Pasteurization

• 1. Introduction

  ›

  Pasteurization is a mild heat treatment for packaged foods, and living without it once carried a measurable cost. Between 1912 and 1937, some 65,000 people in England and Wales died of tuberculosis that they contracted from drinking milk. The process that now guards against such deaths heats food to less than 100 degrees Celsius, well short of a full boil. That is enough to destroy or deactivate many of the microorganisms and enzymes behind spoilage and disease. It is not enough to kill most bacterial spores. Several questions run through this story. How did a technique named for a French microbiologist come to be proven first on wine? Why did milk follow only after many years? How does a factory know the heat did its work? And what does the heat take from the food in exchange?

  2324

• 2. Heat Before Pasteur

  ›

  In AD 1117, people in China were already heating wine to preserve it. Japan has its own record, the diary Tamonin-nikki, which a series of monks wrote between 1478 and 1618. The first experiment that showed why heat works came in 1768. The Italian priest and scientist Lazzaro Spallanzani boiled meat broth for one hour and sealed the container the moment boiling stopped. The broth did not spoil, and it showed no microorganisms. A product, he had demonstrated, could be made sterile by heat.

  In 1795, the French military offered a cash prize of 12,000 francs for a new way to preserve food. That same year, a Parisian chef and confectioner named Nicolas Appert began his own experiments. He succeeded with soups, vegetables, juices, dairy products, jellies, jams, and syrups. His method was to pack food into glass jars, seal them with cork and sealing wax, and place them in boiling water. After some 14 or 15 years of work, he submitted the invention and won the prize in January 1810. Later that year he published L'Art de conserver les substances animales et végétales, meaning The Art of Preserving Animal and Vegetable Substances. It was the first cookbook on modern food preservation methods.

  At Massy, a town near Paris, the House of Appert became the first food-bottling factory in the world. Appert filled thick, wide-mouthed glass bottles with everything from beef and fowl to eggs, milk, and prepared dishes. He left an air space at the top, and a vise pressed the cork in firmly. Each bottle was wrapped in canvas for protection, dunked into boiling water, and boiled for as long as Appert judged right to cook the contents thoroughly. He patented the method, which is sometimes called appertisation in his honor.

  The method was simple enough to spread fast. In 1810, the British inventor and merchant Peter Durand, who was also of French origin, patented a version that used a tin can. That created the modern process of canning. In 1812, the Englishmen Bryan Donkin and John Hall bought both patents and began producing preserves. A decade later, Appert's canning method had reached America. Tin cans stayed uncommon until the start of the 20th century, partly because opening one took a hammer and chisel until Robert Yeates invented a can opener in 1855. A less aggressive method was still to come, and it began with a holiday in 1864.

  8104614

• 3. A Summer in Arbois

  ›

  During the summer of 1864, the French chemist Louis Pasteur was on holiday in Arbois, where the local aged wines were often too acidic. He tested a remedy and found, experimentally, that heating a young wine to only about 50 to 60 degrees Celsius for a short time killed the microbes. The wine could then be aged without sacrificing its final quality. The process is known as pasteurization in his honor.

  Pasteur's research in the 1860s showed that thermal processing deactivates unwanted microorganisms in wine. The same treatment also inactivates spoilage enzymes. At first, the job of pasteurization was to stop wine and beer from souring. Milk would have to wait many years.

  The state of milk in that era explains why it needed help. In the United States in the 1870s, before milk was regulated, it was common for milk to contain substances intended to mask spoilage.

  51213143

• 4. Dangerous Milk

  ›

  The US Centers for Disease Control and Prevention says improperly handled raw milk is responsible for nearly three times more hospitalizations than any other food-borne disease source. Milk is an excellent medium for microbes, and at ambient temperature bacteria and other pathogens soon proliferate. Pasteurization prevents tuberculosis, brucellosis, diphtheria, scarlet fever, and Q-fever. It also kills Salmonella, Listeria, Yersinia, Campylobacter, Staphylococcus aureus, and Escherichia coli O157:H7, among others.

  Before industrialization, dairy cows were kept in urban areas so that little time passed between milking and drinking. As cities grew denser and supply chains stretched from country to city, raw milk often arrived days old. It came to be recognized as a source of disease. Tuberculosis was hard to pin on milk because it has a long incubation period in humans. In 1892, the chemist Ernst Lederle inoculated guinea pigs with milk from cows with tuberculosis, and the animals developed the disease. In 1910, Lederle, by then Commissioner of Health, introduced mandatory pasteurization of milk in New York City.

