Presentacion parte1b Agua - Conceptos_Generales.pdf

8/12/2019 Presentacion parte1b Agua - Conceptos_Generales.pdf

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Material de Estudio

Curso: Qumica de AlimentosAvanzadaCdigo: 745012

Escuela de Ingeniera de Alimentos

Posgrado en Ingeniera de Alimentos

Harold Acosta Z., MSc., Ph.D.[[email protected]]

Grupos de Investigacin:

Seguridad Alimentaria, Innovacin y Competitividad [SEGALINNCO]Investigador asociado del Grupo de Investigacin en Calidad y Productividad en las

Organizaciones [GICPO]

Miembro del Instituto Colombiano de la Panificacin y los Cereales, Medelln

Water-Solute Interactions

Macroscopic Level (Water Binding,Hydration, and Water HoldingCapacity)

Molecular Level: Bound Water:

equilibrium water content atappropriate T and low relative

humidity.

Conceptos generales

Fuerzas de estabilidad

Sistemas coloidales hidroflicos:

Estabilidad por hidratacin [agua atrada hacia la superficie de las partculas formanbarrera que impide el contacto entre estas].Estabilidad de partculas hidrfobas:

Doble capa elctrica [partculas suspendidas en agua tienen carga elctrica(generalmente negativa) que atrae iones de carga opuesta. Forma capa de iones decarga contraria (contraiones) que se mantienen por fuerzas electrostticas].

Superficie negativa, cationes en solucinatrados; al aumentar distancia desde lapartcula, atraccin disminuye, generandocapa difusa de iones en solucin.

Potencial electrosttico disminuyeexponencialmente desde la superficie de lapartcula hasta cero, cuando concentracionesde cationes y aniones son iguales.

Espesor de la capa doble muy pequeo vs.dimetro de la partcula (10-8 m).


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Fuerzas de inestabilidad

Movimiento browniano:

Partculas de dimetros de 10-6 m o menos en agua, en constante movimientorpido, desordenado y al azar. Fuente de energa: colisiones de partculas conagua y T, puede contribuir a la estabilidad, pero probable contacto conpartculas en agregados mayores que dependern de la desestabilizacinelectrosttica. Colisiones entre partculas favorecidas por gradienteshidrulicos, mezclando o creando turbulencia. Despreciable cuando aumentael tamao de partcula.

Fuerzas de London- Van derWaals:

Fuerza atmica cohesiva entre tomos. Radio de accin muy reducido. Encoloides se amplia hasta dimensiones comparables con el coloide. Si larepulsin se reduce para permitir contactos entre s , habr adhesin departculas y aglomeracin progresiva (floculacin). La coagulacin (y posteriorfloculacin) de coloides en el agua se logra antes que el potencial Z se reduzcaa cero, ya que este valor oscila entre -5 y -10 mV.

Instability: Water - Nonpolar Solute Interactions

Water non-polar molecules Thermodynamically unfavorable - decrease in entropy Formation of a rigid ice-like clathrate at interface Dipole induced dipole Hydrophobic interactions - association of apolar and

nonpolar groups in aqueous environments

Amphiphilic

molecules

Co-existence at

molecular level

Physical Properties of Water and Ice

Phase transition properties

Melting point at 101.3 k Pa (1 atm) 0.000CBoiling point at 101.3 k Pa (1 atm) 100.000CCritical temperature 373.99CCritical pressure 22.064 MPa (218.6 atm)

Triple point 0.01C and 611.73 Pa (4.589 mm Hg )Enthalpy of fus ion at 0C 6.012 kJ (1 .436 kcal)/molEnthalpy of vaporization at 100C 40.657 kJ (9.711 kcal)/molEnthalpy of sublimination at 0C 50.91 kJ (12.16 kcal)/mol


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Unit cell of ordinary ice at 0C. Circles represent oxygens of water molecules.Nearest neighbor internuclear O-O distance is 2.76 ; is 109.

