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Subsections
Drying is a method of food preservation that works by removing water from the
food, which inhibits the growth of microorganisms and hinders quality decay.
Drying food using sun and wind to prevent spoilage has been practised since
ancient times. Water is usually removed by evaporation (air drying, sun drying,
smoking or wind drying) but, in the case of freeze-drying.[1] Cereals are often dried to 14% w/w, while oilseeds,
to 12.5% (soybeans), 8% (sunflower) and 9% (peanuts). Drying is carried out
as a requisite for safe storage, in order to inhibit microbial growth.
However, low temperatures in storage are also highly recommended to avoid
degradative reactions and, especially, the growth of insects and mites. A good
maximum storage temperature is about 18°.
Spray drying is used for milk powders, freeze-drying for instant coffee, solar
drying in combination with salting is ideal for dried fish. [2]
[3]
Maftoonazad 2010 describes the technology of osmotic dehydration used as a
preparation step to further processing of foods. The foods are immersed in
osmotic solutions, resulting in partial drying. During or after osmotic
treatment microwave heating, vacuum, high pressure, pulsed electric field are
used to to improve dehydration. Mild temperatures are employed. The moisture is
removed by a liquid diffusion process, and phase change occurring in the other
drying processes are avoided, resulting in improved quality of the final
product. [4]
Nonthermal processing of fruit and vegetable has been revealed as a useful tool
to extend their shelf-life and quality as well as to preserve their
nutritional and functional characteristics. Sánchez-Moreno and colleagues 2009
describe the development on the last ten years leading to appropriate
processing parameters for a safe use of these technologies.
Knorr and colleagues 2002 stress that hydrostatic pressure, electric fields,
ultrasonics, supercritical CO2, in combination with conventional processes may
result in improved food preservation methods. [5]
Non-thermal processes such as high-intensity pulsed electric field (HIPEF)
treatments may be applied to pasteurize plant-based liquid foods to preserve and
extend their shelf-life and retain minor bioactive compounds. Elez-Martínez
and colleagues 2009 discuss the effects of HIPEF treatments on components of the
Mediterranean diet. [6] [7]
Crapo and colleagues 2010 report the production of freeze-dried cubes of three
species of Pacific salmon. Moisture content was less than10% and a(w) below 0.4.
Freeze-drying time of 9 h resulted in cubes which may be used for ready-to-eat
soups, as snack food, salad topping, and baby finger-food. [8]
Nail and colleagues 2002, analysing frozen systems, recommend to keep the
formulation as simple as possible and reduce the content of buffer and salt to
a minimum. Proteins may be stabilised by an amorphous excipient, such as a
disaccharide, and a crystallizing excipient, such as glycine. Advices on how
to improve freezer construction and heat transfer is being given, eliminating
metal trays. Lower pressures have to fit the temperature at which the
freeze-drying is being accomplished, best vapour exchange take place avoiding
filling of the container about more than half of its volume.
Analytical methodology for characterization of frozen systems and freeze-dried
solids should be developed, and the temperature dependence of glass
transition-associated mobility, particularly at temperatures below the glass
transition should be looked on. Controlling the degree of supercooling during
freezing could improve product quality. [9]
Mejia-Meza and colleagues 2010 looked at dried raspberries prepared by freeze
drying, microwave-vacuum, hot-air drying, and a combination of hot-air drying
and microwave-vacuum drying methods. The authors found that ellagic acid and
quercetin were present in the largest concentrations, however, antioxidant
activity, compared to fresh rasperries, was reduced by dehydration. Extracts
from dried raspberries by the combination of hot-air drying and
microwave-vacuum drying method presented the best results of all other methods
regarding the activity of the polyphenols.
Leusink and colleagues 2010 report highest retention of anthocyanins and
antioxidant activity of dehydrated cranberries prepared by vacuum-microwave
drying and freeze-drying than dried by hot air drying. [10] [11]
Supplementation in nutrient media of magnesium and calcium may improve the
stability of cell membranes and dehydration stress tolerance of Saccharomyces
cerevisiae, improving the production of active dry yeast preparations for food
and fermentation industries.
Rappoport and colleagues 2009 suggests to incubate Saccharomyces cerevisiae
culture in 0,75 M lactose solution to increase the stability of the cells
during dehydration. The authors report an increase of viability and a decrease
in plasma membrane permeability during rehydration, which reduces leakage from
the cells.[12]
Trehalose is responsible for the survival of anhydrobiotic organisms
protecting proteins and membranes from damage caused by freezing, high
temperatures and dehydration. Yeasts accumulate large amounts of trehalose.
