The concept of aw has been very useful in food preservation and on that basis many processes could be successfully adapted and new products designed. Water has been called the universal solvent as it is a requirement for growth, metabolism, and support of many chemical reactions occurring in food products. Free water in fruit or vegetables is the water available for chemical reactions, to support microbial growth, and to act as a transporting medium for compounds. In the bound state, water is not available to participate in these reactions as it is bound by water soluble compounds such as sugar, salt, gums, etc. (osmotic binding), and by the surface effect of the substrate (matrix binding). These water-binding effects reduce the vapour pressure of the food substrate according to Raoult’s Law. Comparing this vapour pressure with that of pure water (at the same temperature) results in a ratio called water activity (aw). Pure water has an aw of 1, one molal solution of sugar - 0.98, and one molal solution of sodium chloride - 0.9669. A saturated solution of sodium chloride has a water activity of 0.755. This same NaCl solution in a closed container will develop an equilibrium relative humidity (ERH) in a head space of 75.5%. A relationship therefore exists between ERH and aw since both are based on vapour pressure.
The ERH of a food product is defined as the relative humidity of the air surrounding the food at which the product neither gains nor loses its natural moisture and is in equilibrium with the environment.
The definition of moisture conditions in which pathogenic or spoilage microorganisms cannot grow is of paramount importance to food preservation. It is well known that each microorganism has a critical aw below which growth cannot occur. For instance, pathogenic microorganisms cannot grow at aw <0.86; yeasts and moulds are more tolerant and usually no growth occurs at aw <0.62. The so-called intermediate moisture foods (IMF) have aw values in the range of 0.65-0.90 (Figure 3.1).
Figure 3.1 Relationship of food deterioration rate as a function of water activity.
The relationship between enzymatic and chemical changes in foods as a function of water activity is illustrated in Figure 3.1. With aw at 0.3, the product is most stable with respect to lipid oxidation, non-enzymatic browning, enzyme activity, and of course, the various microbial parameters. As aw increases toward the right, the probability of the food product deteriorating increases.
According to Rahman and Labuza (1999), enzyme-catalyzed reactions can occur in foods with relatively low water contents. The authors summarized two features of these results as follows:
1. The rate of hydrolysis increases with increased water activity but is extremely slow with very low activity.
2. For each instance of water activity there appears to be a maximum amount of hydrolysis, which also increases with water content.
The apparent cessation of the reaction at low moisture cannot be due to the irreversible inactivation of the enzyme, because upon humidification to a higher water activity, hydrolysis resumes at a rate characteristic of the newly attained water activity. Rahman and Labuza(1999) reported the investigation of a model system consisting of avicel, sucrose, and invertase and found that the reaction velocity increased with water activity. Complete conversion of the substrate was observed for water activities greater than or equal to 0.75. For water activities below 0.75, the reaction continued with 100% hydrolysis. In solid media, water activity can affect reactions in two ways: lack of reactant mobility and alternation of active conformation of the substrate and enzymatic protein. The effects of varying the enzyme-to-substrate ratios on reaction velocity and the effect of water activity on the activation energy for the reaction could not be explained by a simple diffusional model, but required postulates that were more complex:
1. The diffusional resistance is localized in a shell adjacent to the enzyme.
2. At low water activity, the reduced hydration produces conformational changes in the enzyme, affecting its catalytic activity.
The relationship between water content and water activity is complex. An increase in aw is usually accompanied by an increase in water content, but in a non-linear fashion. This relationship between water activity and moisture content at a given temperature is called the moisture sorption isotherm. These curves are determined experimentally and constitute the fingerprint of a food system.
Many methods and instruments are available for laboratory measurement of water activity in foods. Methods are based on the colligative properties of solutions. Water activity can be estimated by measuring the following:
Vapour pressure
Osmotic pressure
Freezing point depression of a liquid
Equilibrium relative humidity of a liquid or solid
Boiling point elevation
Dew point and wet bulb depression
Suction potential, or by using the isopiestic method
Bithermal equilibrium
Electric hygrometers
Hair hygrometers
3.1.4.1 Vapour pressure
Water activity is expressed as the ratio of the partial pressure of water in a food to the vapour pressure of pure water with the same temperature as the food. Thus, measuring the vapour pressure of water in a food system is the most direct measure of aw. The food sample measured is allowed to equilibrate, and measurement is taken by using a manometer or transducer device as depicted in Figure 3.2. This method can be affected by sample size, equilibration time, temperature, and volume. This method is not suitable for biological materials with active respiration or materials containing large amounts of volatiles.
