The Freezing and Storage of Meat

The Freezing and Storage of Meat
By: Eben van Tonder
17 December 2018

frozen meat

Introduction

Freezer stock and shelf life are two issues often seen as peripheral in a meat factory, especially in smaller companies that lack the manpower to adequately manage and investigate it. Shelflife of various products are sometimes inherited from predecessors in the factory or is set at what the opposition in the market has it at.  Besides this, clients often prescribe what they want shelf life to be which should be referred back to the NPD manager but often the adjustment is made without scientific rigor.  The NPD process was in such a case actually flawed right from the start.  There may not be an NPD Manager or NPD process in the company, especially in small and mid-size organisations.  Freezer management is likewise many times seen as intuitive – something to be taken lightly since freezing of meat is something we have all been exposed to from childhood.  What is there about freezing that is hard to understand? Sometimes the advice given by consultants to start up companies are just wrong. Sometimes shelf life is in error assigned to finished goods only and goods, stored for later use escape detailed shelflife considerations. All these things make this article very relevant.

Shelf life and freezer stock are in reality closely connected and its management intertwined. The goal of the study is to glean practical points of application from the most recent studies on the subject, incorporating old and time-tested techniques.  The work of freezing and storing of meat products are considered to bring about the desired result of good quality raw material and final products by understanding what happens to meat in a freezer, assigning a correct shelf life to each product class and managing the stock accordingly. In considering it, as always, work is viewed as a metaphysical concept and involves the organization of labour and the design of processes and procedures that will bring about the desired end goal.

Determining Product Shelf Life in Frozen Conditions

The first and most critical question is how long can meat be stored frozen.  What is the shelf life of frozen meat and what factors impact it?  The shelf life of meat can be defined as the time period within which the food is safe to consume and/or has an acceptable quality to consumers.  Frozen storage and distribution of meat now takes front and center stage as one of the key factors in shelf life management of frozen products.

The old saying that freezing arrest decay is not true.  Shelf life of frozen food is not in years, but in months. At best, one year for a limited class of products (which does not include pork). Just like any other food, frozen meat deteriorates during storage.  The activity of most bacteria are arrested in frozen meat, but decay happens through other mechanisms.  (Fu and Labuza)  The first important consideration for managing a freezer is to consider all the products that will be packed in it, environmental factors likely to prevail such as temperature and temperature fluctuations and based on these, to develop an understanding of the shelf life each of the products.

In an excellent chapter on shelf life determination for frozen foods and its mathematical modeling using kinetic modeling techniques, Fu and Labuza review the different models.  In so doing, they touch on many of the key considerations.  Here I systematically and in an overview fashion work through their work. Whether one actually does the mathematical modeling or not will, in the end, be determined by the economic need for such work, but working through the chapter fixes the different aspects that are brought to bear upon the matter firmly in one’s mind with the aid of the rigor of mathematical modelling.

The activity of microorganisms in a freezer is not something to be ignored. Freezer coils have many times been the source of listeria contamination, for example, but it is definitely less of a problem than in the rest of the factory and a good quarterly or annual freezer deep clean should suffice.  Since bacteria is not a factor in the deterioration of frozen foods, it is not primarily a health issue, but rather a quality issue.  If freezer coils contaminated the freezer with microorganisms, this will become a problem during thawing.  Generally, food that has been stored frozen for a long time is safe to consume but develops objectionable characteristics through other mechanisms in its frozen state.

Fu and Labuza lists the main ways that freezer meat deteriorates.  Enzymes are the first big culprit, “which can cause accelerated deterioration reactions in meat and poultry (enzymes released from disrupted membranes during precooking).  In processed meat, cell damage or protein and starch interactions during freezing cause drip and mushiness upon thawing. Discoloration could occur by nonenzymatic browning, bleaching, and freezer burn.  For any specific frozen product, which mode determines its shelf life, depends on the product characteristics (raw materials, ingredients, formulation), pre-freezing treatment, freezing process, packaging film and processes, and of course storage conditions. All of the quality deterioration and potential hazards are usually exaggerated or complicated by a fluctuating time-temperature environment (e.g. freeze/thaw cycle) during storage.”

