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HEAT TREATMENT OF MEAT PRODUCTS

Heat treatment of processed meat products serves two main purposes:

The heating parameters to be applied in meat processing can vary considerably in temperature and time depending on the type of product. Heat treatment methods cause various physical-chemical alterations in meat, which result in the beneficial sensory and hygienic effects on the processed products.


When mankind learned to use fire for food preparation, the aspects of palatability were clearly important. Heat treatment became the common way of making meat palatable for consumption. The impact of high temperatures induces coagulation and denaturation of meat proteins and structural and chemical changes of fats and carbohydrates, which make meat tastier and also more tender. In addition, the absorption of nutrients from heat treated meats in the digestive tract of humans is improved.

In modern times, with longer distribution channels for meat and the popularity and steadily growing quantities of processed meat products on the markets, the hygienic aspects of heat treatment of such processed meats, which result in germ reduction, became increasingly important.

Heat treatment for microbial control

Contrary to meat dishes (see box page 90), which are usually consumed hot immediately after preparation, most processed meat products are heat treated during manufacture and cooled down in a next step, as they undergo shorter or longer cold storage periods for distribution and sales. Hence, processed products must have an adequate shelf life, which can only be achieved if their microorganism content is low or practically zero. During slaughtering, subsequent meat cutting and initial processing steps, the numbers of microorganisms in meat are steadily increasing. The thermal treatment at the end of the processing stage is therefore important for microbial control. It is the effective tool to reduce or eliminate the contaminating microflora (see Fig. 452).

Enhancement of texture, flavour and colour through heat treatment

Firstly one should distinguish between heat treatment as part of the processing (here called “treatment A”) and heat treatment immediately before consumption (here called “treatment B”). For some processed meat products only (A) is required and such products are consumed cold. Other products, which were submitted to (A) during manufacture, are warmed-up again before consumption (B) and eaten hot.

For products of the cured-cooked type (e.g. cooked ham, Fig. 116) (see page 171, 177) or of the raw-cooked sausage type (e.g. frankfurter or bologna type sausage, Fig. 115, 120) (see page 127), heat treatment (A) applied in the final processing stage is indispensable in order to achieve



Fig. 117: Meat loaves before heat treatment (in this case baking)


Fig. 118: Meat loaf after heat treatment (core temp. +72°C)


Fig. 119: Meat loaf with firm texture and pink colour after heat treatment


Fig. 120: Large calibre sausage upon heat treatment (core temp. +70.3°C) (see also table 8, page 142)

Precooked-cooked meat products (e.g. liver sausage, blood sausage, corned beef, etc., see page 149), are submitted to two heat treatments (A). The raw meat materials are precooked (Fig. 121) and further processed and after filling in casings or cans, the second heat treatment is applied (Fig. 185, 186). This serves primarily for taste and flavour improvements, but due to germ reduction also for shelf life extension of the final products.


Fig. 121: Precooked lean meat (left) and fat (right) for processing of "precooked-cooked meat products"

Another group, the fresh meat products (such as sausages for frying or burgers, see page 103), are manufactured without any heat treatment. For this type of products, fresh raw ingredients are comminuted and mixed together. Eventually, heat treatment, mostly frying, takes place immediately before consumption (treatment (B), as the products are usually eaten hot (Fig. 122 and 123).


Fig. 122: Fresh sausages before heat treatment


Fig. 123: Fresh sausages after heat treatment

Only two types of meat products exist, which are manufactured and normally also consumed without any heat treatment, raw fermented meat products (such as raw ham, dry sausages, see page 115 and 172) and the raw dried meat products (such as biltong, pastirma, see page 237/238).

Meat dishes

For the cooking of meat for meat dishes, two basic methods are of relevance: dry heat, in which the meat is surrounded by hot air, and moist heat, in which the meat is surrounded by hot liquid.

Dry-heat methods are

  • Broiling (meat is placed in an oven)
  • Pan frying (browned on both sides in the pan)
  • Stir frying (small meat pieces under constant stirring in a wok/ Asian frying pan)
  • Deep fat frying (meat completely immersed in fat)
  • Roasting (meat placed on a grill or in an open roasting pan with the fat side up, no water added)

Moist-heat methods are

  • Braising (water and other ingredients such as milk or vegetable are added),
  • Stewing (cooking in liquid of small meat pieces),
  • Simmering (cooking in liquid of large meat pieces, normally low temperature and long time)

Recommended minimum safe internal temperatures
Poultry (dark meat) 80°C
Poultry (light meat) 71°C
Ground poultry 74-80°C
Ground beef and all types of pork 71°C
Beef/veal/lamb steaks and chops (medium-rare) 63°C

Heating parameters for meat products

For preparation of meat dishes in households or restaurants, exact temperature control is normally not needed and it is only differentiated between low, medium and high dry or moist heat (see box above). Meat dishes are usually consumed immediately after cooking, so the heat treatment is (besides basic food safety aspects1) mainly for sensory reasons. The achievement of a prolonged shelf life is not intended2.