[13]
Leslie and colleagues 1994 write that trehalose is needed to lower the
temperature of the dry gel to liquid crystal phase transition in yeast from
around 60 degrees C to about 40 degrees C. Trehalose and warm water at above
40 degrees C avoid yeast cells to pass through a phase transition during
rehydration. [14]
Saccharomyces cerevisiae cells have a trehalose carrier in the plasma membrane
protecting both sides of the membrane. According to de-Araujo the trehalose
transport through the membranes of the yeast is performed by a system with a
low-affinity uptake component and a high-affinity H(+)-trehalose symporter
regulated by glucose repression.[15] [16]
A study of the survival of Enterobacter sakazakii in milk-based and soybean-based
powdered infant formulas found that the number of inoculated bacteria decreased
significantly in all formulas in the aw range of 0.25 to 0.50 during storage for
1 to 6 month at 21 or 30 degrees C. The number of living bacteria was reduced at
aw 0.43 to 0.50 compared to aw 0.25 to 0.30. The authors of the study, Gurtler
and Beuchat 2007, concluded that death rate of Enterobacter sakazakii in powdered
infant formula is directly proportional to the rise of the storage temperature
and aw. No difference was found between milk-based and soy-based formulas. [17]
A study at a processing plant for powdered infant formula, during 2005 and 2006,
found 40% of the processing environment. (vacuum cleaners and filtering
(sieving) machines, fluids from the drains and swabs from contact surfaces) were
found positiv for Cronobacter spp. Reich and colleagues 2010 stress the
importance of monitoring of the processing environment to insure high safety
and hygiene of powdered infant formulas.
[18]
Oonaka and colleagues 21010 examining powdered infant formula milk found 24.2%
of the samples positive for Enterobacteriaceae, and 6.6% of samples contained Enterobacter sakazakii. Enterobacter sakazakii was highly sensitive to a series
of tested antibiotics. [19]
Cronobacter sakazakii, also known under the older denomination of 'Enterobacter
sakazakii' is found in powdered infant formula and other powdered foods. The
bacteria developed resistance to desiccation and osmotic stresses, surviving more
than two years in the desiccated state. When dried foods are reconstituted, it grows rapidly, becoming a risk to immunocompromised infants. Critical food
production should improved control measures focusing on Cronobacter sakazakii. [20]
Bacillus cereus may cause mild disease with short duration of symptoms. Bacillus
cereus was detected in uncooked pizza bases, cooked pizzas, cooked meat pies,
cooked sausage rolls, processed meats, and raw diced chicken. All samples of skim
milk powder were negative for Bacillus cereus. Eglezos and colleagues 2010
suggest that spores of Bacillus cereus may have been introduced at the final
product by numerous ingredients. [21]
Dehydrated potato were found positive for Bacillus cereus in 10 to 40% of examined samples. B. cereus spores are able to survive drying of the raw
vegetable and may germinate in the rehydrated mashed potato product at
temperatures above 10 degrees C and below 60 degrees C to levels exceeding 10(4)
CFU g(-1) which were responsible for foodborne diseases caused by rehydrated
potato flakes.
[22]
A case of infant botulismus in UK was found to be caused by spores of Clostridium
botulinum from powdered infant formula. In USA 78% in market-purchased samples
of powdered infant formula were found by Barash, Hsia and Arnon 2010 to contain
clostridial spores. Clostridium sporogenes was the most often found, followed by
Clostridium butyricum. More10 other soil-dwelling clostridial species were
identified, but no neurotoxigenic bacteria were found. These findings suggest that neurotoxigenic clostridial spores, however, may also be present in powdered
infant formula of the US market, write the authors.
[23]
Chen and colleagues 2009 verified safe guidelines for manufacturers and
consumers to prepare, handle and store dry infant formula in particular to
reduce the Cronobacter spp.risk.
Contaminated milk powder and dried infant formula are linked to meningitis,
necrotizing enterocolitis and bacteremia in premature babies. To reduce this risk
of contamination with Cronobacter spp., the authors recommend to keep larger
volumes of reconstituted dried infant formula at higher than70 degrees C and
minimize storage time. Unused reconstituted formula should be stored at below 4
degrees C. [24]
Canada asked for a revision of the Code of Practice for Powdered Formulae for
Infants and Young Children at the Codex Alimentarius Committee of Food Hygiene
[25], following contamination incidences with powdered
infant formulae Cronobacter spp (Enterobacter sakazakii) in 2003.
Canada developed Good Manufacturing Practices (GMPs) for Infant Formula in Canada. [26]
The FDA Title 21: Food and Drugs: Current Good Manufacturing Practive in
Manufacturing, Packing, or Holding Human Food contains GMPs for food production
in general. [27] [25]
This Code of Practice focuses on the microbiological hazards, and specifically
on Salmonella and E. sakazakii (Cronobacter species). It is a combination of
control measures should effectively control the identified microbial hazards
in Powdered Formulas (PF).
