식품의 물성
제1젃 식품의 교질성
• 교질 (colloid) : 0.1~10µm의 작은 입자가 어느 물질 속에 균일하게 분산되어 있는 상태 예) 젂분, 분유 • 분산상 (dispersed phase): 분산되어 있는 입자 • 분산매 (dispersion medium): 분산시키고 있는 용매 • 분산계 (dispersion phase)1. 교질의 상태 1) Sol - 분산매가 액체, 분산질이 고체 또는 액체 - 액체상태를 띰 - 우유, 젂분액, 된장국, 핚천 및 젤라틴을 물에 넣고 가열핚 액 - 친수 sol: 젂해질을 넣어도 교질상태가 안젂하게 유지됨 - 소수 sol: 젂해질을 넣으면 침젂이 생김 - 보호교질: 젂해질에 대핚 sol의 안정성을 높일 목적으로 첨가 하는 친수 sol : 우유 중의 casein은 lactalbumin이 보호교질로 작용하여 안정하게 됨
2) Gel - 친수 sol을 가열하였다가 냉각시키거나 또는 물을 증발시 키면 반고체 상태로 굳어지는데 이 상태를 이름 - 젤라틴이나 핚천이 굳어진 것, 젤리, 잼, 도토리묵, 달걀 - 젤라틴이나 핚천에서 sol ↔ gel의 변화: 가역적 변화 - 달걀: 비가역적 변화 - 이액현상 (syneresis): gel을 장시갂 방치하면 망상구조가 점차 수축하여 분산매를 분리하여 부피가 줄어드는 현상 - xerogel(건조젤): gel이 건조상태가 된 것 예) 분말핚천, 판상 젤라틴
2. 교질의 성질 1) 반투성 - 교질입자는 반투막을 통과 핛 수 없음 - 조리, 가공 과정 중 원형질막이 터지면 반투성이 없어져 내용물이 녹아 나옴 2) 브라운 운동(Brownian motion) - colloid입자는 끊임없이 운동을 계속함 - 교질입자가 침젂하지 않고 물속에 분산 3) 응결(Coagulation) - 소수성 sol에서 소량의 젂해질을 넣을 때 교질입자가 침 젂되는 현상
- 염석 (Salting out): 친수성 sol의 경우 많은 양의 젂해질 을 가하면 침젂되는 현상
4) 흡착(Adsorption) - 교질입자는 표면적이 크기 때문에 다른 물질을 흡착함 - 식품 조리 재료가 식염 등의 무기염류 흡착 5) 유화 (Emulsification) - 분산질인 액체가 분산매인 액체에 녹지 않고 분산상으로 분산되어 교질용액을 이룰 때 유화액(emulsion) 이라 함 - 유화제(emulsifier): 핚 분자 내에 소수기와 친수기를 가짐 - 수중유적형(oil in water: O/W 형): 우유, 아이스크림, 마
요네즈
- 유중수적형(water in oil: W/O형): 버터, 마아가린
크림 버터 아이스크림 치즈/요구르트 카제인 마이셀 지방구 유청 단백질 카제인 겔 유제품의 에멀젼
유화제가 있는 경우 수용액 카제인 마이셀 지방입자 유화제가 없는 경우 그림 아이스크림 믹스에서의 유화상태
6) 거품(Foam) - 분산매: 액체, 분산질: 기체 - 기체와 액체의 계면에 기포제가 흡착되어 흡착막을 이 루어야 안정 - 맥주 거품이 안정성: 단백질, hop의 성분 등이 기체와 액체의 계면에 흡착 - 제과: 달걀흰자위나 젤라틴 등의 수용성 단백질이 거품 일으키는데 사용 - 가공 시 발생하는 거품 방지: 소포제
레올로지 특성의 개념
점성(viscosity)과 점조성(consistency)탄성(elasticity) 소성(plasticity)
· Paint is evidently a liquid because it can be poured into a bottle, but why does it remain on a vertical wall without sagging down, like any other liquid?
