Ph and aw

water activity and PH

1. Factors affecting microbial growth in food (a) Intrinsic factors: They include: pH, water activity, oxidation reduction potential, nutrient content, antimicrobial contents, biological structure. (b) Extrinsic factors: Are factors external to the food that affect microbial growth. Temperature,Concentration of gases in the environmentand RH

2. Intrinsic factors: These are inherent in the food. -They include: Hydrogen ion concentration (pH) will see this Moisture content Wa two factors Nutrient content of the food Antimicrobial substances Biological structures

3. Hydrogen ion concentration (pH) Most bacteria grow best at neutral or weakly alkaline pH usually between 6.8 and 7.5 . Some bacteria can grow within a narrow pH range of 4.5 and 9.0 , e.g. Salmonella

4. cont Other microorganisms especially yeasts and molds and some bacteria grow within a wide pH range, e.g. molds grow between 1.5 to 11.0, while yeasts grow between 1.5 and 8.5.

5. Introduction What is PH? It is measure of the acidity or alkalinity of a solution in water. The acidity or alkalinity of a water solution is determined by the relative number of hydrogen ions (H+) or hydroxyl ions (OH-) present.

6. Cont Acidic solutions have a higher relative number of hydrogen ions, while alkaline (also called basic) solutions have a higher relative number of hydroxyl ions. Acids are substances which either dissociate (split apart) to release hydrogen ions or react with water to form hydrogen ions.

7. cont Bases are substances that dissociate to release hydroxyl ions or react with water to form hydroxyl (OH)ions.

8. PH scale

9. Concentration of H and OH Ion

10. cont Increasing the acidity of foods, either through fermentation or the addition of weak acids, has been used as a preservation method since ancient times. In their natural state, most foods such as meat, fish, and vegetables are slightly acidic while most fruits are moderately acidic.

11. cont The pH is a function of the hydrogen ion concentration in the food: i.e. pH = -log10 [H+]

12. pH values of some food products Food type Range of pH values Beef 5.1 - 6.2 Chicken 6.2 – 6.4 Milk 6.3 – 6.8 Cheese 4.9 - 5.9 Fish 6.6 - 6.8 Oyster 4.8 - 6.3 Fruits < 4.5 (most < 3.5) Vegetables 3.0 – 6.1

13. cont Microorganisms that are able to grow in acid environment are called acidophilic microorganisms. These microorganisms are able to grow at pH of around 2.0 Yeasts and molds grow under acidic this env’t.

14. cont Other microorganisms such as Vibrio cholerae are sensitive to acids and prefer alkaline conditions. Most bacteria are killed in strong acid or strong alkaline environment except Mycobacteria .

15. Minimum and maximum pH for growth of some specific microorganism Microorganism Minimum Maximum Escheri c hia coli 4.4 9.0 Salmonella enterica serovar typhi 4.5 8.8 All bacteria 4.0 9.0 Molds 1.5 11.0 Yeast 1.5 8.5

16. cont Examples of high-acid foods include jams and jellies, pickles and most fruits. This type of foods has PH value less or equal to 4.5. Low acidic foods include: vegetables, legumes, beas,peas,carrot,corn,onion and egg w hite which has greater or equal to 4.5.

17. Types of Water in Food Most natural foods contain water up to 70% of their weight or greater unless they are dehydrated. Fruits and vegetables contain water up to 95% or greater. Water in foods and biological materials can be grouped in three categories : free water entrapped water, and bound water

18. cont Free water- is easily removed from foods or tissues by cutting, pressing, squeezing or centrifugation. Entrapped water is- immobilized within the lattices of large molecules, capillaries, or cells, but it is released during cutting or damage, it flows freely.

19. cont It may be entrapped in foods such as pectin gels, fruits, vegetables, and so on. Entrapped water- although not free flowing, does have the properties of free water. Free and entrapped water together may be considered bulk water.

20. cont Entrapped water has properties of free water and no properties of bound water. Bound water usually is defined in terms of the ways it is measured; different methods of measurement give different values for bound water in a particular food.

21. cont This water will behave almost like pure water during food processes. It is easily removed by drying, easily converted to ice during freezing, and available as a solvent.

22. Some characteristics of bound water include: It is not free to act as a solvent for salts and sugars. It can be frozen only at very low temperatures (below freezing point of water). It exhibits essentially no vapor pressure. Its density is greater than that of free water

23. Moisture content The amount of free water in a food medium. The amount of free water is important for growth of microorganisms.

24. cont If there is lack of free water microorganisms will not grow. Water activity is defined as the vapour pressure of a food substance to that of water at the same temperature. ( Aw = VPFood/VPWater)or A w = P / P0

25. cont The water activity of pure water is equal to 1.0 Food products have a water activity of less than 1.0. A saturated salt solution has a water activity of 0.75. Salting and drying reduces the water activity of a food product

26. Water activity of some food products Food Product Water activity Raw meat and milk 0.99- 1.0 Luncheon meat 0.95 Boiled ham, sliced bacon 0.90 Dried grains 0.80

27. cont Growth of microorganisms is greatly affected by the level of water activity (A w ) in the food. Inhibition of growth occurs if the water activity for food is lowered beyond an organism’s minimum level of water activity that is necessary for growth.

28. cont Microorganisms have varying minimum water activity requirements that supports their growth in food.