The first point is, therefore, to understand that there is a problem which is far bigger than just the management of micro.  The response to the dilemma of frozen food deterioration is to extend shelf life through ingredient selection, process modification and change of package or storage conditions.

Shelf life deterioration in frozen meats, poultry, and seafood takes place through rancidity, toughening (protein denaturation), discoloration,  desiccation (freezer burn).  In the reference below, I uploaded the chapter in its entirety which I downloaded from Researchgate. I set out to identify some of the process modifications needed to effectively optimise the shelf life of meat stored in a freezer.

Kinetic Modelling

Lets first understand what they are doing before we look at the content if their work. They rely on kinetic modelling which has proved to be particularly effective in food systems. Van Boekel and Tijskens write about kinetic modelling that “changes in foods as a result of processing and storage lead to a change in quality (usually a quality loss)” which is exactly the issue in frozen meat storage. “The processes involved are mainly (bio)chemical and physical reactions. Such changes proceed at a certain rate and with certain kinetics. Kinetic modelling enables us to describe these changes and their rates quantitatively.  With kinetic modelling we also have a powerful tool that can help to unravel basic reaction mechanisms. The understanding of the basic mechanisms is vital for quality modelling and quality control.”

“To understand the progress of reactions, knowledge of thermodynamics and kinetics (the study of reaction rates) is required. Thermodynamics is helpful in describing and understanding in which direction a reaction will proceed and the energy and entropy changes that are involved. Thermodynamics thus explains the driving force for a reaction. However, thermodynamics cannot tell anything about the speed at which a reaction proceeds. This is the domain of kinetics. The rate with which a reaction proceeds is the resultant of the driving force and the resistance against change. There is thus an intimate link between thermodynamics and kinetics.” (Van Boekel and Tijskens). Understanding why they rely on kinetic modelling, we now turn our attention to the detailed models.

Modelling of Quality Deterioration

Frozen food starts to degrade as soon as it is produced.  Freezing does not arrest all decay.  This includes final product and intermediate products like primals, packed for future processing.  “The rate and the degree of degradation depend on both the composition and the environmental conditions during storage and distribution. In general, the loss of food quality or shelf life is evaluated by measuring a characteristic quality index, “A”. The change of quality index A with time (dA/dt) can usually be represented by the following kinetic equation:

– dA/dt = k An

where k is called a rate constant depending on temperature, product and packaging characteristics; n is a power factor called reaction order which defines whether the rate of change is dependent on the amount of A present. If environmental factors are held constant, n also determines the shape of deterioration curve.”

Quality Deterioration Curves.png

Besides the nature of the quality deterioration curves, pay close attention to the relentless slope downwards over time.  As we will see, this time is not very long.

The rate constant k, is determined by the temperature, product and packaging characteristics.  If meat is packed that have been cured, partially or fully heat treated, with various fat contents such as trim from fatty cutter bellies, packed and stored for future use in making products like salami or sausages, compared with lean trim, stored for future use or trading it out; if these are packed in 20kg bags and sealed vs packed in cardboard boxes, lined with a think plastic liner vs vacuum sealed in a vacuum bag, all these different factors will have a material impact on the rate constant, k.

The alternative version of the basic equation is,

f(A) = k t

where f(A) is the quality function, k and t are the same as above.  (Fu and Labuza)

“The form of f(A) depends on the value of n. When n is equal to zero it is called zero-order reaction kinetics, which implies that the rate of loss of quality is constant under constant environmental conditions (curve (a) in the figure above). If n is equal to one it is called first-order reaction kinetics, which results in an exponential decrease in rate of loss as quality decreases (curve (b) in the figure above)”  (Fu and Labuza)

Meat is a complex environment.  Complex chemical reactions continue to take place in frozen meat.  As in all modeling, we have to make simplifications.  “The reaction kinetics can be simplified into either pseudo-zero order or pseudo-first order kinetics. In the case of complex reaction kinetics with respect to reactants, an intermediate or a final product (e.g. peroxides or hexanal in lipid oxidation) could be used as a quality index.”  (Fu and Labuza)  This was made possible recently when I had the opportunity to physically examine different pork meats, with different characteristics, packed in a variety of different ways and stored under diverse environmental conditions.

