
Committee [ AFDC ]. The Technical Committee responsible for the standard is AFDC 27 - Contaminants.   
The reporting of the result of a test or analysis made in accordance with this Tanzania Standard, if the final 
value, observed or calculated is to be rounded o ff, shall be done in accordance with TZS 4.  
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  Introduction  
Many chemical contaminants are formed during the combustion of fuel  and other materials  both in the smoking 
and in the direct drying process. Examples include polycyclic aroma tic hydrocarbons (PAH s), dioxins, 
formaldehyde, nitrogen and sulphur oxides (relevant for formation of e.g. nitrosamines). Furthermore, heavy 
metals are also found in combustion gases. The types and amount s of contaminants depend on the materials 
used , the temperature and possible other parameters.  
Hundreds of individual PAH s may be formed and released as a result of incomplete combustion or pyrolysis of 
organic matter, during forest fires and volcanic eruptions as well as industrial processes or other hum an activities, 
including the processing and preparation of food. Owing to their mode of formation, PAH s are ubiquitous in the 
environment and therefore can enter the food chain, especially via air and soil. PAHs can be present in the raw 
materials due to e nvironmental contamination from the air by deposition on crops, soils and transfer from water to 
fresh and marine invertebrates. Commercial and domestic food preparation such as smoking, drying, roasting, 
baking, barbecuing or frying are recognized as impo rtant sources of food contamination. Presence of PAH s in 
vegetable oils can also originate from smoking and drying processes used to dry oil seeds prior to oil extraction.  
PAHs are toxic chemical s which when taken through  food and feed can  pose various  health effects  such as 
cancer , skin, eyes, kidneys, and liver  diseases . Animal  studies have shown adverse effects in reproductive and 
general developmental processes. The major route s of exposure to PAHs in the general population are inhalation, 
ingestion, s moking cigarettes, or breathing from open fireplaces.  
 
Contamination of food with PAH s via environmental contamination should be controlled by either source -directed 
measures like filtering the smoke from relevant industries (e.g. cement work, incinerator  and metallurgy) and 
limiting  the exhaust fumes of PAH s from cars. Good practices, including the selection of appropriate 
farmland/fishing waters, could also decrease the environmental contamination of raw materials with PAH. 
However, this contribution to the reduction of PAH s intake from the final food is not included in this Code of 
Practice.   
The major sources  of PAH s are cereals and cereal products (owing to high consumption in the diets) and 
vegetable fats and oils (owing to higher concentrations of P AHs in this food group). Generally, despite their 
usually higher concentration s of PAH s, smoked fish and meats and barbecued foods do not contribute 
significantly, particularly as they are small components of the diet. However, they do make larger contribu tions 
leading to higher PAH s intake where these foods make up a large part of the diet.  
This Code of Practice intends to provide guidance for national authorities and manufactures , and consumers  to 
prevent and reduce contamination of food with PAH s in com mercial smoking and direct drying processes. For this 
purpose, this Code of Practice identifies important points to consider and provides relevant recommendations.   
 
 
 
 
 
 
 
 
 
 
 
 
 
                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  1. Scope  
This Code of Practice provide s guidance on reduction of PAH contaminati on of food during direct and indirect  
smoking , and direct drying processes . It does not cover  PAH contamination originating from the use of herbs and 
spices in the smoking process ; indirect drying;  barbecuing and other types of cooking in private homes or the 
catering sector; and environmental  contamination of raw materials . 
  