Figure 3.2 Vapour pressure manometer.
(Adapted from Barbosa-Cánovas and Vega-Mercado, 1996)
3.1.4.2 Freezing point depression and freezing point elevation
This method is accurate for liquids in the high water activity range but is not suitable for solid foods (Barbosa-Cánovas and Vega-Mercado, 1996). The water activity can be estimated using the following two expressions:
Freezing point depression:
-log aw = 0.004207 DTf + 2.1 E-6 DT2f (1)
where DTf is the depression in the freezing temperature of water
Boiling point elevation:
-log aw = 0.01526 DTb - 4.862 E-5 DT2b (2)
where DTb is the elevation in the boiling temperature of water.
3.1.4.3 Osmotic pressure
Water activity can be related to the osmotic pressure (p) of a solution with the following equation:
p = RT/Vw ln(aw) (3)
where Vw is the molar volume of water in solution, R the universal gas constant, and T the absolute temperature. Osmotic pressure is defined as the mechanical pressure needed to prevent a net flow of solvent across a semi-permeable membrane. For an ideal solution, Equation (3) can be redefined as:
p = RT/Vw ln(Xw) (4)
where Xw is the molar fraction of water in the solution. For non-ideal solutions, the osmotic pressure expression can be rewritten as:
p = RTfnmb(mwVw) (5)
where n is the number of moles of ions formed from one mole of electrolyte, mw and mb are the molar concentrations of water and the solute, respectively, and f the osmotic coefficient, defined as:
f = -mw ln(aw)/nmb (6)
3.1.4.4 Dew point hygrometer
Vapour pressure can be determined from the dew point of an air-water mixture. The temperature at which the dew point occurs is determined by observing condensation on a smooth, cool surface such as a mirror. This temperature can be related to vapour pressure using a psychrometric chart. The formation of dew is detected photoelectrically, as illustrated in the diagram below:
Figure 3.3 Dew point determination of water activity.
(Adapted from Barbosa-Cánovas and Vega-Mercado, 1996)
3.1.4.5 Thermocouple Psychrometer
Water activity measurement is based on wet bulb temperature depression. A thermocouple is placed in the chamber where the sample is equilibrated. Water is then sprayed over the thermocouple before it is allowed to evaporate, causing a decrease in temperature. The drop in temperature is related to the rate of water evaporation from the surface of the thermocouple, which is a function of the relative humidity in equilibrium with the sample.
3.1.4.6 Isopiestic method
The isopiestic method consists of equilibrating both a sample and a reference material in an evacuated desiccator until equilibrium is reached at 25°C. The moisture content of the reference material is then determined and the aw obtained from the sorption isotherm. Since the sample was in equilibrium with the reference material, the aw of both is the same.
3.1.4.7 Electric hygrometers
Most hygrometers are electrical wires coated with hygroscopic salts or sulfonated polystyrene gel in which conductance or capacitance changes as the coating absorbs moisture from the sample. The major disadvantage of this type of hygrometer is the tendency of the hygroscopic salt to become contaminated with polar compounds, resulting in erroneous aw determinations.
3.1.4.8 Hair hygrometers
Hair hygrometers are based on the stretching of a fibre when exposed to high water activity. They are less sensitive than other instruments at lower levels of activity (<0.03 aw) and the principal disadvantage of these types of meters is the time delay in reaching equilibrium and the tendency to hysteresis.
Today we find many brands of water activity meters in the market. Most of these meters are based on the relationship between ERH and the food system, but differ in their internal components and configuration of software used. One of the water activity meters most used today is the AcquaLab Series 3 Model TE, developed by Decagon Devices, which is based on the chilled-mirror dew point method. This instrument is a temperature controlled water activity meter that allows placement of the sample in a temperature stable environment without the use of an external water bath. The temperature can be selected on the screen and is monitored and controlled with thermoelectric components. Most of the older generations of water activity instruments are based on a temperature-controlled environment. Therefore, a margin of error greater than 5% can be expected due to temperature variations. This equipment is highly recommended for measuring water activity in fruits and vegetables since it measures a wide range of water activity.
