Cooldown of Products after Cooking Eben van Tonder 17 February 2022
I am part of a team designing a meat processing factory in Lagos, Nigeria. I considered refrigeration from the standpoint of the effect of long term storage on product quality. In 2018 I did a comprehensive survey of The Freezing and Storage of Meat. I looked at weight loss during chilling and storage of meat. In 2019 I looked at Weight Loss During Chilling and Freezing of Meat. This time, the issue at hand is rapid cooling after cooking. Home Production of Quality Meats and Sausages by John Novak provided great introductory comments and he was kind enough to mail me the relevant chapter of his book.
He summarises the important point as follows: “Cooling down of cooked products is basically done to cross the danger zone (140o – 40oF; 60oC – 4oC) relatively fast. Cooked sausage at 160oF (72oC) so is still basically safe until the temperature drops down to about 60oC. Therefore, the restaurants are required to hold hot food at 140o F (60oC) or higher.” (Novak)
Regulatory Standard and Application
“The following standards come from the Food Safety and Inspection Service (FSIS), United States Department of Agriculture (USDA):
Compliance Guidelines for Cooling Heat-Treated Meat and Poultry Products (Stabilization)
It is very important that cooling be continuous through the given time/temperature control points. Excessive dwell time in the range of 130°F (55oC) to 80°F (27oC) is especially hazardous, as this is the range of most rapid growth for the clostridia. Therefore cooling between these temperature control points should be as rapid as possible.
1. During cooling, the product’s maximum internal temperature should not remain between 130°F (55oC) and 80°F (27oC) for more than 1.5 hours nor between 80°F (27oC) and 40°F (4oC) for more than 5 hours. This cooling rate can be applied universally to cooked products (e.g., partially cooked or fully cooked, intact or non-intact, meat or poultry) and is preferable to (2) below.
2. Over the past several years, FSIS has allowed products to be cooled according to the following procedures, which are based upon older, less precise data: chilling should begin within 90 minutes after the cooking cycle is completed. All products should be chilled from 120°F (48°C) to 55°F (12.7°C) in no more than 6 hours. Chilling should then continue until the product reaches 40°F (4.4°C); the product should not be shipped until it reaches 40°F (4.4°C). This second cooling guideline is taken from the former (“Requirements for the production of cooked beef, roast beef, and cooked corned beef”, 9 CFR 318.17(h)(10). It yields a significantly smaller margin of safety than the first cooling guideline above, especially if the product cooled is a non-intact product.
If an establishment uses this older cooling guideline, it should ensure that cooling is as rapid as possible, especially between 120°F (48°C) and 80°F (27oC), and monitor the cooling closely to prevent deviation. If product remains between 120°F (48°C) and 80°F (27oC) for more than one hour, compliance with the performance standard is less certain.
3. The following process may be used for the slow cooling of ready-to-eat meat and poultry cured with nitrite. Products cured with a minimum of 100 ppm ingoing sodium nitrite may be cooled so that the maximum internal temperature is reduced from 130 to 80° F in 5 hours and from 80 to 45° F in 10 hours (15 hours total cooling time).
This cooling process provides a narrow margin of safety. If a cooling deviation occurs, an establishment should assume that their process has exceeded the performance standard for controlling the growth of Clostridium perfringens and take corrective action. The presence of the nitrite, however, should ensure compliance with the performance standard for Clostridium botulinum.
From FSIS Directive 7117.0
1. Heat-resistant food-poisoning bacteria can grow from 38°F 3oC up to approximately 125°F (49oF); however their range of rapid growth is from approximately 80°F to 125° F. Thus, cooling product quickly through the rapid growth range is more important than cooling through the slow growth range.
2. The rate of heat transfer (cooling rate) from the product’s centre to its surface is directly proportional to the difference in temperature between those two points. Thus, as the product temperature approaches the coolant temperature, the cooling rate diminishes.
3. Traditional cured products, containing high amounts of salt and nitrite, together with low moisture content are more resistant to bacterial growth than similar newer products; some are even shelf-stable. Thus, rapid cooling of these traditional products is not always necessary. However, manufacturers are making fewer products of this type today. Instead, to meet present consumer tastes, most of their cured products contain less salt and more moisture. These changes minimize the inhibitory effect of added nitrite and increase the need to rapidly cool these products.” (Novak)
The best way to cool sausages down is with water. Showering with cold water is a technique universally used. “Water removes heat much faster than air and hot products will drop their temperature fast. If a product was smoked such showering also cleans the surface of any remaining smoke particles and prevents shrivelling.” (Novak)
Let’s delve deeper into the science behind cooling. To do this we use Watts per meter-Kelvin (W/mK) or what is known as the ‘k Value’. It is the measure used to compare thermal conductivity. The k value, or Thermal Conductivity, is the rate of heat transfer in a homogeneous material. A k value of 1 means that 1m cube of a material will transfer heat at a rate of 1 watt for every degree of temperature difference between opposite faces. This will be given as 1 W/mK. The lower this value is, the less heat the material will transfer.