“There are few cases where neither zero nor first order kinetics applies. Curve (c) in the figure above shows the degradation curve for a 2nd order reaction (with single reactant), which also shows a straight on a semi-log paper. A fractional order should be used to describe the curve (d) in the figure above.  (Fu and Labuza)

The experience of many butchers is that there is no deterioration in meat quality if meat is frozen.  Curve (e) may apply.  It indicates the presence of an induction period or lag time before the quality deterioration begins (e.g. browning pigment formation in the Maillard reaction or a microbial growth lag phase). The length of the lag depends on many factors, but temperature is a predominant factor.  (Fu and Labuza)  The lag phase is induced by temperature which ultimately is overcome by other factors over time and the degradation continues relentlessly.  “Given this, modeling of both the induction or lag period and deterioration phase is necessary for accurate prediction of quality loss or shelf life remaining.” (Fu and Labuza)

A non-kinetic approach

A non-kinetic approach, e.g. a statistical data fitting technique can also be used to describe the deterioration curves. “Varsanyi and Somogyi (1983) found that the change in quality characteristics as a function of time could be approximately described with linear, quadratic and hyperbolic functions and that storage temperature and packing conditions affected the shape of the deterioration curves.”  Fu and Labuza find this method difficult to use for predicting shelf life under variable storage conditions, except the linear curve.  The importance of standardizing packing conditions for different products and to select appropriate storage temperatures becomes clear.

Temperature Dependence

The rate of deterioration is largely temperature dependent.  “The Arrhenius relationship is often used to describe the temperature dependence of deterioration rate.  By studying a deterioration process and measuring the rate of loss at two or three temperatures (higher than storage temperature), one could then extrapolate on an Arrhenius plot with a straight line to predict the deterioration rate at the desired storage temperature.”  (Fu and Labuza)

An exponential relationship exists between shelf life and storage temperatures.  These can be expressed as follow:

q = exp(-bT+c)

or

ln q = -bT+c

where q is shelf life at temperature T in °C, b is the slope of the semilog plot of q vs T and c is the intercept or reference temperature.

These are represented as follows:

shelf life plot.png

Q10

An approach that is similar to the Arrhenius equation, is the Q10 approach.  It is also often used for estimation of the temperature acceleration of shelf life, which is defined as :

Q10 = rate @ T1+10 °C / rate @ T1
Q10 = shelf life @T1 / shelf life @T1+10 °C
Q10 = (q10)1.8

where T1 is temperature in °C. If the temperature unit is in °F, then the term q10 is used, which in fact is more often used than Q10 in the frozen food literature.  The magnitude of Q10 depends on the food system, the temperature, and the absolute range. Q10 values from 2 up to 20 have been found for frozen foods (Labuza, 1982) Labuza and Schmidl, 1985.

Of interest is data from data from July (1989) and Labuza (1982).

estimate of the Q10.png

The chart, as quoted by Fu and Labuza.

HQL represents High-Quality Life, a term suggested by the International Institute of Refrigeration (IIR, 1986).  It is defined as “the storage period through which the initial quality was maintained from the time of freezing up to the point where 70% of the trained test panel members are capable of detecting a noticeable difference between the frozen food stored at different temperatures and the corresponding controls stored at – 40 deg C in a triangular sensory test;  therefore this parameter is also known as just Noticeable Difference  (JND)”  (Evans, 2008)

Note the pork at -20 deg C and HQL at 400 days and at – 10, at 50 days.  This is an astronomical difference brought about by 10 degrees! It forever dispels the notion that freezing is freezing! My estimation is that most factory refrigeration in South Africa runs on average neatly between these two temperatures. Then one still has to consider the actual temperatures for stock stored at the back of the freezers vs stock in the front and closest to the door.

