SOP: Determination of Shelf-life of Packaged Foods by the Half-Value Period Method
GIFSQ — Global Institute for Food Safety & Quality
Principle
The shelf-life of a packaged food depends on (i) the reactivity of the food with deteriorating influences and (ii) the capacity of the package to protect the food from these influences. Next to microorganisms, oxygen is the most destructive agent; next in importance is the sensitivity of the food to storage atmosphere and changes in water content. Where retention of low moisture content limits the shelf-life, the tests are: the normal moisture content of the product, the moisture content at which the product becomes unacceptable, and the RH and temperature of the surrounding atmosphere at which the equilibrium moisture content is maintained in the product. The half-value period (HVP) — the time required for the moisture content of the packed product to move half-way between the initial value and the value obtained at equilibrium under the storage conditions — is a constant for a given packaging material, and is used to compare package efficiency or to estimate shelf-life in a relatively short storage period (28 days).
Method A — Direct Weighing Method
This method is useful as a control method when a satisfactory package has been found; the initial moisture content of the product need not be determined.
Prepare 5 packets of the product using the packaging material considered most suitable; note the weight of each packet and find the mean value (W₀)
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Expose the packets to the desired storage conditions (e.g., 25°C and 65% RH, or 38°C and 90% RH); note the gain or loss in weight of each packet at intervals of storage; find the mean value of the weights of the 5 packets at each interval (e.g., W₅, W₁₀, W₁₅, W₂₀)
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Finally, open each packet, expose the contents to the storage atmosphere, allow to equilibrate, and note the weight; find the mean value (Wₑ)
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From Wₑ subtract W₀, W₅, W₁₀, W₁₅, … and find the log of the resulting values (Wₑ − W); plot log(Wₑ − W) against time; find the slope (s) of the resulting line, which gives log(Wₑ − W) per day
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Calculate the HVP from: HVP = log 2 / s = 0.3010 / s
Method B — Product Equivalent Method
In this method the moisture gain or loss of the package in addition to that of the contents is considered: the weight of the package is converted to the equivalent weight of the food material packed, and this is added to the weight of the material packed.
Determine the initial moisture content of the product (M₀); expose the material to the desired RH and temperature (e.g., 75% RH at 25°C) and find the equilibrium moisture content (Mₑ); find the average weight of the empty package (Mₚ)
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Pack the material in the package; expose 5 such sealed packs to the RH and temperature used for determining the equilibrium moisture content; note the loss or gain in weight of each package at definite intervals and find the mean values
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Calculate the average initial effective weight (W₀) from the expression: W₀ = (initial weight of packed product) − (weight of empty package) + (product equivalent weight of package). To find the product equivalent weight of package, note the weight of package as equivalent of the actual weight of the product packed. Note: the printed layout of this expression is garbled; it is built from the book’s own variable definitions.
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Calculate the moisture content (M) of the sample when the weight loss or gain is x g from the expression: M = ((M₀·W₀) − 100x) / (W₀ − x). Note: the printed layout is garbled; it is built from the book’s own variable definitions.
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Find the value of M at the different storage intervals when the loss or gain in weight was noted; subtract Mₑ and find the log of the resulting values; plot log(M − Mₑ) against days, find the slope (s), and calculate: HVP = 0.3010 / s
Method C — Breakdown Method
Used when the product is packed in large containers (e.g., fruit juice powders, instant beverage mixes, dried fruits and vegetables) that are difficult to weigh accurately, or so well packed that the loss or gain in weight is negligible. A large number of containers are stored, a container is withdrawn at intervals, and the moisture content of the product is determined.