Chris and Steve James point out that “the thermal conductivity of processed meat products (including cured sausages and hams) range between 0.272 Wm-1K-1 at 22°C to 0.482 Wm-1K-1 at 80°C (Marcotte et al., 2008), whereas the thermal conductivity of water is 1 Wm-1K-1. Thus, the thermal conductivity of water is more than double that of most processed meats.” Water sprayed onto the surface of a food product will act as a high conductivity path.
Intermitted Spray Cooling
Chris and Steve James made their comments on a blog site related to intermitted spray cooling in response to someone claiming that water on the surface of a product acts as an insulator which is obviously incorrect not the case, as one can see when comparing the K value of the sausages and that of water. They stated that “the reason why intermittent spray cooling can have benefits over continuous spray cooling is that a break in spraying allows the water on the surface of the hot product to evaporate, thus enhancing the cooling effect.”
Panão (2008) investigated the physics involved in the heat transfer process using intermitted spray cooling. Their work has shown that small duty cycles promote heat removal by phase-change. They found that “as the duty cycle evolves toward the continuous spray condition, the cooling system’s thermal response improves, but phase-change is mitigated, affecting the system’s performance. Intermittent spray cooling is also compared with continuous spray cooling experiments and liquid savings has been estimated by 10–90% for the same energetic efficiencies reported in the literature.” They further recommend using shorter impingement distances and low injection pressures. (Panão, 2008)
Craig Habbick who has experience with these systems recommends the following procedure. Brine solution (water and salt) cooled to -5oC and sprayed intermittently altering 2 minutes to 1 minute off with circulation on during the spray off times. The entire cycle takes 15 minutes and reduces core temperature from 72oC to 2oC.
The concentration of salt was high. They used a Baume meter regulator to test the salt. Weight loss was so low that they had to remove water from the recipe due to seepage in the packs after packaging. Nett loss during cool down was around 2%. They used an Alkar brine chiller.
For details on the Alkar Brine Chiller, visit https://alkar.com/liquid-brine-chiller/
The important graphs from the Alkar website are:
Novac remarks that there is no need to grab a water hose the moment the sausage was cooked in a smokehouse to 155° F (69°C) as this temperature lies outside the danger zone (40 – 140°F, 4 – 60°C). “U.S. regulations permit restaurants to hold cooked food at 145°F (63°C) or higher temperatures. However, once when the temperature of the product drops to 140°F (60°C), it should be cooled fast. The surface of a product such as a head cheese or smoked sausage both will benefit from a brief hot shower or immersing them in hot water. This will remove any possible grease from the outside and the product will look better. Then it will be showered with cold water. Some pork products may be cooked to > 137° F (58°C) just to eliminate the danger of contracting Trichinae. Such products should be cold showered immediately as they are already lying within the danger zone.” (3. Traditional cured products, containing high amounts of salt and nitrite, together with low moisture content are more resistant to bacterial growth than similar newer products; some are even shelf-stable. Thus, rapid cooling of these traditional products is not always necessary. However, manufacturers are making fewer products of this type today. Instead, to meet present consumer tastes, most of their cured products contain less salt and more moisture. These changes minimize the inhibitory effect of added nitrite and increase the need to rapidly cool these products.” (Novak)
We initially planned to have a blast chiller, but after the recommendation from Craig Habbick, the intermitted spray cooling system is so effective that there is no need for an air chiller (blast chiller).
The Alkar product Brochure
Jensen, W. K., Devine, C., Dikeman, M. (Editors), (2014), Encyclopedia of Meat Sciences, Second Edition. ISBN: 9780123847317. Elsevier.
Marcotte, M., Taherian, A. R. & Karimi, Y. (2008) Thermophysical properties of processed meat and poultry products. Journal of Food Engineering. Vol. 88:3, pp315-322
Miguel R.O. Panão, António L.N. Moreira, Intermittent spray cooling: A new technology for controlling surface temperature, International Journal of Heat and Fluid Flow, Volume 30, Issue 1, 2009, Pages 117-130, ISSN 0142-727X, https://doi.org/10.1016/j.ijheatfluidflow.2008.10.005.
Novak, J. Home Production of Quality Meats and Sausages