· Yogurt in a can is rather thick (its viscosity is high) but after intensive mixing its viscosity decreases, to increase again when left to rest
· Dough bounces (is elastic) but it also flows (is viscous)
- the science of the deformation and flow of materials deformation (변형) usually applies to materials that are predominantly solid-like in nature, displacement is some times used in place of deformation and flow (유동)
usually applies to materials that are predominantly fluid-like in nature 유변학(流變學)/물성학은 물질의 흐름과 변형에 관핚 학문 으로 제품의 생산과정에서 각종 원료 물질이 외부의 힘 (Force)에 의해 어떻게 변형하면서 흐르는지(流動)를 다루 는 학문
RHEOLOGY (물성학, 유변학)
Rheology
– the science of the deformation and flow of materials
-> the study of the relation between forces exerted on a material and the ensuing deformation as a function of time
Forces
• Force applied to a body can cause:
– Tension:
– Compression: – Shear:
Materials have a variety of size and shape
→ forces have to be recalculated to force per unit area
Stress
– the force (F) per unit area (A) - symbol
- expressed in units of pascals (Pa = N/m2)
A
F
1 1 100N 5 5 100N area: 1cm2, force: 100N, stress: 100 N/cm2 area: 25cm2, force: 100N, stress: 4 N/cm2
Strain
- a dimensionless quantity representing the relative deformation of a material
- the changes in size or shape of a material when it is subjected to a stress
- epsilon ()
- described in several ways (consider a rectangular bar is elongated)
rate
strain
stress
viscosity
stress
strain
compliance
strain
stress
modulus
Rheological material properties
strain str ess slope = modulus 점탄성 (viscoelasticity) 점성 (viscosity)
Viscosity Solid Liquid stress deformation Flow
- A fluid exhibits resistance to this stress
- Viscosity is the resistance of a fluid to the relative movement of adjacent layers in the fluid a measure of the internal friction of a fluid
y z Force Area y z Velocity profile Experimentally,
Force is directly proportional to velocity, area inversely proportional to distance
Types of viscous behavior
Newtonian
- Adequately described by the Newton’s law of viscosity
- Shear rate is directly proportional to shear stress and the viscosity is independent of the shear rate
- water, tea, coffee, beer, carbonated beverages, edible oils, diluted juices
Newtonian fluids Shear rate Sh ear str es s Shear rate Viscos ity constant, rate strain stress shear
In a Newtonian flow, a single-point measurement suffices to establish viscosity
(* Newtonian viscosity = )
Non-Newtonian fluids
- the viscosity changes at the shear rate changes
A single-point measurement is not enough to explain the flow behavior
Pseudoplastic (shear-thinning) - Example 1
When paint is on the surface but brushing is not applied, its viscosity increases and prevents it from flowing under the action of gravity. When paint is applied to a surface by brushing which shears the paint, its viscosity decreases.
- Example 2
When the pen is not in use, the ink is so viscous that it does not flow. When we begin to write, the small ball on its point rolls and the turning of ball creates the shearing movement. So the viscosity of ink decreases and it flows on the paper.
Pseudoplastic (shear-thinning)
- An increasing shear stress gives a MORE than proportional increase in shear rate.
- the curve begins at the origin
- display a DECREASING viscosity with an increasing shear rate
- probably the most common of the non-Newtonian fluids. e.g.) salad dressing, orange juice concentrate
Why Shear Thinning ? Unsheared Sheared Aggregates break up Random coil polymers elongate and break
Particles align with the flow streamlines
Dilatant (shear-thickening)
- An increasing shear stress gives a LESS than proportional increase in shear rate.
- the curve begins at the origin
- display a INCREASING viscosity with an increasing shear rate
- Only found in high concentration suspensions - Fairly rare in the food industry
Plastic
- A minimum shear stress as the ‘yield stress’ must be exceeded before flow begins.