The major advantages of the chilled-mirror dew point method are accuracy, speed, ease of use and precision. The AquaLab's range is from 0.030 to 1.000aw, with a resolution of ±0.001aw and accuracy of ±0.003aw. Measurement time is typically less than five minutes. Capacitance sensors have the advantage of being inexpensive, but are not usually as accurate or as fast as the chilled-mirror dew point method. Capacitive instruments measure over the entire water activity range 0 to 1.00 aw, with a resolution of ±0.005aw and accuracy of ±0.015aw. Some commercial instruments can complete measurements in five minutes while other electronic capacitive sensors usually require 30 to 90 minutes to reach equilibrium relative humidity conditions.
Traditional intermediate moisture foods (IMF) can be regarded as one of the oldest foods preserved by man. The mixing of ingredients to achieve a given aw, that allowed safe storage while maintaining enough water for palatability, was only done, however, on an empirical basis. The work done by food scientists approximately three decades ago, in the search for convenient stable products through removal of water, resulted in the so-called modern intermediate moisture foods. These foods rely heavily on the addition of humectants and preservatives to prevent or reduce the growth of microorganisms. Since then, this category of products has been subjected to continuous revision and discussion.
Definitions of IMF in terms of aw values and moisture content vary within wide limits (0.6-0.90 aw, 10-50% moisture), and the addition of preservatives provides the margin of safety against spoilage organisms tolerant to low aw. Of the food poisoning bacteria, Staphylococcus aureus is one of the organisms of high concern since it has been reported to tolerate aw as low as 0.83-0.86 under aerobic conditions. Many of the considerations on the significance of microorganisms in IMF are made in terms of aw limits for growth. However, microbial control in IMF does not only depend on aw but on pH, Eh, F and T values preservatives, competitive microflora, etc., which also exert an important effect on colonizing flora.
The application of IMF technology has been very successful in preserving fruits and vegetables without refrigeration in most Latin American countries. For instance, the addition of high amounts of sugar to fruits during processing will create a protective layer against microbial contamination after the heat process. The sugar acts as a water activity depressor limiting the capability of bacteria to grow in food. As described in Figure 3.1, IMF foods are those with aw in the range of 0.65 to 0.90 and moisture content between 15% and 40%. Food products formulated under this concept are stable at room temperature without thermal processing and can be generally eaten without rehydration. Some processed fruits and vegetables are considered IMF foods. These include cabbage, carrots, horseradish, potatoes, strawberries, etc.; their water activities at 30°C follow:
|
Foods |
aw |
|
Cabbage |
0.64 |
|
0.75 |
|
|
Carrots |
0.64 |
|
0.75 |
|
|
Horseradish |
0.75 |
|
Potatoes |
0.75 |
|
0.64 |
|
|
Strawberries |
0.65 |
|
0.75 |
Under these conditions, bacterial growth is inhibited but some moulds and yeast may grow at aw greater than 0.70. In addition, chemical preservatives are generally used to inhibit the growth of moulds and yeasts in fruits and vegetables.
Advantages:
Intermediate moisture foods have an aw range of 0.65-0.90, and thus water activity is their primary hurdle to achieving microbial stability and safety. IMF foods are easy to prepare and store without refrigeration. They are energy efficient and relatively cheap. They are not readily subject to spoilage, even if packages have been damaged prior to opening, as with thermostabilized foods, because of low aw. This is a plus for many developing countries, especially those in tropical climates with inadequate infrastructure for processing and storage, and offers marketing advantages for consumers all over the world.
Disadvantages:
Some IMF foods contain high levels of additives (i.e., nitrites sulphites, humectants, etc.) that may cause health concerns and possible legal problems. High sugar content is also a concern because of the high calorific intake. Therefore, efforts are been made to improve the quality of such foods by decreasing sugar and salt addition, as well as by increasing the moisture content and aw, but without sacrificing the microbial stability and safety of products if stored without refrigeration. This may be achieved by an intelligent application of hurdles (Leistner, 1994).
Fruit products from intermediate moisture foods (IMF) appear to have potential markets. However, application of this technology to produce stable products at ambient temperature is limited by the high concentration of solutes required to reduce water activities to safe levels. This usually affects the sensory properties of the food.
