Panoramix,
It is true that canned and packaged foods leaves a person more tamasic than sattvic. Depending on that person, and their personal doshic and gunic state of being, this can be an significant issue of concern, while for others it will be barely noticeable. Over time, a diet out of balance will likely lead to a gunic imbalance, which is why choosing sattvic foods as often as possible keeps us healthier in all ways.
I wanted to speak to this same line of IA’s: [quote=InnerAthlete;17110]Diets should not be balanced but balancing and some students may actually need to eat tamasic foods in order to be balanced human beings.[/quote]
From my experience, it is unlikely that intentionally eating tamasic foods will bring any person into balance on the first kosha, or the any of the subtler bodies; this is true even for a person who is current in state of high rajas. True balance (not just symptom care) can only be approached through increasing sattvic, tridoshic and high-pranic foods, not by introducing or increasing an imbalanced diet on the opposite end of the gunic spectrum.
[I]In response to your question, we are speaking about foods and the gunic qualities they are likely to impart on the eater[/I], but in also working with foods and the doshas, we again look to choose foods that move the person to balance. Depending on what is needed for that person, foods and practices will ideally bring imbalances toward balance–again, this is done by dosha-pacifying choices, not by trying to increase the opposite dosha. It is inadvisable, under any circumstances, to try to[I] increase[/I] a particular dosha or to increase rajas or tamas as an Ayurvedic treatment or care protocol. This is because all 3 doshas, and the 3 gunas, are always present in our constitutions, it is only the ratios and states of being that change. Bringing one dosha toward balance is balancing to the other 2 as well—balance moving toward balance, moving toward balance. Focusing on sattva elevates tamas and rajas into sattva.
I personally do not drink the dairy-alternatives that you listed, because I enjoy and do well with limited dairy, but I do use canned veggies in conjunction with fresh veggies when I cook at home. I limit the canned things because my relationship to them is different. It is such a pleasure, a real pleasure, to pick out the red pepper that I [B]want [/B]to eat at the store. It is pleasure to wash it and cut it up—these things are balancing and nourishing to me too, not just the eating of them. Opening a can just doesn’t “spin my wheels”
in the same way and that is because they lack the prana that the fresh foods hold and impart to the consumer.
Since you are conscious of foods and their imparting qualities, here is something that you may find interesting. From August until about mid-January, I had been traveling in India, Nepal and Tibet. In five months, there was only [B]one[/B] place that had a refrigerator, so our meals where always cooked fresh from local items, never stored or reheated for later meals. It is now being back home, going on 2 months, that I am seeing clearly for myself just how tamasic- and kapha-increasing left-overs are to my koshas, gross to subtle. As a therapist, I know these things, yes, because of my studies and work, but it was difficult to be really clear on their personal impact because the very aspects of tamas or kapha imbalance make it difficult to be highly discerning–getting that break for these many months, allowed me enough space to clear up those imbalances and to therefore feel the impact of them now upon my return.
I love questions like this one because it elevates our forum and fosters awareness and thoughtfulness. Thank you for starting this thread and hopefully more members will add to it 
All happiness,
Nichole