  Great Britain in the 18th century used a traditional form of pasteurization, scalding and straining cream to improve the keeping qualities of butter. The practice reached Boston by 1773, though it was not widely followed in the United States for the next 20 years. In 1886, Franz von Soxhlet suggested pasteurizing milk itself. In the early 20th century, Milton Joseph Rosenau set standards while at the United States Marine Hospital Service. His low-temperature, slow method heated milk at 60 degrees Celsius for 20 minutes, and he set it out notably in The Milk Question, published in 1912.

  The first US state law making dairy pasteurization mandatory came in 1947. In 1973, the federal government required pasteurization of any milk used in interstate commerce. In developed countries, milk is now considered a safer food. Even so, the CDC found that between 1998 and 2011-79% of dairy-related disease outbreaks in the United States were due to raw milk or cheese products. That meant 148 outbreaks and 2,384 illnesses, with 284 people requiring hospitalization, and two deaths.

  3320303128

• 5. Time and Temperature

  ›

  At 71.5 degrees Celsius for 15 seconds, milk receives what is known as high-temperature short-time pasteurization, or HTST. That treatment ensures the milk is safe and gives it a refrigerated shelf life of about two to three weeks. Ultra-pasteurized milk can last two to three months. In ultra-high-temperature processing, or UHT, milk is sterilized at 135 degrees Celsius for one to two seconds. With special packaging, that extends shelf life to three months without refrigeration. Add sterile handling and aseptic packaging, and UHT milk can be stored unrefrigerated for up to 9 months.

  The dividing line for how much a food needs is a pH of 4.6. Acidic foods at or below it, such as fruit juice and beer, are treated to inactivate enzymes and destroy spoilage microbes. In juice the enzymes are pectin methylesterase and polygalacturonase, and the microbes are yeast and lactobacillus. Pathogens cannot grow in such acid, and shelf life stretches by several weeks. Less acidic foods, such as milk and liquid eggs, are treated to destroy pathogens and spoilage organisms, namely yeast and molds. Not every spoilage organism dies, so refrigeration must follow.

  In the early 20th century, nobody had robust knowledge of which time and temperature combinations would inactivate the pathogens in milk, so several different standards were in use. By 1943, studies of complete thermal death, as best it could be measured then, confirmed two. One was HTST at 72 degrees Celsius for 15 seconds. The other was batch pasteurization at 63 degrees Celsius for 30 minutes. Later work showed the process fully inactivates Coxiella burnetii, once thought to cause Q fever through infected milk, and Mycobacterium tuberculosis.

  Until the 1960s, microbiological techniques could not count how far bacteria were actually reduced. Researchers answered by spiking milk with high levels of the most heat-resistant strains of the main milk-borne pathogens, then heat-treating it and counting survivors. In a 15-second treatment, Salmonella Typhimurium fell by more than 6.9 log10 at 61.5 degrees Celsius. Listeria monocytogenes fell by more than 6.9 at 65.5 degrees. A reduction of 6 to 7 log10 means one bacterium in a million to one in ten million survives.

  The Codex Alimentarius Code of Hygienic Practice for Milk asks for at least a 5 log10 reduction of Coxiella burnetii. Its minimum conditions equal heating every particle of milk to 72 degrees Celsius for 15 seconds in continuous flow, or 63 degrees for 30 minutes in a batch. To reach every particle, flow in the heat exchangers should be turbulent, meaning the Reynolds number should be sufficiently high. That matters because test-tube studies without flow show less inactivation than larger experiments that mimic commercial plants. Modern HTST systems add flow-rate restriction and divert valves so no part of the milk gets a shorter time or a lower temperature. Temperatures commonly run 1.5 to 2 degrees Celsius above 72.

  Pasteurization is not sterilization, and it does not kill spores. Double pasteurization adds a second heating that kills spores that have germinated. Where it is allowed, milk gets its first heating at collection from the farm so it does not spoil before processing. Many countries bar the label pasteurized for such milk but allow thermized, which refers to a lower-temperature process.

  32454642

• 6. Inside the Plant

  ›

  When food sits in glass, the plant uses hot water rather than steam, to avoid cracking the glass through thermal shock. Food can be pasteurized before or after it reaches its container. In-container treatment generally uses steam or hot water. With plastic or metal packaging the risk of thermal shock is low, so either works.