Food preserved by combined methods (hurdles) remains stable and safe even without refrigeration, and is high in sensory and nutritive value due to the gentle process applied. Hurdle technology is the term often applied when foods are preserved by a combination of processes. The hurdle includes temperature, water activity, redox potential, modified atmosphere, preservatives, etc. The concept is that for a given food the bacteria should not be able to “jump over” all of the hurdles present, and so should be inhibited. If several hurdles are used simultaneously, a gentle preservation could be applied, which nevertheless secures stable and safe foods of high sensory and nutritional properties. This is because different hurdles in a food often have a synergistic (enhancing) or additive effect. For instance, modified foods may be designed to require no refrigeration and thus save energy. On the other hand, preservatives (e.g., nitrite in meats) could be partially replaced by certain hurdles (such as water activity) in a food. Moreover, a hurdle could be used without affecting the integrity of food pieces (e.g., fruits) or in the application of high pressure for the preservation of other foods (e.g., juices). Hurdle technology is applicable both in large and small food industries. In general, hurdle technology is now widely used for food design in making new products according to the needs of processors and consumers. For instance, if energy preservation is the goal, then energy consumption hurdles such as refrigeration can be replaced by hurdles (aw, pH, or Eh) that do not require energy and still ensure a stable and safe product.
The hurdle effect is an illustration of the fact that in most foods several factors (hurdles) contribute to stability and safety (Leistner, 1992). This hurdle effect is of fundamental importance for the preservation of food, since the hurdles in a stable product control microbial spoilage and food poisoning as well as undesirable fermentation.
Increasing consumer demand for fresh quality products is turning processors to the so-called minimally processed products (MP), an attempt to combine freshness with convenience to the point that even the traditional whole, fresh fruit or vegetable is being packaged and marketed in ways formerly reserved for processed products (Tapia et al., 1996). According to these authors, the widely accepted concept of MP refrigerated fruits involves the idea of living respiring tissues. Because MP refrigerated products can be raw, the cells of the vegetative tissue may be alive and respiring (as in fruits and vegetables), and biochemical reactions can take place that lead to rapid senescence and/or quality changes. In these products, the primary spoilage mechanisms are microbial growth and physiological and biochemical changes, and in most cases, minimally processed foods are more perishable than the unprocessed raw materials from which they are made.
The technology for shelf-stable high moisture fruit products (HMFP) is based on a combination of inhibiting factors to combat the deleterious effects of microorganisms in fruits, including additional factors to reduce major quality losses from reactions. In order to select a combination of factors and levels, the type of microorganism and quality loss from reactions that might occur must be anticipated (Tapia et al., 1996). Minimal processing may encompass pre-cut refrigerated fruits, peeled refrigerated whole fruits, sous vide dishes, which may include pre-heated vegetables and fruits, cloudy and clarified refrigerated juices, freshly squeezed juices, etc. All of these products have special packaging requirements coupled with refrigeration (Tapia et al., 1996). These products, apart from special handling, preparation, and size reduction operations, might also require special distribution and utilization operations such as Controlled atmosphere/Modified atmosphere/air flow rate/vacuum storage (O2, CO2, N2, CO, C2H2, H2O controls), computer controlled warehousing, retailing and food service, communications network, etc. HMFP fruits are less sophisticated than MPR fruits and should be priced lower when introduced commercially (Tapia et al., 1996). Careful selection of these processes should of course be made to find the appropriate methods suited to a particular rural or village situation.
An example of the hurdle technology concept is presented in Figure 3.4, in which a comparison of HMFP, IMF and MPR fruits in terms of hurdle(s) involved is made. Example A represents an intermediate moisture fruit product containing two hurdles (pH, and aw). The microorganisms cannot overcome (jump over) these hurdles, thus the food is microbiological stable. In this case, aw is the most relevant hurdle exerting the strongest pressure against microbial proliferation of IMF. In the preservation system of HMFP (example B), it is obvious that aw does not represent the hurdle of highest relevance against microbial proliferation; pH is the hurdle exerting the strongest selective pressure on microflora. As in example A, HMFP does not require refrigerated storage. In example C, the mild heat treatment T(t) is applied and the chemical preservative, P, added affects the growth and survival of the flora. With these considerations in mind, it is possible to understand and anticipate the types of microorganisms that could survive, as well as their behaviour and control in such fruits.