Physical Properties of Ice

Extended structure of ordinary ice. Oxygen atoms shown. Open and shaded circles,respectively, represent oxygen atoms in upper and lower layers of a basal plane

Physical Properties of Ice

Conceptos generales


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Agua Ligada: existe en la vecindad de solutos y otros constituyentes no-

acuosos; exhibe movilidad reducida y sus propiedades difieren significati-vamente de la masa de agua en el mismo sistema y no congela a -40C

Agua de Constitucin: parte integral de constituyentes no-acuosos* < 0.03% del aguatotal* Regionesintersticiales de proteinas;hidratosqumicos

Agua Vicinal: interacciona fuertemente con sitios hidroflicos especficos pormedio de asociaciones agua-ion y agua-dipolo

* 0.5 0.4 % del agua total* Optimizala estabilidad global de laMonocapa

Agua Multicapa: ocupa lossitios remanentes despus de la primera capa yforma varias capas alrededor de grupos hidroflicos por medio de enlaces dehidrgeno w-w y w-s

* 3 2 % delaguatotal* Las velocidades de reaccin aumentan

Agua en los alimentos

Water Activity (aW)

Relationship between water content and perishability Various foods with the same water content differ significantly in perishability

- Water content alone is not reliable Importance of water associations with non-aqueous constituents to support

deteriorative activities Rates of deteriorative changes and microbial growth at normal food storage

conditions often depend on water content and aW.

Water activity is defined as the ratio of the vapor pressure of water in amaterial (p) to the vapor pressure of pure water (po) at the same T.

The water activity (aW): ratio of the water vapor pressure of the food tothe water vapor pressure of pure water under the same conditions

aW =f / fo = p / po = ERH (%) / 100

n1 moles de solvente (agua) y n2 moles de soluto

Thermodynamic principle [pure water (aw = 1.0), std. state]. In the equilibrium:

Presin de vapor

Isoterma de

adsorcin y

desorcin


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Efecto de la temperatura sobre aw

Estructura de

los alimentos

aw Temperature Dependence

Clausius-Clapeyron equation

P is the vapor pressure

P0 is a vapor pressure at a known temperature T0,

H is the heat of sorption (KJ/mol)

R is the ideal gas law constant, 8.2x10-3

T is thetemperature (in Kelvin).

Sorption Isotherms

Plots interrelating water content of a food with its water activity at constant T The curve is established using a microclimate method

Gas-tight jars where RH is fixed with saturated salt solutions Usefulness

Concentration and Dehydration processesease or difficulty of water removal

Assessing stability (shelf-life) of the food Prevent caking and sticking of food powders Determine moisture barrier properties of packaging materials Formulation of food mixtures

Equilibration process

need pure salt solutions problem of volatile contamination large surface area to food must be slurry- need excess salt mixed type samples takes time 7 to 21 days vacuum fluxing toxicity of salts - eg lithium, nitrite, iodide, bromide


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Zones of

moisture

Moisture adsorption isotherms

at different storage temperatures

At any given EMC, aw increases with increasing temperature

Humedad base hmeda, HbhHumedad base seca, Hbsmw, masa del aguams, masa de la materia secamt, masa total

Presiones parciales del vapor de agua

en el aire Pv y en la superficie del grano Pvg


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Isotermas de sorcin de agua del maz

Fenmenos

de adsorcin y

de desorcin

Calculating the Monolayer

Limitation - only use up to 0.55 maximum

BET Monolayer DeterminationLimit between Zones I & II

Water molecules are bonded to the product by strong H-bonds

Higher Stability Brunauer, Emmett and Teller (BET) method

m = water content (dry basis)

mo = water content at monolayer

c = constant related to heat adsorption


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GAB isotherm equation

Model equations for calculating isotherms

aw and Growth of Microorganisms


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Predicting Food Spoilage

Controlling aw in foods

Limitations

on awlowering

Mtodos para reducir aw

Congelacin Secado/deshidratacin Adicin de solutos

Sal (NaCl, KCl, fosfatos) Azcares (sacarosa, glucosa, fructosa, maltosa, lactosa) Polialcoholes (sorbitol, glicerina, manitol, propilen glicol) cidos (fosfrico, ctrico, ascrbico,fumrico) Hidrolizados de protena Aminocidos (alanina, glicina)

Alimentos de

humedad

Intermedia

[IMF]

Alimentos autoestables Contenido de agua de 25-50%

aw de 0.65 a 0.86 Evitar el desarrollo de bacterias

(Staphylococcus aureus) Levaduras mohos pueden crecer


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Estabilidad de los alimentos

Fru to s cl im atri cos Frutos n o cli matri cos

Albaricoque

Melocotn

Manzana

Pera

Aguacate (solo madura

fuera de la planta)

Pltano

Nectarina

MangoChirimoya

Ciruela

Sanda

Tomate

Kivis

Higos(segn la variedad)

