"Service means activity, for when we serve someone, we are acting. When we serve Krsna, we are preaching Krsna consciousness, or cooking, or cleansing the temple, or distributing books about Krsna, or writing about Him, or shopping for foodstuff to offer Him. There are so many ways to serve."
Path of Perfection, Chapter 1
Shopping for bhoga (unoffered foodstuffs) should be an extension of the Krsna Consciousness with which prasada is prepared and served to the Lord. The same principles of cleanliness and orderliness that apply to one's person and kitchen facilities should also be applied to shopping.
Whenever possible, shop for foodstuffs in stores that are as close to 'mode of goodness' as possible. Such stores are clean, the produce is fresh, the environment is open to air and sunlight, and the mood of people there is positive and happy. Here in the west, we are more likely to experience 'mode of passion' shopping venues, where people are frenetically rushing down florescent lit aisles, and consumerism is the religion. Avoid shopping in places that appear 'mode of ignorance', where food is left rotting in the bins, the air smells foul, and the place is dark or dirty.
Shop methodically and carefully, taking the time to read labels and understand exactly what's contained in the foods you purchase. As Krsna says in the Bhagavad-gita, "If one offers Me with love and devotion a leaf, a flower, a fruit, or water, I will accept it." Krsna will accept offerings of foodstuffs prepared from milk products, vegetables, fruits, nuts, and grains. Meat, fish, and eggs are not offerable, and are strictly avoided. Unfortunately, these forbidden items are often included in prepared foods, hidden under names that are indistinguishable from other additives. For example, some brands of yogurt and sour cream contain gelatin, a substance made from the horns, hooves, and bones of slaughtered animals. Many cheese products contain rennet, an enzyme extracted from the stomach tissue of slaughtered cows. In the "Ingredients" section that follows, you will find a detailed list of food additives and some name brand products that contain animal ingredients.
Certain vegetarian items are also forbidden for offering to the Lord, including garlic and onions. Some Vaisnavas consider these items to be in the mode of darkness (ignorance), because they are grown underground. Similarly, mushrooms are considered unbonafide for offering. Some consider onions and garlic to be mode of passion, because they tend to heat up the blood, while others avoid them because the strength of their flavour dulls to palate to finer, more subtle tastes. Vedic cooks replace the tastes of onion and garlic with a spice called Hing (asafetida), which adds a similar rich taste to cooked foods without overcoming the palate.
On the subject of onions, garlic and mushrooms, we have several interesting comments from Srila Prabhupada's letters and conversations:
"Odor, although one, becomes many - as mixed, offensive, fragrant, mild, strong, acidic and so onaccording to the proportions of associated substances. PURPORT Mixed smell is sometimes perceived in foodstuffs prepared from various ingredients, such as vegetables mixed with different kinds of spices and asafetida. Bad odors are perceived in filthy places, good smells are perceived from camphor, menthol and similar other products, pungent smells are perceived from garlic and onions, and acidic smells are perceived from turmeric and similar sour substances. The original aroma is the odor emanating from the earth, and when it is mixed with different substances, this odor appears in different ways."
My Dear Harer Nama, [...] So far as explaining my letter to Kris, I have listed the ingredients which may use for preparing prasadam but these various varieties of foodstuffs may be prepared either in the given recipes or if you like you may invent nice new formulas for offering. The important thing is that your preparations be palatable for Lord Krishna and that the ingredients be within the groups of ingredients already listed. You have asked me about sassafras and yes, it may be offered. Mushrooms are generally not offered, but there is no prohibition, there is no harm in them.
68-12-01 Letter to Harer Nama
My Dear Himavati, Because mushrooms grow in a filthy place, they are not usually offered to Krishna.
71-11-17 Letter to Himavati
Given the lack of specific prohibition against mushrooms coupled with Srila Prabhupada's comments above, the use of mushrooms in preparations being offered to Krsna is certainly an arguable practice. This is particularly true when you consider that in today's agri-food industry in the west, mushrooms are produced in a growing medium that is nearly always pastuerized or sterilized, which means it's 'cleaner' than most field dirt.
In general, if something can be offered to the Lord as part of a prescribed form of worship, then it is acceptable, otherwise not. Stimulants and intoxicants are always rejected, including coffee, cocoa, tea, tobacco, alcohol, and any items including caffeine. Caffeine-free coffee and herbal teas are acceptable.
Some devotees go so far as to reject all vegetarian products that are "meat look-alikes", finding these products too close for comfort to actual animal foods, or not wanting to be tempted to re-awaken their tastes for these items. Srila Prabhupada, however, found such foods to be acceptable for offering to Krsna. In the "Recipes" section, we've included a collection of items Srila Prabhupada enjoyed as 'false meat' substitutes.