Figure 3.4. Schematic representation of hurdles: water activity (aW), pH, preservatives (P), and slight heat treatment, T(t), involved in three fruit preservation systems. (A) an intermediate moisture fruit product; (B) a high-moisture fruit product; (C) a minimally processed refrigerated fruit product. (Adapted from Tapia et al., 1996)
3.3.3.1 Glucose
Glucose is not a very good humectant due to the lower water holding capacity (WHC), which makes it difficult to obtain the isotherm curve at low aw.
3.3.3.2 Fructose
Fructose has a higher water activity reduction capacity and therefore is more desirable as a humectant in stabilizing food products.
3.3.3.3 Sucrose
Sucrose is one of the most studied sugars and is widely used in food systems, in the confectionary industry, both in the U.S. and Europe, but has a lower water activity reduction capacity compared to fructose.
The water reduction capacity of sugar and salts in their amorphous and anhydrous state at different aw is presented in Table 3.1.
Table 3.1. Water activity reduction capacities of sugars and salts.
|
Moisture content (g H2O/100 g Solids) |
||||||||
|
|
Anhydrous |
Amorphous |
||||||
|
Sugars |
aW = 0.60 |
0.70 |
0.80 |
0.90 |
0.60 |
0.70 |
0.80 |
0.90 |
|
Sucrose |
3.0 |
5.0 |
10.0 |
- |
14.0 |
20.0 |
35.0 |
65.0 |
|
Glucose |
1.0 |
3.5 |
7.5 |
12.5 |
1.0 |
3.5 |
8.0 |
22.0 |
|
Fructose |
14.0 |
22.0 |
34.0 |
47.0 |
18.0 |
30.0 |
44.0 |
80.0 |
|
Lactose |
0.01 |
0.01 |
0.05 |
0.10 |
4.5 |
4.7 |
4.7 |
- |
|
Sorbitol (adsorption) |
17.0 |
22.0 |
37.0 |
76.0 |
2 5.0 |
3 5.0 |
5 5.0 |
110.0 |
|
Corn syrup |
- |
- |
- |
- |
1 4.0 |
2 0.0 |
3 0.0 |
54.0 |
|
Salts |
|
|
|
|
|
|
|
|
|
NaCl (adsorption) |
0.1 |
0.1 |
130.0 |
5 85.0 |
- |
- |
- |
- |
|
NaCl (desorption) |
- |
_ |
385.0 |
5 90.0 |
- |
- |
- |
- |
|
KCl (adsorption) |
0.1 |
0.1 |
0.1 |
0.1 |
- |
- |
- |
- |
|
KCl (desorption) |
- |
- |
0.1 |
580.0 |
- |
- |
- |
- |
Source: Sloan and Labuza (1975).
3.3.3.4 Other humectants
Based solely on the water activity reduction capacity (Table 3.1), sorbitol and fructose are the most desirable humectants. Sucrose has the third best reduction capacity and lactose the poorest. The amorphous form absorbs more water at specific aw than the corresponding crystalline form. As seen in Table 3.1, NaCl and KCl salts appear to be superior humectants at a high range of aw. The increased aw lowering ability exhibited by the salts may be explained by the smaller molecular weight, increasing the ability to bind or structure more water (Sloan and Labuza, 1975).
Other sugars used as humectants in food stability include lactose and sorbitol. The amorphous form absorbs more water at specific aw than the crystalline form. Polyols are better humectants than sugars because of their greater water activity reduction capacity and are less hygroscopic than sugars. The most widely used polyols as humectants in foods are 1,3- butyleneglycol, propylene glycol, glycerol, and polyethylene glycol 400.
3.3.4.1 Organic acids
Organic acids, whether naturally present in foods due to fermentation or intentionally added during processing, have been used for many years in food preservation. Some organic acids behave primarily as fungicides or fungistats, while others tend to be more effective at inhibiting bacterial growth. The mode of action of organic acids is related to the pH reduction of the substrate, acidification of internal components of cell membranes by ionization of the undissociated acid molecule, or disruption of substrate transport by alteration of cell membrane permeability. The undissociated portion of the acid molecule is primarily responsible for antimicrobial activity; therefore, effectiveness depends upon the dissociation constants (pKa) of the acid. Organic acids are generally more effective at low pH and high dissociation constants. The most commonly used organic acids in food preservation include: citric, succinic, malic, tartaric, benzoic, lactic, and propionic acids.