Store the containers at the desired temperature and RH (e.g., 25°C and 75% RH); draw samples from a few containers, determine the initial moisture content as percentage on dry basis, and find the mean (M₀)
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Allow the remaining samples to attain equilibrium under the storage conditions and determine the equilibrium moisture content (Mₑ) on dry basis
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Divide the remaining packed samples into 3 or 4 lots; draw one lot at intervals of 7 days, determine the moisture content on dry basis of the sample in each container, and find the mean value (M)
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Subtract Mₑ or M₀ from Mₑ (the printed text is garbled here) and find the log of the values; plot log(Mₑ − M) against days, find the slope (s), and calculate: HVP = 0.3010 / s
Estimation of shelf-life from the HVP
Take (i) the temperature (T°C) used in determining the HVP; (ii) the average temperature (t°C) to which the product will be exposed during transit, storage and use; and (iii) the critical moisture content (M_c), initial moisture content (M₀) and equilibrium moisture content (Mₑ) of the product. The HVP at t°C is calculated using the relation:
- HVP at t°C = HVP at T°C × (vapour pressure of water at T°C / vapour pressure of water at t°C)^k
where k is 1.0 for polyethylene, 1.6 for moisture-vapour-proof cellulose, 0.5 for polyvinyl chloride and 1.9 for polyvinylidene chloride. Note: the printed layout of this relation is garbled; it is built from the book’s own variables and the listed k values. The pressure of saturated water vapour at each temperature is taken from the book’s Table 16-14.
From the value of HVP at t°C, construct the graph log₁₀(Mₑ − M) versus time; the curve is a straight line, so two points are required: one point is 0 on the time scale when the moisture content is M₀; the second point on the time scale is the number of days corresponding to the HVP, with the moisture corresponding to the HVP equal to ½(M₀ + Mₑ).
From the line so constructed, determine the value of log₁₀(Mₑ − M) corresponding to the critical moisture content (M_c); the time corresponding to this value is the shelf-life at t°C.
Alternative confirmation — Weight gain or loss method
Pack the product in the selected packaging materials; note the weight of each package accurately; place the packages in the test atmosphere and re-weigh at suitable intervals over a period preferably long enough to exceed the critical moisture content of the product (the moisture content at which the product becomes unsaleable); plot the moisture gain against days, and express the results as the number of days required for the product to become unsaleable under the conditions of test.
Alternative confirmation — Performance test method
For products that deteriorate with changes in moisture content (e.g., dehydrated fruits and vegetables, fruit juice powders), follow the deterioration by a performance test such as changes in texture, solubility or sensory quality; such a test requires more samples, since the package is destroyed at each withdrawal.
Notes
- The package adopted should not permit the product to reach the danger point, so that there is a safe margin of error.
- For practical purposes, the HVP is a constant dependent on the packaging material used.
References
Ranganna, S., Handbook of Analysis and Quality Control for Fruit and Vegetable Products, 2nd edition.
Adam, J.R. & A.R. Merz, Ind. Eng. Chem., 21, 305 (1929) — cited in the book.
Landrock, L.B., Food Res., 22, 604 (1957) — cited in the book.
Karel, M., Y. Aikawa & B.E. Proctor, Mod. Packag., 29(1), 153 (1955) — cited in the book.
Henderson, S.M., Agric. Eng. (St. Joseph Mich.), 33(1), 29 (1952) — cited in the book.
Wink, W.A., Ind. Eng. Chem. Anal. Edn., 18, 251 (1946) — cited in the book.
Landrock, A.H. & B.E. Proctor, Mod. Packag., 24, (6), 123 (1951) — cited in the book.
Landrock, A.A. & B.E. Proctor, Food Technol. (Chicago), 5, 332 (1951) — cited in the book.
Siddappa, G.S. & A.M. Nanjundaswamy, Food Technol. (Chicago), 14, 533 (1960) — cited in the book.
Paine, F.A., Fundamentals of Packaging, Blackie & Son Ltd., London, p. 275 (1962) — cited in the book.
Hall, E.E., Food, 19, 306 (1950) — cited in the book.
Selby, J.W., Modern Food Packaging Film Technology, The British Food Manufacturing Industries Research Association, Leatherhead, UK, Scientific and Technical Survey No. 39, p. 57 (1961) — cited in the book.
Oswin, C.R., J. Soc. Chem. Ind., 64, 67 and 224 (1945) — cited in the book.