Fluid milk shelflife – from hours to months
Petr Dejmek
Dept of Food Technology, Engineering and Nutrition, LundUniversity,
Box 124, S-221 00, Lund, Sweden
Tel +46 46 2229810, fax +46 46 2224622, mail
Abstract
The development of shelf life of liquid milk and the scientific background of the problem and the technical solutions utilized are followed in a historical perspective. It is concluded that the general increase in affluence of the society and the general improvement of the engineering competence contributed most to the observed increase of fluid milk shelf life.
At a first glance, one can be overwhelmed by the spectacular increase of the shelf life of fluid milk. Sixty years ago, a substantial part of fluid milk was distributed, even in the developed countries, in bulk to retailers and sold over the counter. Such milk had a shelf life of hours and would be boiled in the household to keep until the evening, or overnight. Currently shelf life is counted in weeks and months, Fig 1. The problems caused by short shelflife are since long forgotten by the consumers , and freshness rather than functionality seems to be gaining prominence, so that the average shelf life on the market might be decreasing again.
Milk at ambient temperatures is a selective substrate for lactic acid bacteria and these are always present in the environment. Thus milk sours “spontaneously” and the presence of lactic acid effectively limits the growth of other organisms, apart from some yeasts. No common seriously pathogenic organisms thrive in sour milk. In the Nordic countries, skim milk would be collected throughout the summer season in a large vessel and kept for use over winter - the Icelandic skyr is a vestige of this traditional milk storage system. While the nutritional value of milk was kept intact, the taste was not to everybody’s liking. This is graphically depicted in an Icelandic tale from the Middle Ages, Egil Skallagrimsons saga. Egil, a famous Icelandic poet and warrior, came uninvited to a Swedish farm, and was treated with milk. Hungry, he gorged himself, but when he later found out that there was meat to be had, he nearly killed the unfortunate farmer
Today fermented milks are considered a product completely separate from liquid milk, and from consumer perspective, detectable taste or structure changes to milk limit its shelf life. These are most often caused by the metabolites of growing microorganisms. Nonbacterial changes in the chemistry of milk are mostly caused by oxidation or indigenous enzymes of milk. Physical instability presents an interesting development. As long as milk fat was a most cherished milk component, creaming was not a limitation to shelf life. It was actively promoted, and a large body of research was generated in the search of a thick and distinct creamline. Nowadays, creaming is barely tolerated. Sediment, on the other hand has alwaysbeen a shelf life limiting product fault.
Apart from the changes observable by the consumer, shelf life may be limited by legislation. Many countries impose, in addition to health related criteria, arbitrary limits on some aspect of milk microflora, typically a maximum CFU count using some officially promulgated procedure.
The current view of milk shelf life can be summarized as follows (Walstra, Wouters and Geurts, 2006):
Pasteurized milk can be spoiled by psychrotrophic recontamination, or if that is largely avoided, by pasteurization surviving spores. Itsmaximum shelf life will depend on the temperature in the storage chain, from a few days where storage temperatures of 12°C are legal, to beyond three and up to six weeks at temperatures close to milk freezing point. The shelf life of sterilized milk is less clearly defined, as no easily detectable bacterial growth occurs. Taste deterioration is governed again by storage temperature, being 3 to 6 months in ambient temperature in moderate climates, and by the presence of heat treatment surviving lytic bacterial enzymes. After monthlong storage at moderate temperatures UHT milk may gel as a late effect of proteolytic activity Some formation of a bottom sediment occurs. Oxidation is usually avoided, but Maillard reactions may become prominent at tropical storage temperatures.