For processed meat products exact temperature control is indispensable, as the balance between two opposite requirements has to be found:

Heat treatment of processed meat products will therefore always be a compromise between sensory and hygienic requirements.

In case of difficult hygienic conditions (e.g. tropical environment, highly contaminated raw meat, risk of interrupted cold chain) more intensive heat treatment must be applied. However, this may result in a certain degradation of the eating quality and higher cooking losses. If meat production and meat handling conditions are good (e.g. moderate climate, fresh hygienic raw materials, excellent processing and storage conditions), the heat treatment can be less intensive, which results in better sensory quality, but in hygienically more sensitive products.


1) Naturally, basic food safety aspects play also a role in heat treatment of meat dishes, such as elimination of potentially food poisoning microorganisms.

2) Exception: For supplying canteens, supermarkets, etc. with pre-packed cooked and afterwards chilled ready-to-eat dishes, which have to be reheated before consumption, exact temperature control during cooking is necessary as the product will be stored.

“Hurdle technology” of heat treated products


Fig. 124: Hurdle technology

In modern meat processing, the effect of heat treatment can be supported by the application of additional “hurdles”, which have the potential to slow down microbial growth. Such “hurdles” allow keeping the heat treatment of sterilized products at lower temperature levels, so that the product quality is less affected (see page 294 “Commercially sterile products”). Alternatively, this technology can be used to produce shelf-stable products of the non-sterilized type through heat treatments below 100°C. This kind of heat treatment alone would not be enough to stop microbial growth, but the additional “hurdles” complete the effect. This kind of meat preservation is called hurdle technology.

Frequently used “hurdles” are lowering of water activity (aw) (see page 323) or acidity (pH) (see page 321) in a product, or the utilization of chemical preservatives (see page 74), to which amongst many others also the commonly used nitrite curing salt (see page 68) belongs. All these measures on their own would not stop microbial growth, but some or all of them in combination with heat treatment account for a number of “hurdles”, which cannot be overcome by microorganisms surviving in the product (see Fig. 124). The result of such “built-in hurdles” is that meat products can be moderately heated, but surviving microorganisms can not grow. In most efficient combinations of such “hurdles”, microorganisms do not even grow under ambient (“room temperature”) storage conditions. Such products do not need refrigeration, they are shelf-stable, but much less heat treatment was needed than for fully sterilized canned products (see page 294). Naturally, in the meat sector the range of products, which can be made shelf-stable according to the hurdle technology, is limited but may be of significance in certain circumstances, in particular if no uninterrupted cold chain is available.

Examples:

Meat mixes of the raw-cooked type (see page 127), with high amounts of coarsely cut lean meat pieces (about 90%) and the rest raw-cooked batter for binding purposes, are filled into permeable casings (see page 264) and pasteurized. Built-in hurdles are the pasteurization temperature, nitrite curing salt (and possibly other preservatives) and most importantly low aw. The low aw is achieved through smoking and drying of the sausages in hot air/hot smoke. Such sausages or pieces are vacuum-packed in synthetic films and heated again in the package. The second heat treatment may be close to 100°C or slightly above and eliminates unwanted spoilage bacteria in the sausage and secondary contamination caused through the manipulation by vacuum packaging. Correct arrangements of all hurdles make the product shelf-stable.

Meat mixes of the precooked-cooked type (see page 149), such as liver sausage, possess due to relatively high fat contents (about 30%) relatively low aw-values. If this aw-hurdle is combined with nitrite curing salt or common salt (and/or other preservatives) and heat treatment in the range of 100°C or slightly above, such sausages can be made shelf-stable. Precondition is to fill such sausages in impermeable heat resistant casings, which sustain the mentioned heat treatment.

Important hurdles for meat preservation
High temperature: Heat treatment
Low temperature: Cooling, freezing
Water activity (aw): Drying, salt, sugar, fat
Acidity (pH): Acidification
Redox potential: Decrease oxygen (vacuum, ascorbate)
Preservatives: Sorbate, nitrite etc.
Competitive flora: Fermentation (only applicable for non-heat-treated products)

Types of heat treatment

Principally, for heat treatment (also called “thermal treatment”) of meat and meat products, it can be distinguished between products which undergo

  1. Heat treatment at temperatures below 100°C, mostly in the temperature range of 60 to 85°C, also called “pasteurization” or simply “cooking”.
  2. Heat treatment at temperatures of above 100°C, also called “sterilization”.