2. Normative references  
There are no normative references in this document.  
 
3. Terms and definitions  
For the purposes of this Tanzania standard , the following terms and definitions  apply : 
3.1 Contaminant  
any substance not intentionally added to food, which is present in such food as a result of the production 
(including operations carried out in crop husbandry, animal husbandry and veterinary medicine), manufacture, 
processing, preparation, tre atment, packing, packaging, transport or holding of such food or as a result of 
environmental contamination. The term does not include insect fragments, rodent hai rs and other extraneous 
matter  
 
3.2 direct drying  
  drying process where the combustion gas es co me into direct contact with the foods    
   
3.3 sun drying  
direct drying process where sunshine and wind are used for drying under circumstance s open to the environment  
 
3.4 indirect drying  
drying process where the combustion gases do not come into direct contact  with the foods, where the hot air is 
heated via a heat exchanger , electricity or by other means  
  
3.5 Hazard Analysis Critical Control Point ( HACCP )  
system which identifies, evaluates, and controls hazards which are significant for food safety   
  
3.6 other plant materials  
covering other types of fuels than woods used in the smoking or drying process, e.g. bagasse, corn c obs, saw 
dusts , dry grasses , rice husks , coconut husk s, and shell s 
 
3.7 Polycyclic aromatic hydrocarbons (PAH s)  
group of contaminants that constitu te a large class of organic compounds containing two or more fused aromatic 
rings made u p of carbon and hydrogen atoms   
  
3.8 pyrolysis  
chemical decomposition of organic materials by heating in the absence of oxygen or any other reagents, except 
possibly stea m  
3.9 smoke  
gaseous products of burning materials especially of organic origin consist ing of suspended liquid and solid 
particulates       
                                                                                                            AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  3.10 smoke condensates  
products obtained by controlled thermal degradation of wood in a limited supply of oxygen (pyrolys is), subsequent 
condensation of the resulting smoke vapours, and fractionation of  the resulting liquid products  
  
3.11 smoking of food  
process used as a preservation method to prolong the shelf life of food due to components of the smoke inhibiting  
growth of s ome microorganisms   
 
3.12 direct smoking  
smoking process, where the smoke is developed in the chamber in which the food is proce ssed  
  
3.13 indirect smoking  
process where smoke generators are used, and the smoke is being developed in a chamber, separat e from 
where  the food is smoked    
 
4. General principles for reducing PAH contamination in food  
The food producer should be aware of the conditions under which higher levels of PAH s are generated and 
wherever possible, should control those conditions to minimize their f ormation. To accomplish this, an analysis of 
important points to consider in processes used or intended to be used in food production with smoking or direct 
drying should be carried out.  
  
The first step of the analysis is to identify important points to consider. Possible major important points to consider 
include :  
a) Possible sources of PAH s from the environment and the process;  
b) Possible effects on consumer health;  
c) Controllability; and  
d) Possible measures to reduce PAH contamination.     
  
The producer sh ould take appropriate measures to control the identified important points for reducing PAH s, 
based on the results of the analysis and other legitimate factors relevant for human health protection and 
economic activities such as :  
a) The microbiological status  and possible risks from other contaminants;  
b) The organoleptic properties and quality of the final product (the ideal method would have no adverse 
effects on the appearance, flavour, taste or nutritional properties of the product); and  
c) Feasibility and eff ectiveness of controls (cost, commercial availability, occupational hazards).  
  
The producer should monitor the effects of the implemented measures and should review them if necessary.  
 
5. Evaluation of compliance with relevant legislation s  
Processed food shall be in compliance with relevant national legislation s and standards, including general 
requirements for consumer protection. Furthermore, food shall be produced in accordance with relevant national 
codes of practice. Some of these may contain further information about drying or smoking, which should also be 
considered.  
  
The formation of PAH s during smoking and direct drying is dependent on a number of variables, including:  
a) Fuel (woods and other plant materials, diesel, gases, liquid/solid waste and other fuels);  
b) Smoking or drying method (direct or indirect);                                                                                                                 AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  c) Smoke generation process in relation to the temperature of pyrolysis and to airflow in the case of a 
smoke generator (friction, smouldering, thermostated plates) or in relation with other methods such as 
direct smoking or regenerated smoke by atomizing smoke condensate (liquid smoke);       
d) The distance between the food and the heat source;       
e) Position of the food in relation to the heat source;        
f) Fat content of the food and what ha ppens to it during processing;  
g) Duration of smoking and direct drying;  
h) Temperature during smoking and direct drying;  
i) Cleanliness and maintenance of equipment; and 
j) Design of the smoking chamber and the equipment used for smoke/air mixture (which influence s the 
smoke density in the smoking chamber).  
  
