13504-00 AN, Water Activity The Key to Pet Food Quality ... - AquaLab

6. ročník – PřírodopisOdpovědi:Téma: Prvoci, žahavce, ploštěnci, hlístiA- na V pobřeží, kterého kontinentu se nachází Velký bariérovýútes - AustrálieB- zapouzdřená larva tasemnice v mase skotu - boubelC- klidové stádium, při kterém se organismus zapouzdří do -cystyČ- části těla tasemnice - článkyD- jaký způsob života mají žahavci - dravýE- vztahy mezi živou a neživou přírodou se zabývá vědní oborEkologieF- děj, při kterém se přeměňuje oxid uhličitý a voda na glukosu akyslík se nazývá - fotosyntézaG- vyhledávač na internetu- googleH- červy s kulatým tělem - hlístiCH- hnědá řasa - chaluhaJ - období v roce, kdy se rozmnožuje trepka nepohlavně - jaroK- mořští živočichové mající vápenité schránky a tvořící útesykoráliL- kdo sestavil mikroskop - LeeuwenhoekM- ploštěnec, který se živí krví ovcí v játrech - motoliceN- přisedlý žahavec, žijící ve sladké vodě - nezmarO - hermafrodit - obojetníkP - nervová soustava ploštěnců - provazcovitáR - hlíst parazitující u dětí o velikosti 1 cm - roup dětskýŘ – vodní rostliny - řasyS - sladkovodní dírkonožci - slunivkyŠ - 25 cm parazit člověka, psů, koček, který má kulaté tělo -škrkavkaT - cizopasník s plochým tělem - ploštěnciU - druh houby s plodnicí - ucháčV- z vajíčka přímo dospělý jedinec, jde o přímý…vývojW- anglicky voda - waterZ - chlorofyta - zelené řasyŽ - kmen živočichů, který loví kořist žahavými buňkami - žahavci

Table 2. Water activities of common pet food products as measured usingDecagon Devices’ AquaLab chilled mirror water activity instrument.Product Water Activity Moisture Content(% d.b.)Moist Canned Pet Food 1 0.994 79.6Moist Canned Pet Food 2 0.830 24.0Intermediate Moisture Pet Food 1 0.823 13.70Intermediate Moisture Pet Food 2 0.791 14.40Intermediate Moisture Pet Food 3 0.679 8.43Intermediate Moisture Pet Food 4 0.669 13.0Intermediate Moisture Pet Food 5 0.525 9.21Dry Pet Food 1 0.493 8.59Dry Pet Food 2 0.459 7.79Dry Pet Food 3 0.236 4.40handling, packaging and storage to maintain propertextural qualities.Because water activity is a measure of the energystatus of the water, differences in water activity arethe driving force for moisture migration. Bydefinition, water activity dictates that moisture willmigrate from a region of higher a w to a region oflower a w , but the rate of migration depends on manyfactors. Many pet foods are multi-componentproducts with heterogeneous regions. If theseregions are at different water activities, water willmigrate between the components until they reach anequilibrium water activity, regardless of theirmoisture contents (Brandt, 1996;Katz and Labuza,1981). For example, if two components of a multicomponentpet food product both have 15%moisture content but are different water activities,moisture will be exchanged, even though theirmoisture contents are the same. This moisturemigration could lead to textural or microbialspoilage problems.There are a number of products on the market withmulti-textured characteristics that use wateractivity. These products combine a hard, dry-bakedpet food and a soft, moist pet food. The hard, drycomponent has the advantage of teeth cleaning, butis less palatable than a soft, moist food. The soft,moist component may be highly palatable, but lacksthe abrasive teeth cleaning property. When the twocomponents are mixed, they equilibrate to acommon equilibrium water activity during storage.This equilibrium a w must allow the dry componentto remain hard and crunchy while leaving the softcomponent moist and tenderMeasurement of WaterActivityWater activity is measuredby equilibrating the liquidphase water in the samplewith the vapor phase waterin the headspace of a closedchamber and measuring thevapor pressure of theheadspace. New instrumenttechnologies have vastlyimproved speed, accuracyand reliability ofmeasurements. Twodifferent types of wateractivity instruments arecommercially available. Oneuses chilled mirror dewpoint technology while theother utilizes relative humidity sensors that changeelectrical resistance or capacitance; each hasadvantages and disadvantages. The methods vary inaccuracy, repeatability, speed of measurement,stability in calibration, linearity, and convenience ofuse.In a chilled mirror dewpoint system, a sample isplaced in a sample cup which is then sealed againsta sensor block. Inside the sensor block is adewpoint sensor, an infrared thermometer, and afan. The dewpoint sensor measures the dewpointtemperature of the air and the infrared thermometermeasures the sample temperature. From thesemeasurements, the relative humidity of theheadspace is computed as the ratio of dewpointtemperature saturation vapor pressure to saturationvapor pressure at the sample temperature. When thewater activity of the sample and the relativehumidity of the air are in equilibrium, themeasurement of the headspace humidity gives thewater activity of the sample. The fan is to speedequilibrium and to control the boundary layerconductance of the dewpoint sensor.The major advantages of the chilled mirrordewpoint method are speed and accuracy. Chilledmirror dewpoint is a primary approach tomeasurement of relative humidity based onfundamental thermodynamic principles. Chilledmirror instruments make accurate (±0.003a w )measurements in less than 5 minutes. Since themeasurement is based on temperaturedetermination, calibration is unnecessary, butrunning a standard salt solution checks properfunctioning of the instrument. If there is a problem,