All such products will achieve a more or less prolonged shelf life through reduction or complete destruction of microbial populations by the heating process (thermal reduction/thermal destruction).

Both groups of products have the following in common: They are

The difference between the two groups (a) and (b) of heat treated meat products lays in their microbial status achieved, which determines how these products can be stored after thermal treatment:

Practically all meat products in hermetically sealed containers (tin cans, glass jars, retortable pouches) are sterilized products and can be stored at ambient temperature (chapter “Canning”, page 277). In the rare event of only pasteurizing meat products in cans, glass jars or retortable pouches, a clear indication on their label must inform consumers that storage under refrigeration is mandatory. It is of utmost importance that meat processors, food handlers and consumers are aware of the difference between pasteurized and sterilized products. The presence or absence of spore forming microorganisms, which depends on the intensity of the heat treatment, decides on the classification “pasteurized” or “sterilized” products.

Reactions of microorganisms to thermal treatment

Vegetative microorganisms are living bacterial cells. Each cell is surrounded by a cell wall, which does not provide strong protection against adverse conditions (high or low temperature, dry environment), with the result that such microorganisms will be killed or damaged to such an extend that no further growth is possible.

Spores are strong resistant capsules, which are formed by bacterial cells of genus Bacillus and Clostridium only. Spores contain all vital structures of the microorganisms. In dry, cold or hot environment, where the bacterial cell will be destroyed, the spore has a much stronger resistance against such adverse conditions. The spores remain dormant (without growth) as long as the unfavourable conditions prevail. Under more favourable conditions (sufficient water/humidity and temperatures in the range of 10-40°C), spores will transform again into vegetative bacterial cells capable of multiplying and fast growing to high numbers, which can spoil and/or intoxicate food.

Bio-physically the heat inactivation of microorganisms is relatively complex. The heat destruction of a population of microorganisms does not occur instantly but gradually. Mathematically, it can be expressed by the term “decimal reduction time” (also called D-value, see page 290), i.e. after a defined heat impact period (constant heat) 10% of the original population will survive, after the same impact period again 10% and so on.

Example:

Salmonella species, 100000 (105) microorganisms per gram
Treatment temperature 65°C
Decimal reduction time 6 sec

      6 sec        6 sec        6 sec       6 sec       6 sec
105 -----→ 104 -----→ 103 -----→ 102 -----→ 101 -----→100 = 30 sec

(In this example the temperature impact of 30 seconds at 65°C is needed for the elimination of the microbial load of originally 105/g).


Table 4: Examples for heat resistance/ decimal reduction times of selected microorganisms (experimental results from various sources)

Vegetative organisms

50°C

55°C

60°C

65°C

70°C

75°C

80°C

E. coli 4-7 min            

Salmonella ssp.

(average)
      0.02-
0.25 min
1.2 sec    

Salmonella typhimurium

      0.06 min      

Salmonella senftenberg*

      0.8-1 min      

Salmonella typhi

          1 sec  

Mycobacterium tuberculosis

      12-18 sec   5 sec  

Listeria monocytogenes

    5-8 min   0.1-
0.3 min
   
Staph. aureus       0.2-2 min     2 sec
Campylobacter 1.1 min            
Enterobacter           3 sec  

Lactobacillus spp.

      0.5-1 min      

Spoilage bacteria, yeasts, moulds

      0.5-3 min      

Bacterial spores

100°C

105°C

110°C

121°C

Bacillus spp. 0.1-
0.5 min
     

Bacillus cereus

5 sec     0.5 sec

Bacillus anthracis

15 min      

Bacillus stearothermophilus

    <300 min 4-5 min

Cl. botulinum type E

0.01 min <1 sec    

Cl. botulinum spp.

50 min     0.1-
0.2 min

Cl. sporogenes

      0.1-
1.5 min

* = most heat resistant Salmonella type

As can be seen in table 4, vegetative microorganisms can all be destroyed at temperatures below 100°C, basically in the temperature range of 60°C to 85°C (depending on the type of microorganisms). Only those microorganisms capable of forming spores (which all belong to the groups of Bacillus and Clostridium) can survive temperatures of 100°C and above.

The above data on heat resistance of microorganisms clearly demonstrate the importance of accurately applying heat treatment temperatures and times recommended for specific meat products. So called undercooking, which means that recommended temperature/time parameter were not reached, must be avoided. Equally important is the need for strict refrigeration for certain products after mild heat treatment (pasteurization) because of the surviving more heat resistant microorganisms. Non-compliance with these basic rules may result in economic losses through product spoilage and/or public health problems through food poisoning.

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