The early scientific understanding of milk shelf life
As so often in food science, practical solutions are often found to problems for which causes are not understood. Famously, canning, invented by Nicholas Appert in 1804, produced long life food, even if Appert the whole of his life firmly believed that it was the exclusion of air that caused the success of his method. Similarly in milk. It was found that evaporation concentrated canned milk will keep well. Gail Borden, with only one year of formal schooling, experimented with ways to concentrate and can milk, obtained US patent for his vacuum evaporation of milk in 1856 (Borden 1856), and went on to become a household name. In his patent application , Borden mentions that “scalding milk to improve its preservative qualities has long been known”. The Swiss pediatrician Henri Nestle independently experimented with ways of providing safe food for infants, and his factory for condensed milk in Cham started in 1866.
Bacteria as a cause of milk souring was proposed as early as 1844. Nevertheless, in the early days of scientific inquiry into milk, the attribution of milk souring to microorganisms was for a long time in doubt. Due to the almost unavoidable contamination of milk with lactic bacteria, it was believed that souring of milk is a milk inherent reaction, similar to fruit ripening, in Borden’s words “ milk is a living fluid, and as soon as drawn from the cow begins to die, change and decompose”. It was only when Joseph Lister about 1874 succeeded in drawing sterile milk from the udder and isolated “Bacterium Lactis” (Lactococcus lactis subsp lactis)[1] that the argument was finally settled (Ford, 1928).
Louis Pasteur is usually credited for the general idea of microorganisms as the cause of spoilage, and the use of moderately high temperatures for the inactivation of microorganisms, based on his research on spoilage of wine around 1860-1865(P.F.F,.1898). There is no known evidence that Pasteur applied his method to milkThe use of Pasteur’s namecould have originated as a marketing gimmick to capitalize on Pasteur’s fame. The first mention of pasteurization in English is rather late, 1886, in The Times.
In the later part of the nineteenth century tremendous progress was achieved in general, medical and even dairy bacteriology. The basic science based strategies to increase milk shelflife – to avoid contamination, to inactivate, to keep temperatures low - were well established long before the turn of the 20thcentury. The existence of spores was also known by then, and it was well known that temperatures above atmospheric boiling point of water are needed to inactivate them, as seen in a few sentences from a lecture reported in Science 1890, Fig 2.
Even the the basic chemistry of non-bacterial milk deterioration had proceeded fairly far by then.In 1897 Russel and Babcock (of Babcock test fame) found that noncontaminated milk contains an enzyme which degrades caseins, this proteolytic milk indigenous enzyme was named “galactase”. Encyclopedia Britannica 1911. Similarly an indigenous lipasewas demonstrated by Moro in 1902. Even oxidation was studied, and the effect of light as a prooxidant was shown by Hanuš in 1899. Further development occurred in the thirties. Heat stability of milk lipase was studied by Nair, 1930. The effect of milk fat globule membrane damage on lipolysis was determined by Dorner and Widmer in 1931, the effect of copper and iron on autoxidation byGuthrie et al in 1931, and the effect of deareation by Dahle and Palmer in 1937. The basic mechanism of the chain reaction in autooxidation was explained in 1943 by Farmer and Sutton, and the prooxidant role of riboflavin by Patton and Josephson in 1953. (Josephson 1954)
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History of milk heat treatment
Heat treatment is the basis, or an important part, of all current shelf life improving methods. Commercial heat treatment ofliquid milk is well over 100 years old. In Paris, heat treated milk was apparently sold in 1875 (Jensen, 1950).
Before the advent of refrigeration and mechanized transport, the market for liquid milk was limited to farms in the immediate neighborhood of cities. Milk was used primarily for butter production, and skim milk returned to farms as feed. It was the shelf life improvement of this milk by heating (from 12 to 36h at room temperature, according to a Danish experimental dairy report from 1883) that gave impetus to the fast spread of pasteurization in Europe. Only later the health implications (for the health of the cow herd, not the consumer!) were understood.