Outbreaks of E. sakazakii (Cronobacter species) infections have led to the
link with PF, especially in the context of neonatal intensive care setting. E.
sakazakii (Cronobacter species) is known to be present at low concentration in
a proportion of powdered formulas.
For infants at greatest risk, e.g. neonatal intensive care settings,
commercially sterile liquid infant formula should be used if available unless
the attending physician recommends otherwise. If a non commercially sterile
feeding option is chosen, an effective point-of-use decontamination procedure should be used.
There are four routes by which E. sakazakii(Cronobacter species) and
Salmonella can enter PF: 1) through the ingredients added in dry mixing
operations during the manufacturing of PF, 2) through contamination of the
formula from the processing environment in the steps during or following the
drying, 3) through contamination of the PF after the package is opened, and 4)
through contamination during or after reconstitution by the caregiver prior to
feeding. E. sakazakii(Cronobacter species) may be found in many environments
such as food factories, hospitals, institutions, day-care facilities and homes.
In manufacturing, the organism may gain access to the processing line and
product, since current technology cannot completely eliminate this organism from
the manufacturing environment. [26]
These Good Manufacturing Practices (GMPs) establish general requirements for
effective control of ingredients, formulations, processes, facilities and
equipment used for production of infant formula products.
Formal HACCP and ISO 9000 programs are not mandatory for infant formula
establishments at this time unless required by regulation. However, all infant
formula fabricators and manufacturers are required to have in place effective
GMPs and related quality control procedures which provide equivalent results, and
which satisfy all applicable regulatory requirements. This GMP standard
encourages the application of HACCP and ISO 9000 principles and programs in
infant formula establishments as a means to identify and control critical control
points, to prevent contamination and failure incidents, and to continuously
improve products and processes. [28]
Asefi and Mosaffari studied preservation methods of fried red Azershahr
variety onion chopped up in 2mm slices. Frying was conducted at 150 degrees C,
followed by drying to a humidity of 3-4% using hot-air dryer at 70 degrees C,
and microwave, packed in aluminium foil under nitrogen gas, sealed and stored
at room temperature and at -18 degrees C.
The authors report high bacterial counts after 6 month and sensory properties
and vitamin C content decreased according to the bacterial count. The best sample was oven-dried, packed in aluminium foil under inert gas, and kept in a freezer
up to 6 month. [1]
Vega and Roos report that sodium caseinate (NaCas) is more effective than whey proteins (WP) to stabilise emulsions of dehydrated dairy and dairy-like products. Lactose improves the emulsion stabilisation effect of sodium caseinate, when used in 1:1 ratio, but does not improve whey protein stabilised powders. Lactose forms solid-like (or glassy) capsules during sudden dehydration. Crystallisation of lactose affects the storage stability of dehydrated emulsions at humidities of 75% and up. Maltodextrins or gum arabic may improve storage stability but alters the emulsion droplet size after reconstitution.
According to Vignolles and colleagues 2009 the emulsion droplet size and droplet aggregation depended on the homogenizing pressures and were also affected by spray atomization which results in increased viscosities due to the resulting higher aggregation. Powders from unhomogenized emulsions showed greater free fat content. The authors stress that the free fat content seemed to have a greater influence than surface fat on powder physical properties, except for wettability. [29] [30]
According to Hussain and colleagues 2011 rehydration of casein powder was strongly influenced using 3 and 6% NaCl and 0.75 and 1.5% CaCl(2) (wt%) as rehydration media. The authors found that low salt concentration provides quick wetting, swelling, and long dispersion stage. High salt concentration, however, presented a short dispersion stage. Salt concentrations had less impact on rehydration of whey protein powders which rehydrated normally, however, at high concentrations of CaCl(2) no turbidity stabilization was observed due to protein denaturation, report the authors. [31]
Lactose glass transition is mainly responsible for milk powders ageing. Lactose crystallization modifies the microstructure and chemical composition of the surface of powder particles, decreasing flowability, solubility, emulsifying, and foaming properties. The particles collapse and caking occur. Mechanical stresses trigger proteins unfolding. Ageing due to storage temperature, relative humidity time, milk components, and physical state increase molecular mobility Maillard reaction and oxidation increase protein interactions and aggregations reduces solubility, emulsifying and foaming properties. [32]
SilalaiP and Roos 2010 determined glass transition temperature (T(g)), sticky-point temperatures, water sorption isotherms, lactose contents and identified proteins of milk powders. Solids composition and water were found to affect the T(g) and stickiness behaviour. Precrystallization of lactose decreased the sticky point temperature when reduced protein was present. Increased protein decreased stickiness at all water activities. The authors propose to use glass transition to describe time-dependent stickiness and crystallization phenomena. Glass transition may be used to control and reduce stickiness of dairy powders.
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