Meln (segnla variedad)

Uva

Cereza

Fresa

Pia

Naranja

Limn

Pomelo

Pepino

MelnHigo

Litchi

Estabilidad de los alimentos - frutas

Estabilidad de los alimentos - secado


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Estabilidad de los alimentos - secado

Estabilidad de los alimentos - liofilizacin

Estabilidad de los alimentos - liofilizacin


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Maximal freeze concentration, no solute crystallization, constant pressure, no time dependenc e.Tm

is the melting point curve, TE is the eutectic point, TmS is the solubility curve. Tg is the glass

transition curve, and Tg is the solute-specific g lass transition temperature of a maximally freeze

concentrated solution. Heavy dashed lines represent conditions of metastable equilibrium. Allother lines represent conditions of equilibrium.

State

Diagram

Binary

System

Another view of a State Diagram

Influencia

de aw y delpH en la

estabilidad

de los

alimentos


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The moisture in food has a direct influence on its weight, and weight of materialis related to its value in dollars.

If the weight of fr uit in a packing house decreases by 5% due to moisture lost tothe air, the owner has lost 5% of his product, which is 5% of gross income.

Moisture in fruit has significant monetary value. Moisture in any food has thesame value as the going unit price of the food. For example, if apples are sellingfor $2.00 per kg and part of the weight is lost in storage due to low humidity air,the lost water has the value of $2.00 per kg!o For every 1,000 kg of $2.00/kg product shipped having 5% moisture loss, the

owner will be paid $100 less than the cost of the product that was placedinto storage.

o Minimizing weight loss is important in maintaining amount of product aswell as its quality.

Money value of moisture in foods

Moisture has a tendency to move from a location of higher concentration to a region oflower concentration. More specifically, moisture in the air at a high vapor pressure will gotowards a location of lower vapor pressure. This movement of moisture is a naturalphenomenon, commonly occurring in foods, and has numerous ramifications.

Fruit left open to room air will quickly dry, lose moisture, and eventually becomeinedible.

Preventing or restricting the movement of moisture is a major objective of the wrappingmaterials business.

Foods are wrapped with plastics to prevent the exchange of moisture. Foods are coveredwith plastic wraps before being heated in the oven to retain moisture in the food.

The availability and variety of packaging materials has grown significantly since the1980s. Materials, primarily plastics, have been developed with unique moisture and gastransmitting properties to meet the defined food requirements (Fellows, 1988).

Water movementand need for

specific packaging

The electrical properties of biological materials have long beenutilized for rapid, non-destructive measurement of moisturecontent. The moisture in agricultural grains is routinely sa mpledfor making decisions about drying, storing, a nd marketing.

Moisture content can be measured rapidly, with low costelectronic sensors, as food products flow through processingoperations. For these reasons, moisture is the primary qualityfactor used in the agricultural grain trade. Electrical conductanceand resistance of water differs significantly from dry matter sothat property can be utilized in determining moisture in foodmaterials (Figure 2.10). Knowledge of these properties providesfor rapid assessment of current quality and storage potential in amodern marketing system.

Dielectric properties of food materials are utilized in microwave heating. Homemicrowave ovens became so popular in the 1980s that now most U.S. homes use themfor thawing, heating, and cooking food. Home microwave ovens use the 2450 MHZfrequency. Industrial usage of microwave ovens has not been as widespread. Microwaveheating results from absorption of electromagnetic waves. Absorption of microwaveenergy depends primarily on the composition of the material. Water and high moisturefoods are excellent absorbers of microwave energy while dry materials and ice are poorabsorbers. Microwave heating, therefore, works best to heat unfrozen foods of highmoisture. Microwave differs from convection heating where heat must move from theoutside inward. Microwave energy can penetrate deeply into the product, although theintensity of this energy decreases with depth. For example, when heating raw potatoesat 2450 MHz, half the total energy is absorbed by the first 0.93 cm of depth (Table 2.04).Despite this depletion of energy, heating occurs rapidly at all locations in a productexposed to microwave energy. This heating may not be uniform throughout due tospatial variation in material properties and in the microwave radiation. The interaction ofthe food material and the microwave heat source makes it even more i mportant to knowthe dielectric properties of the food. Table 2.04 lists dielectric properties of selected foodproducts and related materials.

Water dielectric

properties &

microwaving

Presentacion parte1b Agua - Conceptos_Generales.pdf

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