  Most liquid foods skip the container and pass through a continuous process. They move through a heating zone, then a hold tube that keeps them at pasteurization temperature for the required time, then a cooling zone. Only after that is the product filled into packaging. Plate heat exchangers often handle low-viscosity products such as animal milk, nut milk, and juices. They are built from many thin vertical stainless steel plates that separate the liquid from the heating or cooling medium.

  Ketchup and baby foods are non-Newtonian fluids, and so are dairy products. Shell and tube heat exchangers often serve them. These are concentric stainless-steel tubes, with food in the inner tube or tubes and the heating or cooling medium circulating in the outer one. For very viscous material, a scraped-surface exchanger adds an inner rotating shaft with spring-loaded blades. The blades scrape away whatever builds up on the tube wall.

  Heating in an exchanger before packaging beats in-container treatment on four counts. It treats the product more uniformly, handles a wider range of products, transfers heat more efficiently, and offers greater throughput. After the hold tube sits a flow diversion valve. If the temperature or time falls short, the valve sends the under-processed product back to the raw product tank. Adequately processed product is cooled in a heat exchanger and then filled.

  Alkaline phosphatase is how a dairy proves its work. Direct microbiological testing is the ultimate measure of pathogen contamination, but it is costly and slow, so shelf life has already shrunk by the time results arrive. Pasteurization denatures this enzyme, and its destruction ensures the destruction of common milk pathogens. For liquid eggs, the equivalent signal is the residual activity of α-amylase.

  34950239

• 7. What Heat Takes

  ›

  Vitamin B2 occurs in cow's milk at about 1.83 milligrams per liter, and the recommended adult intake is 1.1 milligrams a day. That makes milk a major contributor, and pasteurization significantly lowers its B2 concentration. It is the one vitamin where the loss is a real concern.

  A systematic review and meta-analysis examined the other vitamins. The heat is mild, so shelf life grows by only a few days or weeks and heat-labile vitamins change only slightly. Pasteurization appeared to reduce vitamins B12 and E, though research on those was limited, and it appeared to raise vitamin A. It significantly decreased vitamin C and folate. Milk is not an important source of B12, E, C, or folate in the North American diet, so the effect on adult intake is negligible. Milk is an important source of vitamin A, and because pasteurization appears to raise it, that is no major public health concern.

  In fruit juices, pasteurization may cause a loss of volatile aroma compounds. Juice is deaerated before pasteurization, and that step may be responsible for the loss. Deaeration does help limit the loss of nutrients such as vitamin C and carotene. Volatile recovery can restore quality, although it is costly.

  Chlorophylls, anthocyanins, and carotenoids, the pigments of plant and animal tissues, do not change much under pasteurization. In juice, polyphenol oxidase, or PPO, is the main enzyme behind browning, and removing oxygen in the deaeration step deactivates it. Milk looks whiter after pasteurization than raw milk because of homogenization beforehand, which emulsifies its fat and water-soluble components. In vegetables, color degradation depends on the temperature and the length of heating.

  The ceiling for vegetables is 80 degrees Celsius. Above it, enzymatic and non-enzymatic changes in the structure of pectin can cause textural loss. Below it, tissue softening is not a concern with mild-heat pasteurization. Reducing exactly these effects drove the search for ways to pasteurize without heat.

  515455352

• 8. Beyond Heat

  ›

  Pascalization, better known as high pressure processing, does its work without heat. Pasteurizing in the broader sense means any method that cuts microbes by an amount, a log reduction, equivalent to Pasteur's process. Developers created novel thermal and non-thermal processes to spare heat-labile nutrients and sensory qualities. Non-thermal examples in commercial use include pulsed electric field, ionising radiation, UV decontamination, pulsed high intensity light, high power ultrasound, oscillating magnetic fields, and streamer plasma.

  Microwave volumetric heating, or MVH, is the newest available pasteurization technology. It uses microwaves to heat liquids, suspensions, or semi-solids in a continuous flow. Because it delivers energy evenly and deeply into the whole flowing product, the heating can be gentler and shorter. Almost all heat-sensitive substances in the milk are preserved.

  Medical equipment, notably respiratory and anesthesia equipment, is often disinfected with hot water as an alternative to chemicals. The temperature is raised to 70 degrees Celsius for 30 minutes. More thorough sterilization takes an autoclave, which uses higher temperatures and pressures.

  Today the list of commonly pasteurized products runs from beer, canned food, eggs, and juices to syrups, vinegar, and water. Wine, the drink Pasteur treated in Arbois, is on it too.

  565534501