Temperature and Relative Humidity of fruits and vegetables stored in Deep freezers

[mathew kurian]

Dear Friends,

I would like to design a multi chamber deep freezer for fruits and vegetables, hence I would like to know or get any article to select the product temp and RH. I would also like to know how to calculate the refrigerated room temp and RH.


Mathew Kurian
mathew.kurian@hotmail.com

[hookster]

Hi Mathew
Wow you open up a huge can of worms! e.g. soft fruits/hard fruits, leaf - root vegetables!
It is far easier to select a target commodity and design around that. The RH will be important but extremely difficult to manipulate in a Frozen chamber.

Personally I would stick to ASHRAE for my information required but majority of what you are requesting seems to be a cross over between chill & frozen fundamentals. Specific heat and respiration rates for product with the mass of stored product will be your base for design. Target temperatures will be determined via product storage temperatures and cool down times required.

Sorry if this is vague but your question is a mass of incorporated design features required.

[mathew kurian]

Hey Thank you for the reply.

I am new to refrigeration field. I am doing this project as part of my studies.

I was able to find storage temp and Rh of fresh fruits and vegetables in ASHRAE but I understand that those temperature cannot be taken for deep freezers.
for example I select Apple as the product which as a storage temp of -1 deg Celsius and maintained at 90-95 % RH but I should store the product in a deep freezer according to my project. Hence I would like to make assumption of storing the apples in deep freezer of -25 Deg Celsius but in that case what will be RH.

[hookster]

You always need to ask yourself the question when last did I buy a frozen apple?
Well you would not have, it would only be supplied processed and frozen. Apples have very good life cycle storage and with Controlled Atmosphere that is extended even further.

RH in a freezer is always assumed to be 100% (there are variances but this is due to external changes ie defrost, air ingress etc) The cooler the air the higher the RH but the lower the actual moisture content entrained.

Freezer temperature variations will cause RH fluctuations with detrimental effect on product. Product containers will reduce moisture loss and a fast blast freeze limits moisture loss from the outer skin while core temperature is reduced to required storage temperature, reducing ice damage to the fruit cells due to expansion.

Do they want you to include product treatment in your case study i.e Citric acid treatment to reduce browning and ascorbic acid for preservative?

All in all it sounds a nice project.

[chemi-cool]

The only fruit you can freeze are dates.
All the other fruit and vegetables will be destroyed once they are frozen. Is it a new start-up?

I suggest that you look for other material for deep freezing, such as meat, ice-cream, Industrialized food, etc, etc.

[mathew kurian]

Thank you for the valuable informations.
I got a outline idea to start off my project but I need to do lot of research on the topic and take advices from experienced people. I will not be considering product treatment, instead I will go for a deep freezer to store sliced vegetables like carrots, green peas, and corn in one chamber, in another chamber I will consider strawberries.

[chemi-cool]

Good luck.

[hookster]

Carrots, peas & Corn are ideal frozen products.
If you solve the frozen strawberries issues please contact me! I would love to be involved in reaping those millions $$$$$ :D

[install monkey]

liquid nitrogen will freeze veg without destroying the insides-job done!:o

[hookster]

Liquid Nitrogen is great for snap freezing!
Now we need to be able to remove the time lag when changing state back to desired conditions!

[mathew kurian]

Dear Friends,

Using Cryogents and Accelerated freeze drying are good option, but I want to do it with a deep freezer. Kindly suggest me a manufacture of compressor that works at evaporating temperature low as -30 deg celsius.

[mikeref]

Mathew, you say this is for studies only? Theoretical or practical?
Once most fruit is frozen, even by a few degrees below 0 C, the high concentration of water within the produce freezes and expands, damaging the fruit. When thawed out, all that is left is a soggy mess. Any percentage of relative humidity is not going to make a difference to the fruit interior.

[chemi-cool]

Most food freezing is done in a frizzing tunnel, you don't just throw a bag full of vegetables into a freezer.
Read carefully what Mike had written and search google for ways to freeze veggies.

All the compressor manufacturers make compressors for -40C.

[Peter_1]

Once most fruit is frozen, even by a few degrees below 0 C, the high concentration of water within the produce freezes and expands, damaging the fruit. When thawed out, all that is left is a soggy mess. Any percentage of relative humidity is not going to make a difference to the fruit interior.

Depends how and more, how fast the product was frozen. We have a client who freezes several tons/day fruit and vegetables.

[mathew kurian]

Dear Friends,

My project is theoretical but I want to know the practical problems from experinced people because I belive that this feild in learnt from experince than from literatures. I have read that we should go for faster cooling because it leads to smaller crystals and retaining the properties of the product.

[mathew kurian]

Dear Friends,

I hope that the packet of sliced vegetables we buy from super markets are just sliced and then frozen in deep freezers, without any kind of treatments.

[sterl]

Slow freezing vegetables will create large ice crystals and disrupted cell walls. Slow freezing fruit does much the same but bulk of fruit is not built like that of a vegetable so the damage to fruit tends to be greater. In addition: Cutting corn or carrots to make small particles is OK. Peas and beans are already small particles. Mostly we would cook these before eating. RE: Small particles: So are cranberries and blueberries but they rupture their skin when frozen slow....Ok if you are going to make a pie, not so good to snack on.

Fruits come in a lot of different sizes and some are more delicate than others but a lot of them have either a core or a hollow in the middle. Some are compound structures: like grapes, they have an outside skin that is give or take tough, and a core that is pretty delicate. Slicing them up is usually not conducive to suitability for consumption in the normal sense. Most of us not too interested in bananas turned into muck brown stuff....

As to time limits and similar: contact freezers (otherwise plate freezers) can do a reasonable job on small fruit that is "rugged" and finish freeze in about an hour. A big IQF freezer can freeze most particle vegetables in 8 to maybe 20 minutes. Put whole strawberries through the IQF and they will come out frozen; they will still be mush when you try to thaw them. To avoid that: forget moving air as part of the mechanism, put them directly into a bath of cryogen. They, and blueberries and cranberries and raspberries and so on freeze quite nicely; but they still won't thaw to the original structure and texture....Its all pie filling at that point.