water activity of dehydrated foods

1. MICROBIAL SPOILAGE MECHANISM OF DRIED/DEHYDRATED VEGETABLE
PRODUCTS DUE TO HIGH WATER ACTIVITY DURING STORAGE
Singham Pragati*, Genitha T. R**., Birwal Preeti
*M.Tech Food Process Engineering and Management, Department of Food Engineering, (NIFTEM, Haryana)
** Assistant Professor, Department of Food Process Engineering, (SHIATS, Allahabad)
*pragati.niftem@gmail.com , **genitha@ahiats.edu.in
Table 1. Relation between drying method and microbial reduction for vegetables
Drying method Reduction in
microbial count
Type of feed Examples
Tray 1.5 log Diced, sliced, chunks Pumpkin, carrot, tomato,
garlic, onion, cauliflower,
lettuce, ginger, celery,
mushroom
Drum 2 log Puree, paste, slurry Potato, tomato
Fluidized 2 log Whole(uniform), Peas,
Freeze 3 log Sliced, diced, juices, Tomato, carrot,
Spray 4.5 log Juices Tomato juice, carrot juice
Microwave-Augmented Freeze
Drying
3-4 log Diced, slices, paste, juices Tomato, peas, broccoli,
Ultrasonic 4.5 log Whole, sliced Onions, garlic
Explosion puffing 5.2 Chunks Mushroom
Refractance window 4-6 log Puree Tomato, sugarbeet,
Table 2. Water activity of selected vegetables (Okos et.al, 1992)
Dehydrated vegetables Moisture content (% d.b) aw Temperature (°C)
Potato 5
10
15
0.12
0.47
0.69
36
Onion 5
10
15
0.40
0.62
0.74
27
Celery 5
10
15
0.43
0.59
0.69
25
Tomato 10
20
30
0.44
0.61
0.69
27
Peas 5 0.25 25
Dried vegetable mix 5 0.21 25
aw + 0.1 = Shelf life – 2x or 3x
Microbes ability to regain moisture i.e tolerance
for low water activity
(Osmoregulation / Osmoadaptation)
Microbial invasion as a result of
hygroscopic nature
Water migration from environment
to cytoplasm via water channels
Maintenance of membrane
turgor pressure
Establishment of internal
equilibrium “Homeostasis”
Microbes start mutiplying i.e
ceased lag phase become active