In general, changes in processing techniques can in some cases reduce the amount of PAH s formed during 
processing. Indirect drying or smoking processes result in lower PAH contents than direct drying or smoki ng. Also 
the use of smoke condensates, selection of fuel such as type of wood and adjusting times and processing 
temperatures influences the PAH formation. Addition of activated carbon to coconut oil at the right dosage during 
the refining process can comp letely remove PAH contamination.  
  
Application of an HACCP system in accordance with the principles and steps as recommended is one of options 
for reducing PAH s.  
 
6. Smoking  
Smoking  has been used for centuries as a method for preserving meat and fish. Smok ing impregnates the high -
protein food with aromatic components, which lend flavour and colour to the food, and also play a bacteriostatic 
and antioxidant role.  
  
6.1 Fuel used in smoking  
6.1.1  For smoking of food, woods are normally used, but other types of fuels like bagasse (plant material s from 
sugarcane), corn cob s and coconut husk s and shell s are used. The fuel used is an important point to consider for 
the potential contaminants of the food, e.g. the PAH contamination of food varies with type of fuel used for  
smoking . PAH contamination of oil seed is higher when using coconut husk s compared to coconut shell s   due to 
higher  lignin content of the husk s.  
  
6.1.2  The type of fuel used  has an influence on PAH formation. It is recommended that the different  types of 
fuel should  be evaluated in relation to PAH formation before use. The use of fuels other than wood and other 
plant materials for the purpose of smoking foodstuffs should be avoided . Fuels like diesel,  rubber (e.g. tyres) or 
waste oil must not be used even as a partial component, as they may lead to significantly increased PAH levels.  
 
6.1.3  Resinous wood  is not preferred for smoking processes. Moreover, w oods treated with chemicals such as 
for preserving, waterproofing, fireproofing etc. should not be used for smok ing or the production of smoke 
condensates. Such treatments may result in tainting of the food as well as the introduction of other contaminants, 
e.g. dioxin from woods treated with pentachlorophenol (PCP).  
  
6.2 Foodstuffs  
6.2.1  The position of the food in the  smoke chamber and the distance between the food and the heat source is 
an important point to consider in the smoking process. As PAH s are particle bound, a greater distance from the 
smoke source to the smoked food might reduce content of PAH s in the food.  
  
6.2.2  During direct smoking, fat dripping from the food into the source of the smoke, e.g. glowing wood or other 
plant materials might increase the content of PAH s in the smoke and thereby in the smoked food. In order to                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  avoid an increase in the PAH content thr ough fat drippings into the open fire, perforated metal sheets can be 
installed between the food to be smoked and the heat source.  
  
6.2.3  The   quality of the final food product must be evaluated to ensure compliance with set standards .     
  
6.2.4  The organoleptic properties of the final products are an essential part of their characteristics.    
  
6.3 Processing  
6.3.1  Four types of smoking  techniques  are generally recognized: smouldering, thermostated plates, friction 
processes, and smoking with smoke condensates. Friction processes allow smoke to be produced by pyrolysis of 
wood sawdust, wood chips, and wood logs. Smoke condensates may be used by exposing food to smoke which 
is reproduced or regenerated by atomizing smoke condensate (liquid smoke) in a smoking chamber.  
   
6.3.2  Smoke is produced by pyrolysis of the fuel at temperatures of around 300 -450 °C in the glow zone. To 
produce smoke for smoking food, flames should be avoided, including by adjusting airflow.  
  
6.3.3  Differences in the smoking processes can lead to highly variab le PAH levels in the final food product. The 
choice of technology for processing is very important for the final concentration of PAH s. Identifying the 
parameters critical for PAH formation in a specific process may potentially be useful to control PAH lev els. Direct 
smoking requires less equipment than indirect smoking but can result in higher levels of PAH in the final food 
product.  
   
