Sustainability Corner: The Science of Food Preservatives
By: Jean-Michel Boudreau
On September 27, 2006, reporters and photographers stood by in amazement – David Corbin, chairman of the Texas-based Sadex Corporation, was about to knowingly eat spinach inoculated with 5 million colonies of E. coli O157:H7 bacteria per gram, the nasty bugs that had terrorized the spinach-eating world that same year’s summer. “You would have been better off to have a cow come and dump on it”, he proclaimed.
Evidently, Corbin was not worried and for good reason – prior to their digestion, his spinach had been subject to irradiation with a beam of microbe-destroying electrons or gamma rays, a method commonly known as electronic pasteurization. This method reduced the number of E. coli colonies on his spinach to only 50 – 70 colonies per gram – not enough to make humans sick. Needless to say, Corbin experienced no ill effects in one of history’s more famous acts of food preservation. Food preservation is the process of treating and handling food to stop or slow down food spoilage, loss of quality, edibility, or nutritional value and thus allowing for longer food storage.
What it means to preserve food
The act of preservation has impacted a variety of different cultures. Some practices can even date back to over 12 000 B.C from drying methods used by Middle Eastern cultures. Preservation can be done in a variety of ways: freezing, drying, fermenting, pickling, curing, canning or making jams. Environmentally, preservation reduces the amount of food that spoils during peak seasons of production. By preserving them, we can keep food intact for longer periods of time, allowing us to eat certain foods even during off-peak seasons. It also reduces transportation costs and its associated environmental impacts as it limits the need to import produce from warmer countries during our cold Canadian winters.
Here we introduce you to some of the key superstars of preservation from a scientific lens:
Sodium Chloride (i. e. Table Salt)
Electronic pasteurization is just one of the many methods developed in the preservation of foods. For example, one of the earliest methods for preserving food involved coating them in salt. When the salt concentration outside a cell is higher than inside it, water is drawn out of the cell to reduce the outside salt concentration in a process known as osmosis. Thus, creating these high salinity environments on the surface of foods would cause the water in bacterial and fungal cells to be secreted, which would dehydrate and kill them in the process. Additionally, a variation of this in which food is immersed in brine solution, more commonly known as pickling, takes preservative measures one step further; it increases the acidity of the brine. The mechanism through which this occurs proceeds via metabolic processes of Lactobacillus plantarum, a hardy, yet harmless, bacterium regularly found in anaerobic plant matter, certain cheeses and saliva. When present in the brine, the aerotolerant L. plantarum’s enzyme lactate dehydrogenase employs coenzyme NADH to catalyze the conversion of pyruvate to lactic acid making the brine even more inhospitable for other microbes.
Another food-preserving tactic for meats, essential for preventing growth of Clostridium botulinum, the culprit behind botulism, a rare and potentially fatal illness that involves the most toxic substance known to mankind, the botulinum toxin (BTX), is done by curing them through adding nitrite salts. In an acidic environmental, in many cases by addition of lactic and/or acetic acid, an equilibrium is established between the salt’s nitrite and nitrous acid which depends upon the pH of the solution. Theoretically, at the usual pH of meats (pH = 5.2 – 7.0), only a small quantity of the added nitrite exists as nitrous acid. Nevertheless, they are added in sufficient amounts for adequate production of the true bioactive compound, nitric oxide, via nitrite decomposition. The most important mechanism by which it destroys C. botulinum proceeds by inhibition of the iron-sulfur clusters of ferredoxins, enzymes necessary for their energy production, by nitrosylation (i. e. the addition of nitric oxide molecules) of their iron ions. Interestingly, nitric oxide also reacts with the myoglobin, an oxygen-binding protein found in the muscle tissue of meats to form nitrosomyoglobin which, not only improves their flavor, but is also responsible for their characteristic reddish-pink color.
Sulfur dioxide is an antimicrobial agent mainly used in the wine industry. There are a few mechanisms by which sulfur dioxide prevents the microbe growth in wine. Perhaps, it is best summarized by A. Larry Branen and colleagues in Food Additives: First, it reacts with end products or intermediate products inhibiting enzyme chain reactions. Second, it cleaves essential disulfide linkages in proteins and induces changes in the molecular conformation of enzymes. This modifies enzyme active sites and destroys the coenzymes. Furthermore, it destroys the activity of thiamine and thiamine-dependent enzymes by cleavage and produces cytotoxic effects by cross-linking individual nucleic acid residues or nucleic acid residues and proteins. Additionally, it damages cell metabolism and membrane function by peroxidizing lipids (i.e. degrades lipids by free radical reactions resulting in the substitution of hydrogens to peroxide groups). Sulfur dioxide also has the additional advantage of being an antioxidant; it can react with dissolved oxygen to form sulfates. This is very useful because some of the vinegar-forming bacteria are resistant to sulfur dioxide, but the need for oxygen to convert ethanol to acetic acid can ruin a wine’s flavor!
Benzoates and Propionates
Benzoates and propionate salts are another two classes of food-preserving agents. The latter is regularly used in the production of breads and bakery products where they prevent moulding via inhibition of fungal growth. It is generally accepted that the undissociated benzoic and propionic acid (i. e. protonated benzoate and proionates, respectively) are the active antimicrobial agents. Although no definite theory has been yet proposed to explain their antimicrobial effect, they are both believed to be related to the high lipoid solubility of the undissociated form which allows it to accumulate on the cell membranes or on various structures on the surface of micro-organisms, effectively inhibiting cellular activity.
It may cause some alarm that viruses are employed to preserve meats. However, the virus currently approved by the Food and Drug Administration only invades bacteria, and more specifically, only Listeria monocytogenes. This nasty bacterium is responsible for listeriosis, an awful flu-like illness that potentially induces blood poisoning (septicemia) and meningitis and may be fatal if left untreated. The mode of action of these listeria killing viruses, appropriately named bacteriophages (i.e. bacteria-eaters), initiates when the bacteriophage docks a protein on the surface of a L. monocytogenes cell and injects its genetic material into it. Following this, it hijacks the cell’s proteins and orchestrates the assembly and accumulation of more viruses. This ultimately results in the bursting of the microbe, destroying it and ignites a chain reaction in which the newly biosynthesized virus are free to repeat the same antimicrobial process.
So far, the methods described involve the addition of tiny particles to food. This is not true for all food-preserving techniques, however. Canning, for instance, is a process in which food in sealed containers are heated and pressurized to destroy micro-organisms, though, its employment in industry occurs only after food has been processed.
As noted above, electronic pasteurization is the process of killing microbes through the irradiation of food with a beam of electrons or gamma rays. The usage of this method may raise the eyebrows of some, however, put simply, eating irradiated food does not expose consumers to radiation. The insects and microbes that contaminate our food supply can testify to the lethal effects of radiation exposure, but the food does not become radioactive. Now, that isn’t to say that there aren’t any novel toxins created in the process of electronic pasteurization. In fact, the carcinogen class consisting of 2-alkylbutanones (2-ACBs) have been found only in irradiated foods. Fortunately, however, it is only present in negligible abundance. Furthermore, numerous feeding studies of irradiated foods have been carried out, in many cases using extreme amounts. For example, dogs, cats and mice have been fed irradiated chicken that made up 35% of their diet with no effect.
The Sustainability Office will be hosting various food preservation workshops throughout the Fall semester, preserving our campus-grown produce over the winter which includes pickling cucumbers and beans as well as canning tomatoes.
*Published in The Underground V.36 I.02 pg. 30-31