文档介绍：Chapter 9 Quick freezing of fruits and vegetables
Introduction: advantage: foods of convenience
1872,French Charles Tellier
1928 ,quick frozen food , . ,
1937 vegetables . IQF (individual quick freezing )
The first section Principle of freezing
一、Definition
1. Quick freezing
When the centre temperature of food is cooled from -1℃to -5℃, the time maintains less 30 minutes, the process names quick freezing.
process where the temperature of the food passes through the zone of maximum ice crystal formation(-1℃ to -5℃) in 30 min or less.
2. zone of maximum ice crystal formation (最大冰晶生成区):
refers to the temperature range of -1℃—-5℃.During the period, over 80% water will be frozen and turned into ice.
The first section Principle of freezing
二、Principles
The preservation of fruits and vegetables by freezing depends on two principles:
(一) Activity of anisms is inhibited at low temperatures.
Freezing will kill some anisms, those that survive freezing will not proliferate during frozen storage. Critical temperature of inhibiting anisms propagating is about -12℃.
The first section Principle of freezing
(二) Activity of the enzymes naturally present is slowed down.
Enzymic activity in frozen foods is very low at subfreezing temperatures, so the biochemical reaction is very low, we can use it to store the foods.
Temperature of -18℃ was used to control the activity of the enzymes usually.
The first section Principle of freezing
三、Ice crystal formation
Under standard conditions: water
Living cell of fruits and vegetables
The freezing point
The first section Principle of freezing
(一)The crystallization process occurs in two steps
1. nucleation or crystal formation (晶核的形成)
The temperature of the water slurry(悬浮液)returns to freezing point, thermal motion of water molecule slow down, after the supercooling point, it begin to form the nucleation(晶核), which is the center of the crystal.
The first section Principle of freezing
2. crystal growth(冰晶体的生长)