Also, of interest to the meat processing plant is the low HQL days of pork sausage and ground burgers.  These studies can be used when setting preliminary shelf lifetimes while more rigorous work is done for the different products either sold frozen or stored in freezer rooms.

Fluctuating temperatures

Fluctuating temperatures over time can be accommodated in various ways in predictive models.  In discussing different models, Fu and Labuza introduce the following relevant concepts to our discussion.

They state that “a widely fluctuating temperatures may cause freezer burn or in-package desiccation (July 1984).  Ledward and MacFarlane (1971) looked at freezer burn and showed that both lipid oxidation and metmyoglobin formation depends on the treatment of meat prior to and during frozen storage.  Meat subjected to freeze cycles were the least stable and meat frozen quickly was most stable.  Therefore, during prolonged aerobic frozen storage delay in freezing should be avoided as well as thawing and refreezing on the surface.  (James and James, 2000)

Certain chemical reactions, enzymatic as well as nonenzymatic, could even proceed more rapidly at temperatures below freezing. This is called a negative effect of temperature (Singh and Wang, 1977), which could be caused by one or more of the following factors:

(1) a freeze concentration effect;   This effect results from the concentration of solutes in the unfrozen water phase.  A consequence of this is that certain chemical reactions exhibit a rate increase in foods when frozen.  Frelka, et al. used colour as the only quality indicator and their work indicate that the oxidation of myoglobin follows traditional Arrhenius first-order kinetics at temperatures but only at temperatures above freezing. Below the freezing point, an increase in rate was observed with a maximum rate around −15°C. (Frelka, et al, 2015)  In general, freeze concentration causes great stress on protein stability.  “It has even been shown to cause protein unfolding at the ice: aqueous interface and the aggregation of unfolded proteins.”  (Avacta)

Freeze concentration may lead to precipitation.  Zachariah and Satterlee (1973) studied the relationship between frozen storage temperatures and oxidation rate for bovine, ovine and porcine myoglobin.  Measurements were done between – 5 deg C and – 27 deg C. They found that rates were highest between – 11 deg C and – 12 deg C, and lowest below – 18 deg C.  “The autooxidation of porcine myoglobin was faster than ovine or bovine myoglobin.  Porcine myoglobin is precipitated by freezing which leads to the conclusion that the more rapid rate for this protein is due to a combination of autoxidation and precipitation.”  Red colour is, therefore, best preserved at temperatures below – 18 deg C..  (James, S. J., James, B., 2000)   Fennema (1975) has shown that freeze concentration effect can cause rates of chemical reactions to increase dramatically just below the freezing point. (Fu and Labuza)

(2) the catalytic effect of ice crystals;  The groundbreaking work of Buttkus (1967) offers a good case in point for both the concentration effect of freezing and the catalytic effect of ice crystals.  “He demonstrated the interaction of myosin, a structural protein, with malonaldehyde, measuring the extent of the interaction by the number of free ε-amino groups in the protein molecule.”  He evaluated the reaction at a range of different temperatures namely + 20 deg C, 0 deg C and – 20 deg C.  At + 20 deg C, almost 60% of the ε-amino groups of lysine were rendered unavailable after 4 days, 40% having interacted after 8 hours.  The reaction was considerably reduced at 0 deg C.  At – 20 deg C, the reactions at – 20 deg C was about the same as at + 20 deg C.  Grant et al (1966) suggested the results were due to the concentration effect “resulting in a closer association of the molecules in the reaction mixture due to freezing as well as to the result of a catalytic effect in which the ice crystals were thought to participate.  “Further work by Buttkus (1967) demonstrated that storing a mixture of malonaldehyde and myosin at – 20 deg C for 6 days resulted in the participation of other amino acids in addition to lysine. The order of reactivity was found to be methionine, lysine, tyrosine, and arginine. (Eskin, et al, 1971)