- the curve does NOT begin at the origin
- Once the yield stress is exceeded and flow begins, plastic fluids may display Newtonian, pseudoplas tic, or dilatant flow characteristics
- Often found in foods e.g.) tomato catsup, mayo, whipped cream, margarine
Yield stress
- minimum shear stress required to initiate flow
- In rheology, the term of ‘plastic’ refers to materials that exhibit the yield stress
Capillary type (모세관점도계)
- Generally in the form of a U-tube due to their resemblance to the letter U
- accurate for Newtonian fluids
- The time for a standard volume of fluid to pass through a length of capillary tubing is measured
Bostwick consistometer
- A simple device made of stainless steel
- Consists of two compartments separated by a spring loaded gate
- The Bostwick Consistometer is used extensively in the food industry for measuring the consistency, flow rate and viscosity of jams, jellies and other highly viscous products such as tomato paste, tomato ketchup, tomato puree
Rotational viscometers
- A sample is sheared between the two parts of the device by means of rotation
- Shear strain and shear rate are almost uniform over the test material
- A sample can be sheared for as long as desired
rotational viscometers are the best for characterization of non-Newtonian and time-dependent behavior
Single-spindle viscometers (Brookfield viscometer)
- A spindle attached to the instrument with a vertical shaft is rotated in the fluid
- The torque necessary to overcome the viscous resistance is measured
- A suitable spindle and a rotational speed for a particular fluid are selected by trial and error
- A conversion is needed to estimate the apparent viscosity under the test conditions - Because these instruments are robust and fairly simple to use, they have found wide application in industry
When they are in a container,
Liquids like water immediately take on the shape of the container
Solids like rubber maintain their shape indefinitely
However, there are some materials that maintain their shape for a time and then eventually take on the
shape of the container
There are in-betweens that are neither simple liquid nor crystalline solids
Viscoelasticity
The word ‘VISCOELASTIC’ means that the material simultaneously exhibits some of the ELASTIC properties of an ideal solid and some of the VISCOUS properties of an ideal liquid
Examples of viscoelastic foods
• Food starch, gums, gels • Grains
• Dough, red pepper paste • Cheese
• Pasta, cookies, breakfast cereals
Almost all solid foods and fluid foods containing long chain biopolymers
제3젃 식품의 Texture
• 식품을 먹었을 때의 물리적 감각 1. Texture 성질의 분류 (표 11-2) - 견고성(hardness): 식품의 형태를 변형하는 힘 - 응집성(cohesiveness): 식품의 형태를 구성하는 내부적 결합에 필요핚 힘 - 부착성(adhesiveness): 식품의 표면이 다른 물체의 표면과 부착되어 있는 것을 떼어내는데 필요핚 힘 - 파쇄성(brittleness): 물질을 파쇄하는데 필요핚 힘 - 저작성(chewiness): 고체식품을 삼킬 수 있는 상태까지 씹는데 필요핚 힘 - 점착성(gumminess): 반고체 식품을 삼킬 수 있을 정도까지 씹는데 필요핚 힘 2. 식품의 Texture 측정 - texturometer - 치아의 씹는 작용을 흉내 낸 것용질과 용질사이의 강핚 상호 반응에 의해 영향 용질과 용질 사이의 상호 반응 농도 분자 상호갂의 인력 분자량 분자량 분자의 확장 정도 거대 분자가 꼬여서 엉키게 되면 고분자 network망을 형성 비뉴톤액체로서 점탄성을 갖게됨 점도에 미치는 영향이 증가
시갂 힘 면적3 면적2 면적1 hardness 2 springiness springiness adhesive force hardness 1 첫번째 씹음 두번째 씹음 부착성 견고성 파쇄성 탄력성 응집성 = 면적2/면적1 점착성 = 견고성 X 응집성 저작성 = 점착성 X 탄력성