Citric acid is present in citrus fruits. It has been demonstrated that citric acid is more effective than acetic and lactic acids for inhibiting growth of thermophilic bacteria. Also, combinations of citric and ascorbic acids inhibit growth and toxin production of C. botulinum type B in vacuum-packed cooked potatoes.
Malic acid is widely found in fruits and vegetables. It inhibits the growth of yeasts and some bacteria due to a decrease in pH.
Tartaric acid is present in fruits such as grapes and pineapples. The antimicrobial activity of this acid is attributed to pH reduction.
Benzoic acid is the oldest and most commonly used preservative. It occurs naturally in cranberries, raspberries, plums, prunes, cinnamon, and cloves. As an additive, sodium salt in benzoic acid is suitable for foods and beverages with pH below 4.5. Benzoic acid is primarily used as an antifungal agent in fruit-based and fruit beverages, fruit products, bakery products, and margarine.
Lactic acid is not naturally present in foods; it is formed during fermentation of foods such as sauerkraut, pickles, olives, and some meats and cheeses by lactic acid bacteria. It has been reported that lactic acid inhibits the growth of spore forming bacteria at pH 5.0 but does not affect the growth of yeast and moulds.
Propionic acid occurs in foods by natural processing. It is found in Swiss cheese at concentrations up to 1%, produced by Propionicbacterium shermanii. The antimicrobial activity of propionic acid is primarily against moulds and bacteria.
3.3.4.2 Inorganic acids
Inorganic acids include hydrochloric, sulphuric, and phosphoric, the latter being the principal acid used in fruit and vegetable processing). They are mainly used as buffering agents, neutralizers, and cleaners.
3.3.4.3 Fermentation by-products
Fermentation by-products are formed during fermentation of fruits and vegetables, as in sauerkraut processing, pickling, and wine making. One by-product, lactic acid, is formed during fermentation of cabbage or cucumbers. This acid decreases the pH of fruits and vegetables, producing the characteristic flavour of sauerkraut, and acts as a controller of pathogens that may develop in the final fermented product.
3.3.5.1 Sulphites, bisulphites, and metabisulphites
Sodium bisulphite is a potential browning inhibitor in fruit and vegetable products (e.g., peeled potatoes and apples). This preservative when used in food production can delay or prevent undesirable changes in the colour, flavour, and texture of fresh fruits and vegetables, potatoes, drinks, wine, etc. Potassium bisulphite is used in a similar way to sodium bisulphite, and is used in the food industry to prevent browning reactions in fruit and vegetable products.
Sulphites, bisulphites, and metabisulphites of both sodium and potassium together with gaseous sulphur dioxides are all chemically equivalent. Sulphite levels in processed foods are expressed as SO2 equivalents, and range from zero to about 3000 ppm in dry weight. Dehydrated, light coloured fruits (e.g., apples, apricots, bleached raisins, pears, and peaches) contain the greatest amounts in this range. Dehydrated vegetables and prepared soup mixes range from a few hundred to about 2000 ppm; instant potatoes contain approximately 400 ppm. The dose for wine is about 100-400 ppm and for beer about 2-8 ppm. The maximum legal sulphite level in wines permitted by the Food and Drug Administration (FDA) is 300 ppm. In the U.S. most wines have a sulphite level of 100 ppm.
Sulphites are highly effective in controlling browning in fruits and vegetables, but are subject to regulatory restrictions because of adverse effects on health. Sulphites inhibit non-enzymatic browning by reacting with carbonyl intermediates, thereby preventing further reaction. Sulphite levels in foods vary widely depending on the application. Residual levels never exceed several hundred per million but could reach 100 ppm in some fruits and vegetables.
The maximum sulphur dioxide levels in fruit juices, dehydrated potatoes, and dried fruits permitted by the FDA are 300, 500, and 2000 ppm, respectively.
3.3.6.1 Potassium sorbate
Potassium sorbate is a white crystalline powder that has greater solubility in water than sorbic acid, which may be used accordingly in making concentrates for dipping, spraying, or metering fruit and vegetable products. It has antimycotic actions similar to sorbic acid, but usually 25% more potassium sorbate must be used than sorbic acid to secure the same protection.
