Part 4
4. The Frozen Food Chain
The frozen food chain is a system of production units, storage facilities, distribution / transport vehicles, retail outlets and home freezers. The main characteristic of all system elements is that the product temperature is controlled in a way that under normal and standard conditions –18°C is not exceed. The rigorous temperature control aims on optimal quality retention of the frozen products. Weak points in the system are production units, distribution / transportation, retail outlets and home freezers. The frozen food chain is on of the best examples of temperature and time controlled systems of material / product distribution. Over the many years of its existence the frozen food chain in the industrialised countries was always an object of food inspection services and also consumer organisations because of breakdowns of the temperature control or unacceptable long storage times. With the development of sophisticated time-temperature-control equipment, barcode systems and improved refrigeration equipment the problems are getting fewer and fewer and deep frozen products are not any more in the focus of inspection services and consumer advocates. Despite the many improvements the frozen food chain is a very fragile system requires permanent attention. For producers of frozen food especially a detailed understanding of the large variety of conditions to which frozen products are exposed on their way from the production facility to the plate of the consumer is vital. Only the precise knowledge of the temperature conditions in the various elements of the chain allows e.g. to calculate the optimal storage life.
The major elements of the frozen food chain are given in Tab. 4.1 and Fig.4.1.
Unit Number / Element / Temperature /°C / Storage Time1 / Production / -18 to -24 / -
2 / Production storage / -24 to -33 / 1 Month
3 / Long Distance Distribution / -18 / 1 Day to 1 Week
4 / Central Storage / -28 to -33 / 1 Months to 1 Year or more
5 / Long Distance Distribution / -18 / 1 Day to 1 Week
6 / Intermediate Storage / -24 to -33 / 1 Week to 3 Months
7 / Short Distance Distribution / -18 / 1 Day to 3 Days
8 / Distribution Storage / -24 to -28 / 1 Week to 1 Month
9 / Short Distance Distribution / -18 / 5 Hours to 1 Day
10 / Retail Outlet / -18 to -24 / 1 Day to 2 Months
11 / Private Transport / -10 / 1 Hours to 6 Hours
12 / Home Freezer / -18 to -21 / 1 Day to 6 Months
Tab. 4.1 Elements of the Frozen Food Chain
Fig. 4.1 The Frozen Food Chain
The temperature histories of sales units which at the very end arrive in retail storage are very different. If extreme cases are considered it could happen that out of the same production unit sales units have passage times of e.g. 3 months, 2 weeks, 4 days, 5 hours combined with the warmest or coldest passage conditions or have passage times of 1 year (or more), 7 months, 2 weeks, 4 days again combined with the warmest or coldest passage conditions. Besides these cases numerous other combinations are possible.
Special Passage EventsUnit Number* / Warmest Passage/°C / Coldest Passage/°C / Shortest Passage Time / Longest Passage Time
1 / -18 / -24 / -
2 / -24 / -33 / 1 Month / 1 Month
3 / -18 / -18 / 1 Day / 1 Week
4 / -28 / -33 / 1 Months / 1 Year or more
5 / -18 / -18 / 1 Day / 1 Week
6 / -24 / -33 / 1 Week / 3 Months
7 / -18 / -18 / 1 Day / 3 Days
8 / -24 / -28 / 1 Week / 1 Month
9 / -18 / -18 / 5 Hours / 1 Day
10 / -18 / -24 / 1 Day / 2 Months
2 Months, 2 Weeks,
4 Days,5 Hours / 1 Year (or more),
7 Months,
2 Weeks, 4 Days
*For explanations see Tab.4.1 “Elements of the Frozen Food Chain”
Tab. 4.2 Possible passage histories of Deep Frozen Foods in Frozen Food Chain
A detailed understanding of passage conditions is not only of relevance because of quality control in the chain but also because of legal requirements with regard to date marking and giving information on e.g.:
v the end of the High Quality Life (HQL),
v the end of the Practical Storage Life (PSL),
v the date of expiry or
v the so-called „best-before“ date or
v the „consume-before“ date.
The precise conditions in the individual elements of the frozen food chain can only be studied and analysed with the help of electronic time-temperature recorders. Those studies which result in time and temperature distributions for individual products and conditions are rather expensive. Most producers of deep frozen products use therefore a short cut method to determine e.g. the so-called „best-before” date. The assumptions are:
v the quality decay is of 0-order (linear over time),
v all sales units are subjected to the worst case scenario.
A major short coming of the methods used by most producers is that the sensory analysis tests are carried out over rather short periods of time as so called accelerated storage-life-tests, e.g. over 1 month. Since in most cases the quality decay is more pronounced at the beginning than after an initiation period the extrapolated storage times are shorter than the real/possible storage times.