6.3.4  Replacing direct smoking with indirect smoking can significantly reduce contamination of smoked foods. 
In modern industr ial kilns, an external smoke generator can be operated automatically under controlled 
conditions, to wash the smoke from particles before coming in contact with the food and to regulate its flow as it is 
brought into contact with the food. For more traditi onal or smaller scale operations, this may not, however, be an 
option.   
 
6.3.5  Smoking processes are often divided into three groups depending on the temperatures used in the 
smoke chamber during process ing:  
a) Cold smoking with temperature of approximately 18 –25 °C. It is u sed for smoking e.g. some fish species 
and salami -type sausages;  
b) Semi -warm smoking with temperatures of approximately 30 –40 °C. It is u sed for smoking e.g. some fish  
species, bacon and pork loin; and   
c) Warm (or hot) smoking is smoking combined with heating resulting in a temperature of approximately 70 –
90 °C. It is u sed for smoking e.g. some fish species, hams, and frankfurter type sausages.  
       
6.3.6  The type of generator used should be based on an assessment of possible reduction of the PAH cont ent 
in the final food and where possible include washing of the smoke after the generator and before the smoke 
chamber. Good results are achieved by installing baffles after the smoke generator equipped with a device for 
decantation of tar. A more efficien t way is to manage the pyrolysis temperature and decanting of heavy phase 
tanks to a cooling device with baffles.  
 
6.3.7  The scientific background and data to illustrate the exact influence of the use of different types of fuel, 
time, temperature etc. is limite d and specific testing is needed for the identification of important points to consider 
in the individual processes. However, reduction of PAH levels can be achieved using the appropriate fuel, placing 
the food at a distance from the  heat source or using a  barrier between the heat source and food, reducing the 
duration and temperature of the process.  In addition,  other  methods like use of long pipes in the equipment can 
reduce the PAH s.  
                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  6.3.8  As PAH s are particulate bound, a filter may be used to remove particu late material from the smoke. This 
should reduce potential contamination with PAH s.  
  
6.3.9  Oxygen needs to be balanced as both too much and too little oxygen produces PAH s. Adequate oxygen  
for   partial/incomplete combustion of the fue l is needed to produce sm oke. However, too much oxygen may raise 
the temperature in the glow zone and lead to increased formation of PAH s. Lack of oxygen may lead to the 
formation of more PAHs, as well as produc tion of  carbon monoxide, which may be hazardous to operators.  
  
6.3.10  Tempe rature is of importance for the partial/incomplete combustion of the fuel. Generally, PAH formation 
increases with increasing temperature. The composition of the smoke depends on the temperature, which should 
be adjusted to minimize PAH formation. However,  more data is needed to document which temperatures would 
be recommendable.  
  
6.3.11  In principle, the smoking time should be as short as possible to minimize the exposure of the food 
surfaces to PAH -bearing smoke. However, in the case of hot smoking, when the p roduct is being cooked at the 
same time, it will be essential to allow sufficient time for the product to be cooked thoroughly. In case hot smoke 
is the only heat source (traditional smoke houses), the smoking chamber should be heated before the food 
produ cts are placed in it. Smoking time is not an important parameter as long as the source for smoke is well 
managed. Moreover, short smoking times may have an impact on food safety and shelf life. Clearly preventive 
measures cannot be taken in isolation from other considerations and it is vital that they do not adversely impact 
on the sensory properties and consumer acceptance of the product. Additionally, microbiological stability and 
nutritional properties need to remain unimpaired and care needs to be taken  to ensure that other contaminants 
are not inadvertently introduced.  
  
6.3.12  Because smoke condensates are produced from smoke that is subjected to fractionation and purification, 
products made with condensed smoke generally have lower PAH levels than products made with freshly 
generated smoke.  
  