Some European countries legislated obligatory heat treatment of milk used for calf feeding, to contain bovine tuberculosis (Germany 1894, Denmark 1898). The commercial advantage of increased shelflife at a small increase of handling costs then led to a rapid introduction of milk heat treatment in the cities ofEurope. In Sweden 60% of dairies delivered pasteurized milk in 1896. The adoption of heat treatment in USA was much slower, and highly controversial, with only 25% of dairies pasteurizing in 1905. There was a widely held belief that treating milk was unnatural and destroyed some of its essential “goodness” and unheated “certified milk” produced under official supervision and inspection a better alternative.
Sterilized milk was produced commercially in the Netherlands already in 1889, and in Germany and Britain in 1894 and was popular with the customers despite its significantly different appearance and taste (Davies, 1950).
Process and equipment for milk treatment
A profusion of heat treatment equipment was developed and used, a plethora of heating times and temperature combinations existed. Continuous heaters appeared as early as 1882, in connection with continuous centrifugation, but the distribution of residence times was poorly understood and controlled.Gradually, in the early twentieth century, continuous systems lost favor and batchwise heating and heating of retail containers became the rule, as a consequence of public health concerns about the reliability of the operation of continuous systems. Public authorities intervened to impose obligatory rules for times, temperatures and processing conditions. Later, the pendulum swung, first towards asemi-continuous, so called “holder” pasteurization. In true batch pasteurizer, the milk was heated and cooled in a vat. In holder pasteurizer, milk was continuously heated and pumped to a vat, and after specified holding time, typically 30 minutes, pumped out and cooled.The acceptance of true continuous pasteurization, “High Temperature Short Time”, HTST despite its much lower costs varied between countries, in Great Britain continuous pasteurization remained illegal until 1941 (Davies, 1950).
In most countries, multiple pasteurization of milk for public consumption is not allowed. A subpasteurization treatment, thermizaton at 65C for 20s ahead of the final thermal treatment had been introduced by Stadhouders in 1962 and caused an average shelf life increase of a few days . In 1970 Martin and Blackwood found that thermization led to partial spore germination (Blackwood and Martin 1972).
Already the very early milk treatment equipment could be surprisingly modern. So in 1912 Tödt’sMomentanerhitzer,Fig 3, which let milk under pressure pass a narrow channel, heated from both sides with steam,could achieve 130-140C with aresidence time of 5-30 sec. In the same year, Lobeck’s Biorizer,Fig 4, sprayed milk into a steam filled chamber. Several designs of double tube heat exchangers for milk,Fig 5,were developed in the early 20th century (Stassano, Montana, Nielsen) and heat regeneration was widely used in them, in particular as the energy costs increased during the first world war (Weigmann, 1932). In 1923 Seligman’s APV, Aluminium Plant and Vessel Company introduced the plate heat exchangerbut seemed to meet skepsis from health authorities - in Denmark, while in common use for other duties in dairies, plate heat exchangers were permitted for consumer milk pasteurization first in 1939(Jensen, 1950). The basic design of Gaulin’s high pressure homogenizer of 1904 survived with small changes until today. It’s adoption to treat milk before in bottle sterilization was described (Davies, 1950) as a heaven-sent improvement which eliminated “the objectionable glue-like gelatinous mass of butterfat on the neck of the bottle”. The patent application for injection heating followed by flash cooling was filed in 1924 (Grinrod 1929).Wonderful Heath Robinson machines for automatic temperature control (boiling alcohol displacing mercury and the weight of mercury switching a valve) were invented at the turn of the century, however, automatic temperature control began to be generally used in the early 30ies. Stainless steel was introduced in plate heat exchangers just before the second world war, and the material began to be widely used in dairies first after the war, in neutral Sweden with its own steel industry somewhat earlier.
Apart from the classical heating methods, ohmic heating achieved limited success in some markets, and ohmic milk heaters are still being produced. The argument for ohmic heaters is based on the fact that there need not be any surface hotter than the nominal heating temperature.