In case no detailed information on the individual quality decay patterns of a product is available the storage times presented in Table 1. 2 Recommended Storage Times for various groups of frozen product < can be used as a first approximation for calculating quality losses in the Frozen Food Chain.
An example of a detailed procedure to calculate the total quality decay in a straight way is presented in Fig. 4.2 and in Tab. 4.3. The method is also called Time-Temperature-Tolerance (TTT) calculation. For the calculation of the total quality loss each quality decay differential in an element of the chain is subtracted from the relative product quality at the time the product entered the particular element.
The example is based on the storage properties/data of meet, as tabled in the excerpt of Table 1.2 .
°C / Possible Storage Time
Months
Meat Beef / -12 / 5 ./. 8
-15 / 6 ./. 9
-18 / 9 ./. 12
-24 / ~ 18
Excerpt Table 1. 2 Recommended Storage Times for various groups of frozen product
AS outlined in part 1 will the possible storage time end when the end of the Practical Storage Life (PSL) is reached (in total 50% quality loss). The quality loss for each storage temperature can be characterized by a straight line connecting the starting point (High Quality at zero Storage Time) with the relevant end point (end of Practical Storage Life at the appropriate Storage Time) as demonstrated in Table 4.3 for meat (beef)
Characteristic Storage Data / Quality ( Q )% / Storage Time (t) Month / Quality Slope
m = ΔQ/Δ t
Temperature °C
T / Possible max. Storage Time Month / Starting point / End point / Starting point / End point
-12 / 8 / 100 / 50 / 0 / 8 / - 6.25
-15 / 9 / 100 / 50 / 0 / 9 / - 5,55
-18 / 12 / 100 / 50 / 0 / 12 / - 4,16
-24 / 18 / 100 / 50 / 0 / 18 / - 2,77
General Equation for the slope: m = -0,297 T -9,806
Table 4.3 Quality decay characteristics for meat (beef)
Based simple mathematical operations a diagram can be developed with Time (t) as X-axis (Abscissa) and Quality (Q) as Y-axis (Ordinate) and Figure 4.2.
Figure 4.2 Quality decay diagram for meat (beef)
From both representations, for each temperature a characteristic quality decay quotient ΔQ/Δ t can be derived. Mathematically the various quotients are the slopes of the straight lines connecting the starting point of the storage period with the relevant end points of the storage period for the individual storage temperatures.
Figure 4.3 Correlation of Storage Temperatures and Quality Decay Quotient m
In order to calculate the quality decay for any storage temperature between -10°C and -30°C a mathematical function for m can be derived by plotting the m-values for the experimentally analysed temperatures versus the corresponding temperatures. m can now be calculated for any temperature from the equation for the regression line
m = a T + b
The easiest way to obtain the regression line is by best eye-fit
For an assumed passage history the quality at the point of consumption can then be approximated by calculating the quality losses in the individual elements of the Frozen Food Chain, see Table 4.4
UnitNumber / Element / Assumed
Temp.
°C / Assumed
Storage
Time
Month / Quality at
receiving
Qi
% / Quality at
delivering
Qf
% / Quality Loss
ΔQ
%
1 / Production / -18 / - / -- / -- / --
2 / Production storage / -24 / 1 / 100 / 97 / 3
3 / Long Distance Distribution / -18 / 0,25 / 97 / 96 / 1
4 / Central Storage / -24 / 4,5 / 96 / 83 / 13
5 / Long Distance Distribution / -18 / 0,03 / 83 / 83 / --
6 / Intermediate Storage / -24 / 0,5 / 83 / 81 / 2
7 / Short Distance Distribution / -18 / 0,1 / 81 / 81 / --
8 / Distribution Storage / -12 / 0,25 / 81 / 79 / 2
9 / Short Distance Distribution / -18 / 0,03 / 79 / 79 / --
10 / Retail Outlet / -18 / 1,5 / 79 / 72 / 7
11 / Private Transport / -10 / 0,003 / 72 / 72 / --
12 / Home Freezer / -18 / 1 / 72 / 68 / 4
Total quality loss : ΔQ = 32 %
Table 4.4
A graphical method of
Figure 4.4 a Graphical evaluation of the Quality Loss in the Frozen Food Chain
Step 1: Determination of individual Time-Temperature related losses
Figure 4.4 b Graphical evaluation of the Quality Loss in the Frozen Food Chain
Step 2: Aggregation of the Quality losses
It has to be mentioned, that the described approach is in no way precise. It can be seen however that short time exposures to relatively high temperature (~ -12°C) does not really endanger the product quality. The problem with such temperature abuses is related to the fact that usually a rise in the product temperature to ~ -12°C is caused by much higher environmental temperatures which may cause in the worst case a melting of the outer product layers. It is therefore important that especially deep frozen retail products are well protected against short time temperature abuses by insulating packaging materials.