29. Minimum water activity that supports growth of some microorganisms Microorganism Water activity Clostridium botulinum , Bacillus cereus, Pseudomonas aeruginosa , Salmonella spp . 0.95 0.95 0.95 0.95 Staphylococcus aureus (anaerobic), Candida spp., Saccharomyces 0.90 Staphylococcus aureus (aerobic) 0.86 Penicillium spp. 0.82 Most spoilage yeast 0.88 Most spoilage molds 0.80 Osmotic yeast 0.70

30. cont Water activity is a measure of how efficiently the water present can take part in a chemical or physical reaction. If half the water is so tightly bound to a protein molecule that it could not take part in a hydrolysis reaction the overall water activity would be reduced.

31. cont The tightly bound water has no tendency to escape from a food as a vapor and therefore exerts no partial pressure and has an effective water activity of zero.

32. cont Concentration and dehydration processes are conducted primarily for the purpose of: decreasing the water content of a food simultaneously increasing the concentration of solutes and decreasing perishability.

33. cont Water activity refers to the water in the food that is available (free) to support microbial growth. It is measured with a water activity meter in a scale from 0 to 1.

34. cont Foods with values below 0.85 are non-hazardous regardless of their acidity, because they do not support the growth of harmful bacteria. Examples are dried and semidried products.

35. Water activity meter

36. cont A critical factor that determines the stability or shelf life of foods. Most bacteria, for example, do not grow at water activities below 0.91, including pathogens such as Clostridium botulinum.

37. cont Below 0.80 most molds cannot be grown and below 0.60 no microbiological growth is possible. However, there remain a number of food spoilage microbes that can grow within the range 0.8 - 0.6

38. cont E.g. Staphylococcus aureus, a common food poisoning organism, can grow down to this relatively low water activity. Intermediate-moisture foods, which have aw values between 0.6 and 0.9, have drawn considerable attention because they are palatable without the need to rehydrate them.

39. Table: typical growth limits function of w a aw= 0.91-0.95 = most bacteria aw = 0.88 = most yeast aw = 0.80 = most mushroom aw = 0.75 = halophile bacteria aw = 0.70 = osmiophil yeast aw = 0.65 = xerophile mushroom

40. cont Foods which have high level of water activity, the shelf life is limited mainly by microbiological activity. Products with aw levels below about 0.70 may well be stable microbiologically and consequently have a longer shelf life, but enzyme related breakdown processes is occurred.

41. cont It is mainly determine chemical reactions that affects the quality and stability of these foods. Water activity control is an important factor for the chemical stability of foods.

42. cont Most foodstuffs contain carbohydrates and proteins and are therefore subject to non-enzymatic browning reactions (Maillard reaction). The Maillard reaction gets stronger at increasing aw values and reaches its peak at aw = 0.6 to 0.7 with further increase of aw this reaction gets rapidly weaker.

43. cont Most enzymatic reactions are slowed down at aw values below 0.8. Some of these reactions occur even at very low aw values. However, as many foodstuffs are thermally treated during their processing, enzymatic spoilage is usually of very little importance.

44. cont

45. SORPTION PHENOMENA Below moisture content of about 50 % the water activity decreases rapidly and the relationship between water content and relative humidity (water activity) is represented by the sorption isotherms.

46. cont Sorption  is a physical and chemical   process  by which one substance becomes attached to another . Absorption – the incorporation of a substance in one state into another of a different state (e.g., liquids being absorbed by a solid or gases being absorbed by a liquid).

47. cont The adsorption isotherms are required for the observation of hygroscopic products and the desorption isotherms are useful for investigation of the process of drying . The adsorption and desorption processes are not fully reversible.

48. Adsorption and Desorption Isotherms

49. cont Sorption isotherms usually have a sigmoid shape and can be divided in to three areas that correspond to different conditions of the water present in the food. As water is added (resorption), sample composition moves from Zone I (dry) to Zone III (high moisture) and the properties of water associated with each zone differ significantly. .

50. Note : Water present in Zone I of the isotherm is most strongly sorbed and least mobile Water present in zone II is slightly less mobile than bulk water Zone III water is freezable, available as a solvent, and readily supports the growth of microorganism. It is referred to as bulk-phase water.

51. Hysteresis The SI prepared by addition of water (resorption or adsorption) to a dry sample will not necessarily be super imposable on an isotherm prepared by desorption. This lack of superimposability is referred to as “hysteresis”. SIs of polymers, glasses of low molecular- weight compounds, and many foods exhibit hysteresis.

52. cont The magnitude of hysteresis, the shape of the curves, and the inception and termination points of the hysteresis loop can vary considerably depending on factors such as: nature of the food, the physical changes it undergoes when water is removed or added, temperature, the rate of desorption, and the degree of water removal during desorption.

53. Hysteresis of SI

54. a w and food stability a w is: A critical factor that determines the stability or shelf life of foods. Most bacteria, for example, do not grow at water activities below 0.91, including pathogens such as Clostridium botulinum . Below 0.80 most molds cannot be grown and below 0.60 no microbiological growth is possible.

55. cont However, there remain a number of food spoilage microbes that can grow within the range 0.8 - 0.6. Eg. Staphylococcus aureus , a common food poisoning organism, can grow down to this relatively low a w . By measuring a w , it is possible to predict which mos will and will not be potential sources of spoilage. Further, a w can play a significant role in determining the activity of enzymes and vitamins in foods and can have a major impact on their color, taste, and aroma.

56. Note: Many preservation processes attempt to eliminate spoilage by lowering the availability of water to mos. Reducing the amount of free- -or unbound --water also minimizes other undesirable chemical changes that occur during storage.

57. cont The processes used to reduce the amount of free water in consumer products include techniques like concentration, dehydration and freeze drying (lyophilization). Freezing is another common approach to controlling spoilage. Water in frozen foods is in the form of ice crystals and therefore unavailable to mos and for reactions with food components .

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