The common salt of potassium sorbate was developed because of its high solubility in water, which is 58.2% at 20°C (Sofos, 1989). In water, the salt hydrolysis yielded is the active form. Stock solutions of potassium sorbate in water can be concentrated up to 50%, which can be mixed with liquid food products or diluted dips and sprays. Sorbates are effective in retarding the growth of many food spoilage organisms. Sorbates have many uses because of their milder taste, greater effectiveness, and broader pH range (up to 6.5), when compared to either benzoate or propianate. Thus, in foods with very low pH, sorbate levels as low as 200 ppm may give more than adequate protection. The solubility of potassium sorbate is 139 g/100 mL at 20°C; it can be applied in beverages, syrups, fruit juices, wines, jellies, jams, salads, pickles, etc.
3.3.6.2 Sodium benzoate
The use of sodium benzoate as a food preservative has been limited to products that are acid in nature. Therefore, it is mainly used as an antimycotic agent (most yeasts and moulds are inhibited by 0.05-0.1%). The benzoates and parabenzoates have been used primarily in fruit juices, chocolate syrup, candied fruit peel, pie fillings, pickled vegetables, relishes, horseradish, and cheeses. Sodium benzoate is more effective in food systems where the pH is as low as 4.0 or below.
3.3.6.3 Other additives
Other naturally antimicrobial compounds found in fruits and vegetables include:
Vanillin (4-hydroxi-3-methoxybenzaldehyde) is found primarily in vanilla beans and in the fruit of orchids (Vanilla planifola, Vanilla pompona, or Vanilla tahitensis). Vanillin is most active against moulds and non-lactic acid gram-positive bacteria. The effectiveness of vanillin against certain moulds such as A. flavus, A. niger, A. ochraceus, or A. parasiticus has been demonstrated in laboratory media, as well as its effectiveness against yeasts such as Saccharomyces cerevisiae, Pichia membranaefaciens, Zygosaccharomyces bailii, Z. rouxii and Debaryomyces hansenii.
Allicin is an antimicrobial present in the juice vapour of garlic. This compound is effective in inhibiting the growth of certain pathogenic bacteria such as B. cereus, C. botulinum, E. coli, Salmonellae, Shigellae, S. aureus, A. flavus, Rhodotorula, and Saccharomyces.
Cinnamon and eugenol are reported to have an inhibitory effect on the spores of Bacillus anthracis. Also, cinnamon was found to inhibit the growth of the aflatoxin of A. parasiticus. Aqueous clove infusions of 0.1 to 1.0% and 0.06% eugenol were reported to inhibit the growth of germinated spores of B. subtilis in nutrient agar.
Oregano, thyme, and rosemary have been found to have inhibitory activity against certain bacteria and moulds due to the presence of antimicrobial compounds in their essential oils (e.g., terpenes, carvacol, and thymol).
3.3.7.1 The role of heat
The main function of heat in food processing is to inactivate pathogenic and spoilage organisms, as well as enzyme inactivation to preserve foods and extend shelf life. Other advantages of heat processing include the destruction of anti-nutritional components of foods (e.g., trypsin inhibitors in legumes), improving the digestibility of proteins, gelatinization of starches, and the release of niacin. Higher temperatures for shorter periods achieved the same shelf life extension as food treated at lower temperatures and longer periods, and allowed retention of sensory and nutritional properties.
3.3.7.2 Hot water
Hot water plays an important role in the sanitation of food products before processing. Some food products are treated with hot water to eliminate insects, and to inactivate microorganisms and enzymes. Foods are retained in a water blancher at 70-100°C for a specific time and then removed to a dewatering and cooling system.
3.3.7.3 Steam
Steam is a more effective means than hot water for blanching foods such as fruits and vegetables. This method is especially suitable for foods with large areas of cut surfaces. It retains more soluble compounds and requires smaller volumes of waste for disposal than those from water blanchers. This is particularly so if air-, rather than water-cooling is used. Furthermore, steam blanchers are easier to clean and sterilize.
3.3.7.4 Effects of heat on aerobic and anaerobic mesophylic bacteria, yeasts, and moulds
Temperatures ranging from 10 to 15°C above the optimum temperature for growth will destroy vegetative cells of bacteria, yeasts, and moulds. Most vegetative cells, as well as viruses, are destroyed when subjected to temperatures of 60 to 80°C for an appropriate time. Somewhat higher temperatures may be needed for thermophilic or thermoduric microorganisms. All vegetative cells are killed in 10 min at 100°C and many spores are destroyed in 30 min at 100°C. Some spores, however, will resist heating at 100°C for several hours.