the mirror is easily accessible and can be cleaned ina few minutes. For some applications, fast readingsallow manufacturers to perform at-line monitoringof a product’s water activity.Other water activity instruments use resistance orcapacitance sensors to measure relative humidity.These sensors are made from a hygroscopicpolymer and associated circuitry that gives a signalrelative to the equilibrium relative humidity (ERH).Commercially available instruments measure overthe entire a w range with an accuracy of ±0.015a w .Since these instruments relate an electrical signal torelative humidity, the sensor must be calibratedwith known salt standards. In addition, the ERH isequal to the sample water activity only as long asthe sample and sensor temperatures are the same.Accurate measurements require good temperaturecontrol or measurement. Advantages of capacitancesensors include simple design and inexpensiveimplementation.ConclusionWater activity is an effective tool for maintainingthe stability, quality, and safety of pet food. Inaddition, in the intermediate moisture region, whichincludes a majority of the pet food, changes inmoisture content that are undetectable due to thelimited accuracy of moisture content analyses canresult in large changes in water activity andconsequently, changes in stability. This can bedisconcerting when pet food is dried to a moisturecontent specification and stability changes suddenlyoccur even though a moisture content change is notdetected. These stability changes can be predicted ifa water activity specification is used. Water activityis a fast, inexpensive, and accurate way of assuringthe quality and safety of pet food. It can easily beincorporated by any production facility or qualitycontrol laboratory.The pet food and feed industries have long usedwater activity to create novel products and predictshelf life, safety, and quality. Determination ofwater activity during manufacture allows tightcontrol of pet foods and feedstuffs found to be athigh risk for deterioration. Without the use of wateractivity, the pet food industry would have a hardtime developing novel new products or producingnutritious, high quality, stable food.Reference ListBeuchat,L.R. 1983. Influence of water activity on growth, metabolic activities, and survival of yeasts and molds. J FoodProt 46:135-141,150.Brandt,L. 1996. Bound for success. Controlling water activity gives technologists the edge in developing safe, shelf-stablefoods. Food Formulating 2:41-48.Katz,E.E. and T.P.Labuza. 1981. Effect of water activity on the sensory crispness and mechanical deformation of snackfood products. J Food Sci 46:403-409.Lowe,J.A. and S.J.Kershaw. 1995. Water activity-moisture content relationship as a predictive indicator for control ofspoilage in commercial pet diet components. Animal Feed Science and Technology 56:187-194.Scott,W.J. 1957. Water relations of food spoilage microorganisms. Adv Food Res 7:83-127.()*$%& *$+%$,% !&-!$$!+$# !"#$%& % '''#$%& %