Microwave heating was attempted in an ingenious concept by a Swedish startup company,AlfaStar (1986-1996). It tried to use microwave heating of filled and sealed milk packages, and to compensate for the poor penetration depth of microwaves, the packages were flattened for the treatment and later folded back to rectangular shape.However, the attempt failed, it is unclear whether for technical or commercial reasons.
UHT
While milk pasteurization and its equipment have not seen a major change during the last fifty years, there has been a significant development in the production of sterile milk. It was for a long time fairly obvious that inactivation of spores has a steeper dependence on temperature than the taste affecting chemical reactions in milk, and thus that a high temperature, short time treatment would be beneficial. The technical difficulty of reliably achieving a defined residence time shorter than a few second led to an “optimal” temperature range around 140C. This approach received the name Ultra High Temperature treatment, UHT, in an analogy to the HTST flow pasteurization.The problem of UHT was that with hot filling, the packages could not be cooled fast enough, and with cold filling, the sterility could not beguaranteed.
In the current perspective, UHT milk is the obvious flag-bearer of the long shelf life milk concept. A kind of UHT product existed since at least 1951, when preheated milk was filled in cans and sterilized by the Dole process at 220C for 45s , but the origins of UHT are traditionally connected with the Uperization process developed by Sulzer Brothers and Alpura ( later taken over by APV) and the TetraPak sterile carton filler which went commercial at Verbandsmolkerei in Bern, Switzerland in 1961 (Burton H 1967a, b).
From what has been shown earlier it should be clear that the high temperature heating process and the equipment to perform it was just another step in the evolution of more sophisticated and better controlled heat processing lines. Such lines had been commonly used to pretreat milk destined for in bottle or in can sterilization, in fact the Bern plant was originally built with a can filler and a Dole can sterilizing process. All other major equipment manufacturers developed their own variants of the UHT processing line within a few years, either of the same direct steam injection type (Cherry Burrell, Alfa Laval), of steam infusion type, akin to the above mentioned Biorizer (Laguillharre, Breil&Martel, Paasch&Silkeborg, DASI), or with indirect heating (Alfa Laval, APV, Cherry-Burrell, Ahlborn, Sordi, Storck), Anonymous 1967, Burton 1967c,d,e,f,g, Krebs 1975.
In the same way, filler designs had gradually evolved towards more reliable hygienic standards by painstaking focus on detail, be it foaming in the filler, cleanability of the enclosure, overpressure, cleaning procedures and so on. If a single breakthrough should be singled out in UHT milk development, it would probably be the use of hydrogen peroxide to achieve a reliable sterility of the packaging material in the TetraPak aseptic filler which remained the dominant aseptic filler for a long time. The basic concept of wetting the packaging material with a hydrogen peroxide solution and drying it off again remains the dominant sterilization technique for packages which are not sterile produced onsite.
Plastic based sterile packaging appeared first with Prepack’s plastic pouch in 1964 and some ten years later with Formseal’s deep drawn cup and Remy’s blow moulded bottle. The light and oxygen barrier properties of the plastic packages have only slowly improved to match the five-ply sterile carton which, in contrast to the carton used for pasteurized milk, from the very beginning included the impermeable aluminium layer.
The need for longer shelf life of milk was not acutely felt in the US, with its much better developed refrigeration system and common 3 week shelf life of pasteurized milk,. It took twenty years before the first commercial UHT plant appeared. There is a surprising analogy with the slow acceptance of pasteurization.
With the microbial growth eliminated, there was a renewed surge of interest in the reasons why UHT milk would not keep indefinitely. Heat stable bacterial proteases were identified by Mayerhofer, Marshall, White, & Lu, (1973) and heat stable lipases by Driessen & Stadhouders,1974.A new indigenous proteolytic enzyme,Cathepsin D, was identified by Kaminogawa&Yamauchi 1972 and the role of plasmin, plasminogen, and their inhibitor/activator systems clarified in the early 80ies (Grufferty&Fox,1988, Kelly & McSweeney, 2003).