6.4 Post smoking treatment  
6.4.1  There are three types of cleaning steps to be used either during processing or as post process treatment:  
a) During the process smoke may be washed before it enters the smoking chamber. This can b e achieved 
by washing (scrubbing), using a tar condenser, cooling or filtering all of which can remove particle -bound 
PAHs from the smoke;  
b) Post smoking treatment involves the cleaning of the smoked product itself. In this case rinsing the product 
or immer sing it into water may remove soot and particles containing PAH s on the surface of the food. 
This type of cleaning would not be possible to use for all types of products, e.g. not for smoked fish and 
fishery products;  and     
c) The shaving off the surface o f the smoked product itself. In case of solid smoked food   this cleaning can 
reduce PAH s in the final product.       
 
6.4.2  When possible, washing or water -cooling  of smoke should be used to reduce the content of PAH s in the 
final food. Water -cooling is already  used in the meat industry. Washing the product after the process may remove 
PAH-containing particles from the surface of the product.  
6.4.3  Washing of the product should not be used for fishery products as it could result in lower organoleptic 
quality and incr eased microbiological risk s. Fish products are often smoked as the whole fish with the skin, and if 
the skin is not eaten, some contamination is removed together with the skin. The recommendation could be to 
prioritize smoking of fish with skin and, prefer ably, removing the skin before consumption.  
 
7. Important points to consider and recommendations on smoking  
PAH content of smoked foods can be minimized by identifying and evaluating the important points to consider 
mentioned below, and by taking appropriat e measures. An HACCP system might be applied.  
                                                                                                            AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  7.1 Fuel:  
a) The type and composition of wood used to smoke foods, including age and lignin content in the wood 
used  should be considered . In general, conifer woods containing higher lignin contents should be 
avoided;  
b) Monitor the water content of the fuel. Lower water content may lead to rapid burning of fuel and higher 
PAH levels;       
c) When individual types  of woods and other types of plant materials like bagasse (from sugarcane), corn 
cobs and coconut husk s and shells are used, their use should be evaluated in light of PAH contamination;  
d) Do not use woods treated with chemicals;  
e) The use of fuels other than woods and plant materials: Do not use diesel , kerosene , waste products, 
especially rubber tyres and waste oil which may lead to significant levels of PAH s; and      
f) Influence on the taste of the final food.       
 
7.2 Smoke developed and used in the process:  
a) The composition of the smoke depend s on  e.g. the type of wood or other plant materials, the amount of 
oxygen present and the temperature of pyrolysis and possibly the length of time for which the plant 
materials are burned;  
b) The design of the smoking chamber and of the equipment used for smoke/air mixture (e.g. length of the 
pipe in the equipment);       
c) Filtering or cooling the smoke where possible;  
d) Washing off the smoke between a smoke generator and the smoke chamber where possible  and  
e) Install ation of  baffles after the smoke generator equipped with a device for decantation of tar if possible .       
 
7.3 Foodst uffs smoked:  
a) The position of the food in the smoke chamber and the distance between the food and the smoke source;  
b) Chemical properties and composition of food, e.g. the fat content of the food to be smoked;       
c) Deposits of smoke particles on the surfac e and the suitability of the surface for human consumption. For 
fish, the recommendation could be to prioritize smoking of fish with skin;  
d) The quality of product after processing; and 
e) The organoleptic properties of the final food.       
 
7.4 Smoking process:  
a) Whether the smoking process is a direct or indirect process. Replace direct smoking with indirect 
smoking where possible;  
b) Prior assessment of smoke generators by taking account of the resulting PAH content in the smoke;       
c) Adjusting of the airflow to avoid excessive temperatures during smoke generation;  
d) Selecting appropriate smoking chamber and device for treatment of air/smoke mixture;  
e) The accessibility of oxygen during the smoking process;  
f) Smoking time: Reducing the time that food is in contact wi th smoke should take into consideration  for 
microbiological safety and quality ;       
g) Temperatures: Temperature in the glow zone (in the smoke generation step) and temperature of the 
smoke in the smoking chamber;       
h) Perforated metal sheets can be instal led between the food to be smoked and the heat source  to avoid an 
increase in the PAH content through fat dripping into the heat source ;  
i) The cleaning method and schedule applied in the processing unit; and                                                                                                                AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  j) As an alternative to using freshly generated  smoke, manufacturers can consider smoking with 
regenerate d smoke from smoke condensates. They can also produce smoke -flavoured products by 
applying smoke condensates to foods, such as by spraying, dipping, injecting, or soaking.       
  