(3) a greater mobility of protons in ice than in water;  The imperfect nature of ice explains the mobility of protons in frozen water.  The defects in ice are usually of the orientational (caused by proton dislocation accompanied by neutralizing orientations) or ionic types (caused by proton dislocation with formation of H3O+ and OH-) (Fennema, 1996)  An increase in proton mobility is one of the possible reasons given for the degradation of ascorbic acid at refrigerated temperatures.  (Heldman, D. R., Lund, D. B. (Ed), 2006)

(4) a change in pH, up or down with freezing;  “When freezing begins, grains of crystalline ice begins to grow. The solutes are rejected from the ice and concentrated in the interfacial water layer by assistance of the electrostatic force generated by the freezing potential. At a certain stage of freezing, the water layer is completely confined by the walls of some ice grains. Protons move from the ice phase to the unfrozen solution surrounded by the ice walls to neutralize the electric potential generated, and thus the pH of the unfrozen solution decreases.” (Takenaka, et al, 1996)

(5) a favorable orientation of reactants in the partially frozen state; Atoms must “come together” to form chemical bonds. They must be brought to some position or orientation to form a product.  Freezing favours such orientations for many reactants.  One such example is the oxidation of nitrite by dissolved oxygen to form nitrate.  It is known to be accelerated ca. 10times by the freezing of the aqueous solution (Takenaka, et al, 1996) and have important implications for the frozen storage of cured products such as bacon.

(6) a salting in or out of proteins;  According to John Steemson, a researcher working in a molecular biology lab at the University of Auckland, New Zealand, “charge balance (from ions around the protein) is important for protein stability because a protein uses charged residues (as well as other factors) to fold and stay folded. Heaps of ions in solution can mask charges and eliminate or severely curtail those interactions, potentially exposing internal hydrophobic regions and reducing protein solubility.”

He writes in response to a question that “perhaps, more importantly, water-soluble proteins have concentric “shells” of semi-ordered water molecules arranged around them, in much the same way that dissolved salts have associated water molecules making them soluble. If you add too much salt, the waters in the protein solvation shells are stripped out to dissolve the salt, precipitating the protein out of solution. This is sometimes called “salting out”.  The change in positive and negative ions around the protein change the various interactions which are involved in keeping the protein together and helping it to hold it’s structure.  In other words, freezing can denature proteins through this mechanism.

(7) decrease in dielectric constant;  the dielectric constant (ε) is defined as a measure of a substance’s ability to insulate charges from each other.

and

(8) the development of antioxidants at higher temperatures.

Refreezing

Fluctuating temperatures between different freezers as the product moves from the company holding freezer to the refrigerated truck and into the refrigerator at the client’s premises is one reason for fluctuating temperatures.  Another is poor freezer door discipline if it is not managed mechanically – not closing the freezer door and some freeze-thawing cycles may be planned by defrosting products to use in production and refreezing what has not been used.  The question comes up how detrimental this is to product quality and its impact on shelf life.

Provided that thawing is done in such a way not to contaminate the meat through microbes, refreezing of meat does not significantly negatively impact the meat quality.  The biggest impact that one will notice is the loss of water during thawing.  Any attempts to rehydrate the meat to its original water content will be successful only partially.  Thawed meat is also more susceptible to microbial growth because of ruptures cells and increased surface moisture.  (Herren, 2011)  Despite these common-sense deductions, Baker, R. C. et al (1976) demonstrated some surprising results.  They conducted a study where chicken broilers were subjected to 5 freeze-thaw cycles.  They evaluated the meat at the end of the process for drip during thawing, cooking loss, TBA, shear force, total moisture, bacterial counts, visual appraisal for sliminess and bone discoloration, and taste panel appraisal for tenderness, juiciness, and flavor.

