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Freeze Drying in commercial applications, such as pharmaceutical, flowers, and larger food operations, are very expensive and quite complex machines, bordering on "extreme machines". They are equipment intensive, expensive to run, and usually quite high tech.
I have worked on pharma grade freeze dryers as part of my refrigeration trade.
I even looked into purchasing a smaller one to do flowers, or food as you suggested.
Heres one on ebay that could be configured for food, taxidermy, or flowers etc.

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Usually they involve whats know as a "cascade" refrigeration arrangement, where you have a High stage and low stage. The high stage rejects the chamber heat to the air around us, but the low stage depends on the high stage to reject it's heat. Getting all these settings right, and keeping the refrigeration cycle from destroying itself is the trick. Ultra low temperatures cause the refrigeration oil to migrate to the cold spot (evaporator) in the low stage, thus starving the compressor of it's needed oil, then "burnout" occours. Repairs are rediculously high if you have to get service done on them, plus you need a tech that understands the complex cycle.
The other two parts of freeze drying 1) is that usually the shelves inside the freeze dryer have a fluid flowing through them, or electric heaters that make the product "relatively" warm with respect to the cold spot of the chamber (for example cold spot of chamber running at -85 C shelves running at -20C, still frozen, but warmer than the low stage evaporator), this flashes off the water (through sublimation), and the water vapor migrates to the cold spot (evaporator of the low stage known as the condenser to the operator of the freeze dryer, because the vapor freezes, or condenses) and freezes to the cold spot.
2) The freeze dry chamber where the food/pharma/flowers are, is under a deep vacuum from a two stage vacuum pump, this acts essentially the opposite of a "pressure cooker" causing the water to sublime/flash/evaporate at a lower temperature (think boiling water, boils at a lower temperature in the mountains where pressure is lower) so in deep vacuum the water vapor sublimes much easier, and migrates to the "condenser" (low stage evaporator).

here is an article that describes it in much more detail, and probably more correctly than I have described.

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