7.5 Post smoking proce sses:  
The cleaning of the smoked product itself. In this case,  soot and particles containing PAH s on the surface of the 
food may be removed by rinsing the product or immersing it into water. This type of cleaning would not be 
possible to use for all types  of products, e.g. smoked fish and fishery products. Also, washing might lower 
organoleptic quality and increase microbiological risk s.  
 
8. Direct drying  
One of the oldest methods of food preservation is direct drying, as it uses less equipment than indirec t drying. 
Direct drying reduces water activity sufficiently to delay or prevent bacterial growth. Direct drying of food can be 
done either by sunshine or wind or using hot combustion gases. Water is usually removed by evaporation , which 
creates a hard oute r-layer helping to stop microorganisms  from entering the food.  
 
9. Considerations in developing preventive measures to reduce the PAH content of 
dried foods  
9.1  Sun drying  
9.1.1  When drying by sun or wind, the potential source of PAH s is the environment. Contaminat ion can 
originate from soil/dust ,   combustion from industry and   traffic as well as forest fires and volcanic eruptions.  
9.1.2  Sun-drying of foodstuffs has the advantage of using free energy from the sun or wind. However, the 
benefits of greater control over the drying environment and drying time, quicker drying and less contamination 
from dirt, grass and insect particles, coupled with a consumer demand for a cleaner and less contaminated 
product may make artificial drying (dehydration) more attractive.  
9.1.3  A maj or disadvantage of sun -drying is the exposure of foodstuffs to the environment, e.g. exposure to 
undesirable weather conditions and to contaminants . Weather conditions, over which the producer  has no control, 
greatly affect the drying rate. Contamination o f dried foods with foreign matter is a serious concern. Sun -dried 
foods are exposed to contamination by windblown dust, seeds, insects, and rodent  and bird droppings.  
9.1.4  Sun drying of foodstuffs should not take place near industrial point sources of combusti on of gas, such as 
roads with heavy traffic, incinerators, coal -fired power stations, cement works etc., or in the immediate proximity 
of roads with intense traffic. Contamination from drying in such places is expected to be a special problem for 
foodstuff s with a large surface area such as spices. However, covered dryers may protect foodstuffs from 
industrial sources to some extent.  
  
9.2 Direct drying processes, other than sun drying  
9.2.1  Fuel used in direct drying other than sun drying  
Different types of fuel are used in direct drying, e.g. natural gas, peat and mineral oils. For some foods, the effect 
of fuel choice on taste may be the important points to consider in choosing a fuel. In any event, fuels like diesel , 
rubber, tyres, lubricants  or waste oil must not be used even as a partial component, as they may lead to 
significantly increased PAH levels.  
 
9.2.2  Combustion gases  
Drying with combustion gases increase s the contamination by 3  - to 10 -fold; use of coke as fuel result s in much 
less contamination than use  of oil. Direct contact of oil seeds or cereals with combustion products during drying 
processes has been found to result in contamination with PAH s and should therefore be avoided.   
  
9.3 Foodstuffs dried  
Contamination of dried foods e.g. cereals and vegeta ble oils  with PAH s usually occurs during technological 
processes like direct fire drying, where combustion products may come into contact with the food. Direct contact of                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  oil seeds or cereals with combustion products during drying processes has been found to result in accumulation of 
PAHs and should therefore be avoided.   
 
9.4 Direct drying process  
9.4.1  Dehydrators are useful for larger drying yards and growers. Dehydration allows a steady production cycle 
to be maintained, reduces labour costs and is an insurance  against unfavourable weather conditions for sun 
drying. A system using a combination of initial sun drying followed by finish dehydration  using other  dehydrators 
can have considerable advantages without loss of food quality.  
 