They found that very few characteristics in the broilers were affected by rate of freezing, and number of freezing and thawing cycles.  While more moisture was lost as thaw drip, as a result of freezing, less was lost in cooking for refrozen birds so that the total loss was similar regardless of freezing rate and number of thawings.  They concluded that although repeated thawing and freezing is certainly not recommended procedure, it appears from their study that this process does not does not greatly effects various characteristics (including bacterial counts) of chicken broilers.  (Berry and Leddy,  1989)

The Hazard function

The hazard function h(t) of a distribution is defined for t ³ 0 by:

h(t) = f(t)/[1-F(t)]

where f(t) is a probability density function and F(t) is a cumulative distribution function. The h(t) is the conditional probability of failure at time t, given that failure has not occurred before.

Hazard Function.png
Failure Rate as a function of Time

“Early failure should not be taken as a true failure relative to the shelf life of the product unless it represents the normal condition. From t1 to t2 one can expect, barring chance major temperature fluctuations, no failures. This interval represents the true period of the product’s stability. The failure rate is almost constant and small during this time. The hazard or failure rate increases from time t2 to the termination point t3, owing to the true deteriorative changes occurring within the product. The concept of hazard function is important in the analysis and interpretation of the failure times of a product.”  (Fu and Labuza)

“A fundamental assumption underlying statistical analysis of shelf life testing is that the shelf life distribution of a food product belongs to a family of probability distributions and that observations are statistically independent. Parameters of a shelf life distribution are estimated by use of shelf life testing experimental data. Once the parameters of a shelf life model have been estimated, it can be used to predict the probabilities of various events, such as future failures (Nelson, 1972). Five statistical models, normal, log normal, exponential, Weibull and extreme-value distributions were tested for a few food products (Gacula and Kubala, 1975; Labuza and Schmidl, 1988).”   Fu and Labuza found that the Weibull distribution fits best.  I suggest you download the Fu and Labuza chapter from the references below and study the Weibull approach.

“Fail small – Fail early” philosophy

A philosophy crucial in new product development, product reformulation and in fact, equally applicable to the setting up of a new business is the “fail small, fail early” philosophy.  A proper application of the Stage-Gate approach to NPD is an excellent approach that will enforce this philosophy.

One of the most important aspects of product development or reformulation is shelf life.  The shelf life must at least exceed the minimum distribution time required from the processor to the consumer.  It is a mistake not to see that all raw material received is turned into sellable products at the end of the shift which includes intermediate products to be used the next day in production.  All end products at the end of shift must have a product description, batch number, production date, shelf life and packed in designated packaging.

Take trim for example.  If one views the end goal of the production day as incoming meat = products produced at the end of the shift, one would produce the bacon, hams, and sausages on the production schedule and all leftover trim would be stored in a form, ready to be used the next day for sausage production or transferred to the sausage department as an internal client, at the right temperature to guarantee the one day shelf life required.  Such intermediate products would have their own batch number, a production date and a product description attached to it.   If there was not an immediate need for it (such as is often the case with fat), it should be packed it in the right packaging for long-term storage and labeled with batch codes, production dates, best before dated and proper product description.  It should be frozen as per product specifications and stored accordingly.  This way, a product is created that will either be transferred to production at a future date or traded out to clients.   A First In – First Out system will be applied by the freezer manager and a list of products nearing end-of-shelflife dates will be made available every day for action, either to be used in production or traded out.

During the design of the intermediate or final products, shelf life testing can assess problems that the product has in the development stage and corrections can be made.  This process must be repeated periodically.  Such intermitted shelf life tests help to provide assurance that the product remains consistent over time with respect to quality.

“Different shelf life testing strategies are necessary at different stages, as illustrated in the figure below. If the objective is to identify whether pathogens and spoilage microbes will grow in the case of temperature abuse, then a challenge study is necessary. If the objective is to quickly estimate the approximate shelf life of the product then an ASLT can be used, as long as the proper temperature range is chosen. A confirmatory shelf life test may be conducted at the last stage with simulated distribution chain conditions, although in today’s R & D environment, this may be skipped.”

Shelf life testing strategy at different product development stages
Shelf life testing strategy at different product development stages

The detailed treatment of the different strategies is found in the Fu and Labuza chapter.