9.4.2  Common direct drying/heating  operations and applications include drying to remove water (and/or other 
solvents/chemicals) added, left or produced during processing. During direct drying, hot air is blown directly into 
the foodstuffs and combustion products can therefore directly ente r the food. One example of PAH contamination 
from direct drying is contamination of vegetable oils (including olive residue oils) in which oil has been 
contaminated with PAH s during technological processes. Another example can be drying oil seeds prior to oil 
extraction.  
 
9.4.3  Continuous flow drying, where cereals pass the drying area continuously, is a widespread grain drying 
method. This technique can be used for drying cereals for food. Direct drying is mainly used with temperatures up 
to 120 ºC for feeds. F or foods (cereal grains, malt, etc.), indirect drying (external heat generation) with 
temperatures between 65 and 80 ºC are mainly used. The time span for both types of drying is between ½ and 1 
hour, depending on the initial moisture content of the grain.   
  
9.4.4  Dehydration provides a form of insurance against poor weather conditions that can handicap traditional 
sun- and shade -drying. Accurate control of the drying conditions (temperature, relative humidity and air 
movement) essential for efficient dehydratio n is achieved. Many kinds of fresh fruits, vegetables, herbs, meat, and 
fish can be dried.  
  
9.4.5  Too high temperature  (one that causes visible burning of the product) can cause PAH formation. Where a 
system with a burner is being used, the temperature of the burner should be sufficient to allow complete 
combustion of the fuel, as incomplete combustion can lead to PAH s in the drying gases. A good homogeneity of 
the temperature of the air is important to avoid overheating.  
  
9.4.6  The drying time should be as short a s possible to decrease the exposure of the food to the potentially 
contaminating gases as much as possible.  
9.4.7  The use of active carbon is required during refining of the oil as a way to reduce the PAH content after 
direct drying. A monitoring system for the  PAH content should be established and additional refining steps (with 
active carbon) must be used when the PAH level in the food is unacceptable.  
  
9.4.8    Monitor  the combustion gases for CO, monitor the burner (if applicable) for soot accumulation, and 
check  burner settings and burner or fire temperatures.  
  
9.4.9  As drying processes could be a potential source of PAH s in cereals and oil seeds, there is also a need to 
control the levels of PAH s in agriculture crops after harvest, with particular reference to the s ource of 
contamination, as these crops can have a major impact on PAH s intake.  Avoid fire drying of seeds, and seek for 
alternative drying techniques.   
  
9.4.10   Numerous factors, including equipment cost , knowledge, weather  and availability of energy sources 
often result in similar foods being dried in very different ways.  
  
9.4.11   Replacing direct drying with indirect drying can significantly reduce contamination of dried foods.  .  
                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  10. Important points to consider and recommendations on direct drying, except sun 
dryin g  
PAH content of foods directly dried can be minimized by replacing direct drying with indirect drying, if possible or 
by identifying and evaluating the important points to consider mentioned below, and taking appropriate measures. 
An HACCP system might b e applied.  
  
10.1 Fuel:  
a) The type and composition of fuel used to dry foods affect the PAH content;  
b) Do not use woods treated with chemicals, e.g. preserved wood, painted wood;       
c) Monitor the water content of the fuel e.g. wood . Lower water content of wood  may lead to rapid burning 
of the wood and higher PAH levels;  
d) Avoid the use of fuels such as diesel, waste products, especially rubber tyres, olive residues , lubricants  
and waste oil which may increase  significant levels of PAH s; and 
e) The fuel influences t he taste of the final food.  
 
10.2 Drying process:  
a) Temperature of the air should be optimal;  
b) Minimize the time that food is in contact with combustion gases;       
c) Use of active carbon during refining of the oil;  
d) Avoid fire drying of oilseeds;  
e) Avoid direct contact of oilseeds or cereals with combustion products; and 
f) Keep equipment clean and well maintained (especially driers).  
                                                                                                           AFDC 27 (2572) DTZS                                      
 
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© TBS 2024  – All rights reserved  Bibliography  
CAC/RCP 68 -2009, Code of Practice for the reduction of contamination of food with polycyclic   aromatic 
hydrocarbons ( PAH) from   smoking and direct drying processes .