Shelflife Feedback Loups

In managing shelf life of products, a feedback loop must exist between the factory manager and the sales/ accounting department.  Such a feedback loop must exist in terms of sales but also in terms of returns.  My experience is that the monthly returns are something that the accounting department deals with and the factory manager is only brought into the discussion when it goes out of hand.  This is, in my opinion, a mistake.  The vital importance is clear to me that the actual shelf life achieved is communicated to the factory manager and his/ her team on a consistent basis through a study of the number of all returns per month.  Only then will the production team be able to develop an ultimate evaluation of the effectiveness of the processes, product designs, packaging, plant hygiene, quality of raw materials and the validity of assigned shelf life.

In fact, QC and the factory manager should have regular meetings where all matters related to shelf life is discussed.  This includes shelf life issues of stock that are transferred from the freezer to internal clients in the organization.

Sundry Considerations/ Useful Information

What about returns?

Returns must be quarantined in the freezer or chiller for prompt evaluation and discarded.  The first loss of revenue should be the last loss of revenue due to returns.  Attempts to rework it has a long and very dismal history in meat processing, always ending in further losses.

What about reworks?

Products designated for reworks must be treated in then same way as all other other products.  If it is frozen or stored in the chiller, it must be packed with batch number, production date, product description and best before date.  Reworks may include things like bacon shavings that can be used in bacon sausage production.  It must be stored separately from raw products.

What about freezer hygiene?

In South Africa, SANS 10156:2014 applies.

FDA Recommended Shelf Life – Frozen and Chilled

US Food and Drug Administration published a chart indicating recommended shelf life storage chart for various foods.

2018-03-06-FoodStorageCharts-English_

Recommended Further Reading

Conclusion

Understanding the importance of setting shelf-life parameters is probably one of the most important aspects of food production.  It involves every aspect of food quality and factors impacting on it reach back from the production of the animal, through the slaughtering process and processing.  The effect of freezing on shelf life of stored food is critical.  This article is only an introduction to what is a complex subject matter, worthy of detailed study.

References

Avacta Blog. 2015.  Are you freezing or degrading your proteins?

Berry, B. W., Leddy, K. F..  1989.  Meat Freezing: A Source Book.  Elsevier.

Eskin, N. A. M., Henderson, H. M., Townsend, R. J..  1971.  Biochemistry of Foods.  Academic Press.

Evans, J. A..  2008.  Frozen Food Science and Technology.  Blackwell Publishing.

Fennema, O. R..  1996.  Food Chemistry.  Marcel Dekker (Water Minerals – Food Chemistry- O.R. Fennema)

Frelka, J., Phinney, D., Heldman, D. R.  Paper presented at a conference of the International Congress on Engineering and Food, Quebec City, Quebec, Canada.  2015.  Quantification of the freeze-concentration effect on reaction rate in a model food system

Fu, B.,  Labuza, T. P..   1997.  From their book, Quality in Frozen Food.  Chapter:  Shelf Life Testing:  Procedures and Prediction Methods for Frozen Foods (FrozenFoodShelfLife)

Heldman, D. R., Lund, D. B.. (Ed) 2006. Handbook of Food Engineering.  CRC PressVan Boekel, M. A. J. S., and Tijskens, L. M. M..  Kinetic modeling (Kinetic Modelling)

Herren, R. V..  2011.  Science of Animal Agriculture.  4th Edition.  Delmar.

James, S. J., James, B..  2000.  Meat Refrigeration.  Woodhead Publishing

Takenaka, N.,  Ueda, A, Daimon, T, Bandow, H.  Dohmaru, T, and Maeda, Y..  1996.  Acceleration Mechanism of Chemical Reaction by Freezing:  The Reaction of Nitrous Acid with Dissolved OxygenPhys. Chem.1996100 (32), pp 13874–13884, DOI: 10.1021/jp9525806, Publication Date (Web): August 8, 1996, Copyright © 1996 American Chemical Society

Image credit:  http://www.smedunia.in/products/frozen-meat

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